Introduce a function that reads the exact nanoseconds value that is
provided to the guest in kvmclock. This crystallizes the notion of
kvmclock as a thin veneer over a stable TSC, that the guest will
(hopefully) convert with NTP. In other words, kvmclock is *not* a
paravirtualized host-to-guest NTP.
Drop the get_kernel_ns() function, that was used both to get the base
value of the master clock and to get the current value of kvmclock.
The former use is replaced by ktime_get_boot_ns(), the latter is
the purpose of get_kernel_ns().
This also allows KVM to provide a Hyper-V time reference counter that
is synchronized with the time that is computed from the TSC page.
Reviewed-by: Roman Kagan <rkagan@virtuozzo.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
The version field in struct pvclock_vcpu_time_info basically implements
a seqcount. Wrap it with the usual read_begin and read_retry functions,
and use these APIs instead of peppering the code with smp_rmb()s.
While at it, change it to the more pedantically correct virt_rmb().
With this change, __pvclock_read_cycles can be simplified noticeably.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
There is a generic function __pvclock_read_cycles to be used to get both
flags and cycles. For function pvclock_read_flags, it's useless to get
cycles value. To make this function be more effective, get this variable
flags directly in function.
Signed-off-by: Minfei Huang <mnghuan@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Protocol for the "version" fields is: hypervisor raises it (making it
uneven) before it starts updating the fields and raises it again (making
it even) when it is done. Thus the guest can make sure the time values
it got are consistent by checking the version before and after reading
them.
Add CPU barries after getting version value just like what function
vread_pvclock does, because all of callees in this function is inline.
Fixes: 502dfeff23
Cc: stable@vger.kernel.org
Signed-off-by: Minfei Huang <mnghuan@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This reverts commits 0a4e6be9ca
and 80f7fdb1c7.
The task migration notifier was originally introduced in order to support
the pvclock vsyscall with non-synchronized TSC, but KVM only supports it
with synchronized TSC. Hence, on KVM the race condition is only needed
due to a bad implementation on the host side, and even then it's so rare
that it's mostly theoretical.
As far as KVM is concerned it's possible to fix the host, avoiding the
additional complexity in the vDSO and the (re)introduction of the task
migration notifier.
Xen, on the other hand, hasn't yet implemented vsyscall support at
all, so we do not care about its plans for non-synchronized TSC.
Reported-by: Peter Zijlstra <peterz@infradead.org>
Suggested-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
The following point:
2. per-CPU pvclock time info is updated if the
underlying CPU changes.
Is not true anymore since "KVM: x86: update pvclock area conditionally,
on cpu migration".
Add task migration notification back.
Problem noticed by Andy Lutomirski.
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
CC: stable@kernel.org # 3.11+
In certain occasions it is possible for a hung task detector
positive to be false: continuation from a paused VM, for example.
Add a method to reset detection, similar as is done
with other kernel watchdogs.
Acked-by: Don Zickus <dzickus@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>
Implement reset of kernel watchdogs at pvclock read time. This avoids
adding special code to every watchdog.
This is possible for watchdogs which measure time based on sched_clock() or
ktime_get() variants.
Suggested by Don Zickus.
Acked-by: Don Zickus <dzickus@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>
Linux as a guest on KVM hypervisor, the only user of the pvclock
vsyscall interface, does not require notification on task migration
because:
1. cpu ID number maps 1:1 to per-CPU pvclock time info.
2. per-CPU pvclock time info is updated if the
underlying CPU changes.
3. that version is increased whenever underlying CPU
changes.
Which is sufficient to guarantee nanoseconds counter
is calculated properly.
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Gleb Natapov <gleb@redhat.com>
The physical memory fixmapped for the pvclock clock_gettime vsyscall
was allocated, and thus is not a kernel symbol. __pa() is the proper
method to use in this case.
Fixes the crash below when booting a next-20130204+ smp guest on a
3.8-rc5+ KVM host.
[ 0.666410] udevd[97]: starting version 175
[ 0.674043] udevd[97]: udevd:[97]: segfault at ffffffffff5fd020
ip 00007fff069e277f sp 00007fff068c9ef8 error d
Acked-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>
Originally from Jeremy Fitzhardinge.
Introduce generic, non hypervisor specific, pvclock initialization
routines.
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Originally from Jeremy Fitzhardinge.
So code can be reused.
Acked-by: Glauber Costa <glommer@parallels.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Originally from Jeremy Fitzhardinge.
