[RFC v6 00/10] vfio-pci: Allow to mmap sub-page MMIO BARs and MSI-X table

From: Yongji Xie
Date: Mon Apr 18 2016 - 06:55:20 EST

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 it 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 modifies
resource_alignment kernel parameter to enforce the alignment of all
MMIO BARs to be at least PAGE_SZIE so that sub-page BAR's mmio page
will not be shared with other BARs. And we also add shadow resources
to the vfio device and put them into the holes of mmio pages in case
that hot-add device's BARs are assigned into the holes. Then we can
mmap sub-page MMIO BARs safely.

For MSI-X table's unmmapping, we think MSI-X table is safe to access
directly from userspace if hardware supports the capability of
interrupt remapping which can ensure that a given pci device can
only shoot the MSIs assigned for it. But the implenmentation of
this capability is arch-independent. To have a universal way
to test this capability on PCI side for different archs, we introduce
a new bus_flags PCI_BUS_FLAGS_MSI_REMAP.

With this patchset applied, we can get almost 100% improvement on
performance for small block 4k random read when we passthrough a FC
HBA containing sub-page BARs and MSI-X BARs to guest on PPC64 in
our test.

The patch 8 are based on the proposed patchset[2].

Changelog v6:
- Rebase on vfio/next with patchset[2] applied
- Fix some bugs of v5
- Add three patches to make PCI_BUS_FLAGS_MSI_REMAP as
a universal flag to test IRQ remapping

Changelog v5:
- Rebase on vfio/next
- Change the order of patch 1,2,3
- Move the warning "resource_alignment will not work with
PCI_PROBE_ONLY set" from documentation to kernel log
- Add description for parameter "resize"
get minimum alignment
- Add shadow resources to make sure sub-page BAR's mmio
page will not be shared with hot-add BARs.
- Add a new bit to pci_bus_flags to indicate the capbility
of interrupt remapping on PPC64
- Add a property msi_remap to vfio_pci_device to cache the
capbility of interrupt remapping

Changelog v4:
- Rebase on v4.5-rc6 with patchset[1] applied.
- Remove resource_page_aligned kernel parameter
- Fix some problems with resource_alignment kernel parameter
- Modify resource_alignment kernel parameter to support multiple
- Remove host bridge attribute: msi_filtered
- Use IOMMU_CAP_INTR_REMAP to check if MSI-X table can be mmapped
- Add IOMMU_CAP_INTR_REMAP for IODA host bridge on PPC64 platform

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.

[1] http://www.spinics.net/lists/kvm/msg127812.html
[2] http://www.spinics.net/lists/kvm/msg130256.html

Yongji Xie (10):
PCI: Ignore resource_alignment if PCI_PROBE_ONLY was set
PCI: Do not Use IORESOURCE_STARTALIGN to identify bridge resources
PCI: Add a new option for resource_alignment to reassign alignment
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
iommu: Set PCI_BUS_FLAGS_MSI_REMAP if IOMMU have capability of IRQ remapping
PCI: Set PCI_BUS_FLAGS_MSI_REMAP if MSI controller supports IRQ remapping
pci-ioda: Set PCI_BUS_FLAGS_MSI_REMAP for IODA host bridge
vfio-pci: Allow to mmap MSI-X table if interrupt remapping is supported

Documentation/kernel-parameters.txt | 7 +-
arch/powerpc/include/asm/pci.h | 2 +
arch/powerpc/platforms/powernv/pci-ioda.c | 8 +++
drivers/iommu/iommu.c | 15 +++++
drivers/pci/msi.c | 12 ++++
drivers/pci/pci.c | 105 +++++++++++++++++++++++------
drivers/pci/probe.c | 3 +
drivers/pci/setup-bus.c | 9 ++-
drivers/vfio/pci/vfio_pci.c | 65 +++++++++++++++---
drivers/vfio/pci/vfio_pci_private.h | 8 +++
drivers/vfio/pci/vfio_pci_rdwr.c | 3 +-
include/linux/msi.h | 6 +-
include/linux/pci.h | 1 +
13 files changed, 208 insertions(+), 36 deletions(-)