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Jean-Philippe Brucker 91c98fe794 iommu/virtio: Fix interaction with VFIO 
Commit e8ae0e140c ("vfio: Require that devices support DMA cache
coherence") requires IOMMU drivers to advertise
IOMMU_CAP_CACHE_COHERENCY, in order to be used by VFIO. Since VFIO does
not provide to userspace the ability to maintain coherency through cache
invalidations, it requires hardware coherency. Advertise the capability
in order to restore VFIO support.

The meaning of IOMMU_CAP_CACHE_COHERENCY also changed from "IOMMU can
enforce cache coherent DMA transactions" to "IOMMU_CACHE is supported".
While virtio-iommu cannot enforce coherency (of PCIe no-snoop
transactions), it does support IOMMU_CACHE.

We can distinguish different cases of non-coherent DMA:

(1) When accesses from a hardware endpoint are not coherent. The host
    would describe such a device using firmware methods ('dma-coherent'
    in device-tree, '_CCA' in ACPI), since they are also needed without
    a vIOMMU. In this case mappings are created without IOMMU_CACHE.
    virtio-iommu doesn't need any additional support. It sends the same
    requests as for coherent devices.

(2) When the physical IOMMU supports non-cacheable mappings. Supporting
    those would require a new feature in virtio-iommu, new PROBE request
    property and MAP flags. Device drivers would use a new API to
    discover this since it depends on the architecture and the physical
    IOMMU.

(3) When the hardware supports PCIe no-snoop. It is possible for
    assigned PCIe devices to issue no-snoop transactions, and the
    virtio-iommu specification is lacking any mention of this.

    Arm platforms don't necessarily support no-snoop, and those that do
    cannot enforce coherency of no-snoop transactions. Device drivers
    must be careful about assuming that no-snoop transactions won't end
    up cached; see commit e02f5c1bb2 ("drm: disable uncached DMA
    optimization for ARM and arm64"). On x86 platforms, the host may or
    may not enforce coherency of no-snoop transactions with the physical
    IOMMU. But according to the above commit, on x86 a driver which
    assumes that no-snoop DMA is compatible with uncached CPU mappings
    will also work if the host enforces coherency.

    Although these issues are not specific to virtio-iommu, it could be
    used to facilitate discovery and configuration of no-snoop. This
    would require a new feature bit, PROBE property and ATTACH/MAP
    flags.

Cc: stable@vger.kernel.org
Fixes: e8ae0e140c ("vfio: Require that devices support DMA cache coherence")
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/20220825154622.86759-1-jean-philippe@linaro.org
Signed-off-by: Joerg Roedel <jroedel@suse.de>
2022-09-07 15:44:58 +02:00
arch powerpc fixes for 6.0 #4 2022-09-04 11:33:22 -07:00
block block-6.0-2022-08-26 2022-08-26 11:05:54 -07:00
certs Kbuild updates for v5.20 2022-08-10 10:40:41 -07:00
crypto crypto: blake2b: effectively disable frame size warning 2022-08-10 17:59:11 -07:00
Documentation Input updates for v6.0-rc3 2022-09-03 13:09:46 -07:00
drivers iommu/virtio: Fix interaction with VFIO 2022-09-07 15:44:58 +02:00
fs 5 cifs/smb3 fixes, all also for stable 2022-09-02 16:20:24 -07:00
include TTY/Serial driver fixes for 6.0-rc4 2022-09-03 10:34:02 -07:00
init arm64 fixes for -rc3 2022-08-26 11:32:53 -07:00
io_uring io_uring-6.0-2022-09-02 2022-09-02 16:37:01 -07:00
ipc Updates to various subsystems which I help look after. lib, ocfs2, 2022-08-07 10:03:24 -07:00
kernel Networking fixes for 6.0-rc4, including fixes from bluetooth, bpf 2022-09-01 09:20:42 -07:00
lib This push fixes a boot performance regression due to an unnecessary 2022-08-31 09:47:06 -07:00
LICENSES LICENSES/LGPL-2.1: Add LGPL-2.1-or-later as valid identifiers 2021-12-16 14:33:10 +01:00
mm mm: pagewalk: Fix race between unmap and page walker 2022-09-03 10:13:13 -07:00
net net/smc: Remove redundant refcount increase 2022-09-01 10:04:45 +02:00
samples Tracing updates for 5.20 / 6.0 2022-08-05 09:41:12 -07:00
scripts Makefile.extrawarn: re-enable -Wformat for clang; take 2 2022-09-04 11:15:50 -07:00
security Landlock fix for v6.0-rc4 2022-09-02 15:24:08 -07:00
sound ALSA: usb-audio: Add quirk for LH Labs Geek Out HD Audio 1V5 2022-08-28 09:42:14 +02:00
tools s390: 2022-09-04 11:27:14 -07:00
usr Not a lot of material this cycle. Many singleton patches against various 2022-05-27 11:22:03 -07:00
virt KVM: Drop unnecessary initialization of "ops" in kvm_ioctl_create_device() 2022-08-19 04:05:43 -04:00
.clang-format PCI/DOE: Add DOE mailbox support functions 2022-07-19 15:38:04 -07:00
.cocciconfig
.get_maintainer.ignore get_maintainer: add Alan to .get_maintainer.ignore 2022-08-20 15:17:44 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore kbuild: split the second line of *.mod into *.usyms 2022-05-08 03:16:59 +09:00
.mailmap .mailmap: update Luca Ceresoli's e-mail address 2022-08-28 14:02:46 -07:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS drm for 5.20/6.0 2022-08-03 19:52:08 -07:00
Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS gpio fixes for v6.0-rc4 2022-09-03 21:27:27 -07:00
Makefile Linux 6.0-rc4 2022-09-04 13:10:01 -07:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.