Introduce user specified latency in the packet reception path
By exposing the test parameters as part of the debugfs channel
attributes. We will control the testing state via these attributes.
Signed-off-by: Branden Bonaby <brandonbonaby94@gmail.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Based on 1 normalized pattern(s):
this program is free software you can redistribute it and or modify
it under the terms and conditions of the gnu general public license
version 2 as published by the free software foundation this program
is distributed in the hope it will be useful but without any
warranty without even the implied warranty of merchantability or
fitness for a particular purpose see the gnu general public license
for more details you should have received a copy of the gnu general
public license along with this program if not write to the free
software foundation inc 59 temple place suite 330 boston ma 02111
1307 usa
extracted by the scancode license scanner the SPDX license identifier
GPL-2.0-only
has been chosen to replace the boilerplate/reference in 33 file(s).
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Allison Randal <allison@lohutok.net>
Reviewed-by: Kate Stewart <kstewart@linuxfoundation.org>
Reviewed-by: Alexios Zavras <alexios.zavras@intel.com>
Cc: linux-spdx@vger.kernel.org
Link: https://lkml.kernel.org/r/20190530000435.254582722@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Fix a race condition that can result in a ring buffer pointer being set
to null while a "_show" function is reading the ring buffer's data. This
problem was discussed here: https://lkml.org/lkml/2018/10/18/779
To fix the race condition, add a new mutex lock to the
"hv_ring_buffer_info" struct. Add a new function,
"hv_ringbuffer_pre_init()", where a channel's inbound and outbound
ring_buffer_info mutex locks are initialized.
Acquire/release the locks in the "hv_ringbuffer_cleanup()" function,
which is where the ring buffer pointers are set to null.
Acquire/release the locks in the four channel-level "_show" functions
that access ring buffer data. Remove the "const" qualifier from the
"vmbus_channel" parameter and the "rbi" variable of the channel-level
"_show" functions so that the locks can be acquired/released in these
functions.
Acquire/release the locks in hv_ringbuffer_get_debuginfo(). Remove the
"const" qualifier from the "hv_ring_buffer_info" parameter so that the
locks can be acquired/released in this function.
Signed-off-by: Kimberly Brown <kimbrownkd@gmail.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Set "ring_info->priv_read_index" to 0. Now, all of ring_info's fields
are explicitly set in this function. The memset() call is no longer
necessary, so remove it.
Signed-off-by: Kimberly Brown <kimbrownkd@gmail.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Counter values for per-channel interrupts and ring buffer full
conditions are useful for investigating performance.
Expose counters in sysfs for 2 types of guest to host interrupts:
1) Interrupts caused by the channel's outbound ring buffer transitioning
from empty to not empty
2) Interrupts caused by the channel's inbound ring buffer transitioning
from full to not full while a packet is waiting for enough buffer space to
become available
Expose 2 counters in sysfs for the number of times that write operations
encountered a full outbound ring buffer:
1) The total number of write operations that encountered a full
condition
2) The number of write operations that were the first to encounter a
full condition
Increment the outbound full condition counters in the
hv_ringbuffer_write() function because, for most drivers, a full
outbound ring buffer is detected in that function. Also increment the
outbound full condition counters in the set_channel_pending_send_size()
function. In the hv_sock driver, a full outbound ring buffer is detected
and set_channel_pending_send_size() is called before
hv_ringbuffer_write() is called.
I tested this patch by confirming that the sysfs files were created and
observing the counter values. The values seemed to increase by a
reasonable amount when the Hyper-v related drivers were in use.
Signed-off-by: Kimberly Brown <kimbrownkd@gmail.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
fc96df16a1 is good and can already fix the "return stack garbage" issue,
but let's also improve hv_ringbuffer_get_debuginfo(), which would silently
return stack garbage, if people forget to check channel->state or
ring_info->ring_buffer, when using the function in the future.
Having an error check in the function would eliminate the potential risk.
Add a Fixes tag to indicate the patch depdendency.