We can copy the information directly from "struct pvclock_vcpu_time_info",
remove pvclock_shadow_time.
Reviewed-by: Glauber Costa <glommer@parallels.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Originally from Jeremy Fitzhardinge.
pvclock_get_time_values, which contains the memory barriers
will be removed by next patch.
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
If the guest domain has been suspend/resumed or migrated, then the
system clock backing the pvclock clocksource may revert to a smaller
value (ie, can be non-monotonic across the migration/save-restore).
Make sure we zero last_value in that case so that the domain
continues to see clock updates.
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
The scale_delta function for shift / multiply with 31-bit
precision moves to a common header so it can be used by both
kernel and kvm module.
Signed-off-by: Zachary Amsden <zamsden@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
If the HV told us we can fully trust the TSC, skip any
correction
Signed-off-by: Glauber Costa <glommer@redhat.com>
Acked-by: Zachary Amsden <zamsden@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
In recent stress tests, it was found that pvclock-based systems
could seriously warp in smp systems. Using ingo's time-warp-test.c,
I could trigger a scenario as bad as 1.5mi warps a minute in some systems.
(to be fair, it wasn't that bad in most of them). Investigating further, I
found out that such warps were caused by the very offset-based calculation
pvclock is based on.
This happens even on some machines that report constant_tsc in its tsc flags,
specially on multi-socket ones.
Two reads of the same kernel timestamp at approx the same time, will likely
have tsc timestamped in different occasions too. This means the delta we
calculate is unpredictable at best, and can probably be smaller in a cpu
that is legitimately reading clock in a forward ocasion.
Some adjustments on the host could make this window less likely to happen,
but still, it pretty much poses as an intrinsic problem of the mechanism.
A while ago, I though about using a shared variable anyway, to hold clock
last state, but gave up due to the high contention locking was likely
to introduce, possibly rendering the thing useless on big machines. I argue,
however, that locking is not necessary.
We do a read-and-return sequence in pvclock, and between read and return,
the global value can have changed. However, it can only have changed
by means of an addition of a positive value. So if we detected that our
clock timestamp is less than the current global, we know that we need to
return a higher one, even though it is not exactly the one we compared to.
OTOH, if we detect we're greater than the current time source, we atomically
replace the value with our new readings. This do causes contention on big
boxes (but big here means *BIG*), but it seems like a good trade off, since
it provide us with a time source guaranteed to be stable wrt time warps.
After this patch is applied, I don't see a single warp in time during 5 days
of execution, in any of the machines I saw them before.
Signed-off-by: Glauber Costa <glommer@redhat.com>
Acked-by: Zachary Amsden <zamsden@redhat.com>
CC: Jeremy Fitzhardinge <jeremy@goop.org>
CC: Avi Kivity <avi@redhat.com>
CC: Marcelo Tosatti <mtosatti@redhat.com>
CC: Zachary Amsden <zamsden@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
This patch removes one padding byte and transform it into a flags
field. New versions of guests using pvclock will query these flags
upon each read.
Flags, however, will only be interpreted when the guest decides to.
It uses the pvclock_valid_flags function to signal that a specific
set of flags should be taken into consideration. Which flags are valid
are usually devised via HV negotiation.
Signed-off-by: Glauber Costa <glommer@redhat.com>
CC: Jeremy Fitzhardinge <jeremy@goop.org>
Acked-by: Zachary Amsden <zamsden@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
when building 32-bit, I see this ..
arch/x86/kernel/pvclock.c:63:7: warning: "__x86_64__" is not defined
Signed-off-by: Dave Jones <davej@redhat.com>
LKML-Reference: <20090713201437.GA12165@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
arch/x86/kernel/pvclock.c:102:6: warning: symbol 'tsc_khz' shadows an earlier one
include/asm/tsc.h:18:21: originally declared here
Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Avi Kivity <avi@redhat.com>
KVM intends to use paravirt code to calibrate khz. Xen
current code will do just fine. So as a first step, factor out
code to pvclock.c.
Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>
This patch adds structs for the paravirt clocksource ABI
used by both xen and kvm (pvclock-abi.h).
It also adds some helper functions to read system time and
wall clock time from a paravirtual clocksource (pvclock.[ch]).
They are based on the xen code. They are enabled using
CONFIG_PARAVIRT_CLOCK.
Subsequent patches of this series will put the code in use.
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>
