Re: [RFC PATCH v3 0/5] vfio-pci: Allow to mmap sub-page MMIO BARs and MSI-X table on PPC64 platform

[Yongji Xie]

Ping...

Alex, any comment?

Regards,
Yongji Xie

On 2016/1/15 15:06, Yongji Xie wrote:
> Current vfio-pci implementation disallows to mmap
> sub-page(size < PAGE_SIZE) MMIO BARs and MSI-X table. This is because
> sub-page BARs' mmio page may be shared with other BARs and MSI-X table
> should not be accessed directly from the guest for security reasons.
>
> But these will easily cause some performance issues for mmio accesses
> in guest when vfio passthrough sub-page BARs or BARs containing MSI-X
> table on PPC64 platform. This is because PAGE_SIZE is 64KB by default
> on PPC64 platform and the big page may easily hit the sub-page MMIO
> BARs' unmmapping and cause the unmmaping of the mmio page which
> MSI-X table locate in, which lead to mmio emulation in host.
>
> For sub-page MMIO BARs' unmmapping, this patchset adds a kernel
> parameter for PCI resource allocator to enforce the alignment of all
> MMIO BARs to be at least PAGE_SZIE and make it enabled by default on
> PPC64 platform so that sub-page BAR's mmio page will not be shared
> with other BARs. Then we can mmap sub-page MMIO BARs in vfio-pci driver
> with this parameter enabled.
>
> For MSI-X table's unmmapping, we think MSI-X table is safe to access
> directly from userspace if PCI host bridge support filtering of MSIs
> which can ensure that a given pci device can only shoot the MSIs
> assigned for it. So we add a pci_host_bridge attribute to indicate
> if this PCI host bridge supports filtering of MSIs. Then we can mmap
> MSI-X table with this attribute set.
>
> With this patchset applied, we can get almost 100% improvement on
> performance for mmio accesses when we passthrough sub-page BARs to guest
> in our test.
>
> The two vfio related patches(patch 2 and patch 5) are based on the
> proposed patchset[1].
>
> Changelog v3:
> - Rebase on new linux kernel mainline with the patchset[1] applied.
> - Add a function to check whether PCI BARs'mmio page is shared with
>    other BARs.
> - Add a host bridge attribute to indicate PCI host bridge support
>    filtering of MSIs.
> - Use the new host bridge attribute to check if MSI-X table can
>    be mmapped instead of CONFIG_EEH.
> - Remove Kconfig option VFIO_PCI_MMAP_MSIX
>
> Changelog v2:
> - Rebase on v4.4-rc6 with the patchset[1] applied.
> - Use kernel parameter to enforce all MMIO BARs to be page aligned
>    on PCI core code instead of doing it on PPC64 arch code.
> - Remove flags: VFIO_DEVICE_FLAGS_PCI_PAGE_ALIGNED
>                  VFIO_DEVICE_FLAGS_PCI_MSIX_MMAP
> - Add a Kconfig option to support for mmapping MSI-X table.
>
> [1] https://lkml.org/lkml/2015/11/23/748
>
> Yongji Xie (5):
>    PCI: Add support for enforcing all MMIO BARs to be page aligned
>    vfio-pci: Allow to mmap sub-page MMIO BARs if the mmio page is exclusive
>    PCI: Add host bridge attribute to indicate filtering of MSIs is supported
>    powerpc/powernv/pci-ioda: Enable msi_filtered bit for any IODA host bridge
>    vfio-pci: Allow to mmap MSI-X table if host bridge supports filtering of MSIs
>
>   Documentation/kernel-parameters.txt       |    5 +++++
>   arch/powerpc/include/asm/pci.h            |   11 +++++++++
>   arch/powerpc/platforms/powernv/pci-ioda.c |    6 +++++
>   drivers/pci/host-bridge.c                 |    6 +++++
>   drivers/pci/pci.c                         |   35 +++++++++++++++++++++++++++++
>   drivers/pci/pci.h                         |    8 ++++++-
>   drivers/vfio/pci/vfio_pci.c               |   13 ++++++++---
>   include/linux/pci.h                       |    7 ++++++
>   8 files changed, 87 insertions(+), 4 deletions(-)