Fixes: fc96df16a1 ("Drivers: hv: vmbus: Return -EINVAL for the sys files for unopened channels")
Cc: stable@vger.kernel.org
Cc: K. Y. Srinivasan <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
The UIO driver needs the ring buffer to be persistent(reused)
across open/close. Split the allocation and setup of ring buffer
out of vmbus_open. For normal usage vmbus_open/vmbus_close there
are no changes; only impacts uio_hv_generic which needs to keep
ring buffer memory and reuse when application restarts.
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Add comments describing intricacies of Hyper-V ring buffer
signaling code. This information is not in Hyper-V public
documents, so include here to capture the knowledge for
future coders.
There are no code changes in this commit.
Signed-off-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Netvsc has a function to calculate how much ring buffer in percentage is
available to write. This function is also useful for storvsc and other
vmbus devices.
Define a similar function in vmbus to be used by other vmbus devices.
Signed-off-by: Long Li <longli@microsoft.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Fix bugs in signaling the Hyper-V host when freeing space in the
host->guest ring buffer:
1. The interrupt_mask must not be used to determine whether to signal
on the host->guest ring buffer
2. The ring buffer write_index must be read (via hv_get_bytes_to_write)
*after* pending_send_sz is read in order to avoid a race condition
3. Comparisons with pending_send_sz must treat the "equals" case as
not-enough-space
4. Don't signal if the pending_send_sz feature is not present. Older
versions of Hyper-V that don't implement this feature will poll.
Fixes: 03bad714a1 ("vmbus: more host signalling avoidance")
Cc: Stable <stable@vger.kernel.org> # 4.14 and above
Signed-off-by: Michael Kelley <mhkelley@outlook.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
The last use of hv_get_ringbuffer_availbytes in drivers is now
gone. Only used by the debug info routine so make it static. Also, add
READ_ONCE() to avoid any possible issues with potentially volatile
index values.
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
When iterating over incoming ring elements from the host, prefetch
the next descriptor so that it is cache hot.
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Don't signal host if it has disabled interrupts for that
ring buffer. Check the feature bit to see if host supports
pending send size flag.
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Don't need cached read index anymore now that packet iterator
is used. The iterator has the original read index until the
visible read_index is updated.
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
The function hv_signal_on_read was defined in hyperv.h and
only used in one place in ring_buffer code. Clearer to just
move it inline there.
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
With new iterator functions (and the double mapping) the ring buffer
read function can be greatly simplified.
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Here is the big set of new char/misc driver drivers and features for
4.12-rc1.
There's lots of new drivers added this time around, new firmware drivers
from Google, more auxdisplay drivers, extcon drivers, fpga drivers, and
a bunch of other driver updates. Nothing major, except if you happen to
have the hardware for these drivers, and then you will be happy :)
All of these have been in linux-next for a while with no reported
issues.
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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Merge tag 'char-misc-4.12-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/char-misc
Pull char/misc driver updates from Greg KH:
"Here is the big set of new char/misc driver drivers and features for
4.12-rc1.
There's lots of new drivers added this time around, new firmware
drivers from Google, more auxdisplay drivers, extcon drivers, fpga
drivers, and a bunch of other driver updates. Nothing major, except if
you happen to have the hardware for these drivers, and then you will
be happy :)
All of these have been in linux-next for a while with no reported
issues"
* tag 'char-misc-4.12-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/char-misc: (136 commits)
firmware: google memconsole: Fix return value check in platform_memconsole_init()
firmware: Google VPD: Fix return value check in vpd_platform_init()
goldfish_pipe: fix build warning about using too much stack.
goldfish_pipe: An implementation of more parallel pipe
fpga fr br: update supported version numbers
fpga: region: release FPGA region reference in error path
fpga altera-hps2fpga: disable/unprepare clock on error in alt_fpga_bridge_probe()
mei: drop the TODO from samples
firmware: Google VPD sysfs driver
firmware: Google VPD: import lib_vpd source files
misc: lkdtm: Add volatile to intentional NULL pointer reference
eeprom: idt_89hpesx: Add OF device ID table
misc: ds1682: Add OF device ID table
misc: tsl2550: Add OF device ID table
w1: Remove unneeded use of assert() and remove w1_log.h
w1: Use kernel common min() implementation
uio_mf624: Align memory regions to page size and set correct offsets
uio_mf624: Refactor memory info initialization
uio: Allow handling of non page-aligned memory regions
hangcheck-timer: Fix typo in comment
...
Allow driver to get debug information about state of the ring.
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
hv_ringbuffer_read cleanup.
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Don't initialize variables that are then set a few lines later.
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
No need for empty return at end of void function
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This is mostly just a refactoring of previous functions
(get_pkt_next_raw, put_pkt_raw and commit_rd_index) to make it easier
to use for other drivers and NAPI.
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Takes less clock cycles to check for ring wrap and subtract than to
do a modulus instruction.
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Functions that just query state of ring buffer can have parameters
marked const.
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
In order to implement NAPI in netvsc, the driver needs access to
control host interrupt mask.
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
All current usage of vmbus write uses the acquire_lock flag, therefore
having it be optional is unnecessary. This also fixes a sparse warning
since sparse doesn't like when a function has conditional locking.
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
The flag to cause notification of host is unused after
commit a01a291a282f7c2e ("Drivers: hv: vmbus: Base host signaling
strictly on the ring state"). Therefore remove it from the ring
buffer internal API.
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Commit a389fcfd2c ("Drivers: hv: vmbus: Fix signaling logic in
hv_need_to_signal_on_read()")
added the proper mb(), but removed the test "prev_write_sz < pending_sz"
when making the signal decision.
As a result, the guest can signal the host unnecessarily,
and then the host can throttle the guest because the host
thinks the guest is buggy or malicious; finally the user
running stress test can perceive intermittent freeze of
the guest.
This patch brings back the test, and properly handles the
in-place consumption APIs used by NetVSC (see get_next_pkt_raw(),
put_pkt_raw() and commit_rd_index()).
Fixes: a389fcfd2c ("Drivers: hv: vmbus: Fix signaling logic in
hv_need_to_signal_on_read()")
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Reported-by: Rolf Neugebauer <rolf.neugebauer@docker.com>
Tested-by: Rolf Neugebauer <rolf.neugebauer@docker.com>
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Stephen Hemminger <sthemmin@microsoft.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
After the channel is rescinded, the host does not read from the rescinded channel.
Fail writes to a channel that has already been rescinded. If we permit writes on a
rescinded channel, since the host will not respond we will have situations where
we will be unable to unload vmbus drivers that cannot have any outstanding requests
to the host at the point they are unoaded.
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signal the host when we determine the host is to be signaled -
on th read path. The currrent code determines the need to signal in the
ringbuffer code and actually issues the signal elsewhere. This can result
in the host viewing this interrupt as spurious since the host may also
poll the channel. Make the necessary adjustments.
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signal the host when we determine the host is to be signaled.
The currrent code determines the need to signal in the ringbuffer
code and actually issues the signal elsewhere. This can result
in the host viewing this interrupt as spurious since the host may also
poll the channel. Make the necessary adjustments.
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
One of the factors that can result in the host concluding that a given
guest in mounting a DOS attack is if the guest generates interrupts
to the host when the host is not expecting it. If these "spurious"
interrupts reach a certain rate, the host can throttle the guest to
minimize the impact. The host computation of the "expected number
of interrupts" is strictly based on the ring transitions. Until
the host logic is fixed, base the guest logic to interrupt solely
on the ring state.
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
With wrap around mappings for ring buffers we can always use a single
memcpy() to do the job.
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Tested-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Make it possible to always use a single memcpy() or to provide a direct
link to a packet on the ring buffer by creating virtual mapping for two
copies of the ring buffer with vmap(). Utilize currently empty
hv_ringbuffer_cleanup() to do the unmap.
While on it, replace sizeof(struct hv_ring_buffer) check
in hv_ringbuffer_init() with BUILD_BUG_ON() as it is a compile time check.
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Tested-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
For synthetic NIC channels, enable explicit signaling policy as netvsc wants to
explicitly control when the host is to be signaled.
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Implement APIs for in-place consumption of vmbus packets. Currently, each
packet is copied and processed one at a time and as part of processing
each packet we potentially may signal the host (if it is waiting for
room to produce a packet).
These APIs help batched in-place processing of vmbus packets.
We also optimize host signaling by having a separate API to signal
the end of in-place consumption. With netvsc using these APIs,
on an iperf run on average I see about 20X reduction in checks to
signal the host.
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
In preparation for implementing APIs for in-place consumption of VMBUS
packets, movve some ring buffer functionality into hyperv.h
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Use the virt_xx barriers that have been defined for use in virtual machines.
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Use the READ_ONCE macro to access variabes that can change asynchronously.
This is the recommended mechanism for dealing with "unsafe" compiler
optimizations.
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Introduce separate functions for estimating how much can be read from
and written to the ring buffer.
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
On the consumer side, we have interrupt driven flow management of the
producer. It is sufficient to base the signaling decision on the
amount of space that is available to write after the read is complete.
The current code samples the previous available space and uses this
in making the signaling decision. This state can be stale and is
unnecessary. Since the state can be stale, we end up not signaling
the host (when we should) and this can result in a hang. Fix this
problem by removing the unnecessary check. I would like to thank
Arseney Romanenko <arseneyr@microsoft.com> for pointing out this issue.
Also, issue a full memory barrier before making the signaling descision
to correctly deal with potential reordering of the write (read index)
followed by the read of pending_sz.
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Tested-by: Dexuan Cui <decui@microsoft.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
On the channel send side, many of the VMBUS
device drivers explicity serialize access to the
outgoing ring buffer. Give more control to the
VMBUS device drivers in terms how to serialize
accesss to the outgoing ring buffer.
The default behavior will be to aquire the
ring lock to preserve the current behavior.
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
The function hv_ringbuffer_read() is called always on a pre-assigned
CPU. Each chnnel is bound to a specific CPU and this function is
always called on the CPU the channel is bound. There is no need to
acquire the spin lock; get rid of this overhead.
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Currently, there is only one user for hv_ringbuffer_read()/
hv_ringbuffer_peak() functions and the usage of these functions is:
- insecure as we drop ring_lock between them, someone else (in theory
only) can acquire it in between;
- non-optimal as we do a number of things (acquire/release the above
mentioned lock, calculate available space on the ring, ...) twice and
this path is performance-critical.
Remove hv_ringbuffer_peek() moving the logic from __vmbus_recvpacket() to
hv_ringbuffer_read().
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
hv_ringbuffer_peek() does the same as hv_ringbuffer_read() without
advancing the read index. The only functional change this patch brings
is moving hv_need_to_signal_on_read() call under the ring_lock but this
function is just a couple of comparisons.
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Convert 6+-string comments repeating function names to normal kernel-style
comments and fix a couple of other comment style issues. No textual or
functional changes intended.
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Fixes a bug where previously hv_ringbuffer_read would pass in the old
number of bytes available to read instead of the expected old read index
when calculating when to signal to the host that the ringbuffer is empty.
Since the previous write size is already saved, also changes the
hv_need_to_signal_on_read to use the previously read value rather than
recalculating it.
Signed-off-by: Christopher Oo <t-chriso@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
In win8 we have a feature that allows for interrupt driven flow management
for host/guest communication. For instance, if the host were blocked because
there was no space available in the ringbuffer, the host could request that the
guest send an interrupt when space becomes available in the ringbuffer (when
the guest drains the ringbuffer).
While this feature was implemented in the guest a while ago, we had not
advertised that the guest supported this feature. This patch advertises
the support to the host.
For pre-win8 hosts, this has no effect since the size of the ringbuffer
control structure has not changed and all changes have been backward
compatible - unused/reserved space has been used to implement this
feature.
In this version of the patch I have cleaned up the commit log based on
feedback from Greg KH.
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
