Re: [Qemu-devel] [PATCH v2 5/5] vfio/pci: Allow relocating MSI-X MMIO

[Alex Williamson <alex.williamson@redhat.com>]

Hi Eric,

On Fri, 19 Jan 2018 10:50:53 +0100
Auger Eric <eric.auger@redhat.com> wrote:

> Hi Alex,
> 
> On 10/01/18 20:02, Alex Williamson wrote:
> > Recently proposed vfio-pci kernel changes (v4.16) remove the
> > restriction preventing userspace from mmap'ing PCI BARs in areas
> > overlapping the MSI-X vector table.  This change is primarily intended
> > to benefit host platforms which make use of system page sizes larger
> > than the PCI spec recommendation for alignment of MSI-X data
> > structures (ie. not x86_64).  In the case of POWER systems, the SPAPR
> > spec requires the VM to program MSI-X using hypercalls, rendering the
> > MSI-X vector table unused in the VM view of the device.  However,
> > ARM64 platforms also support 64KB pages and rely on QEMU emulation of
> > MSI-X.  Regardless of the kernel driver allowing mmaps overlapping
> > the MSI-X vector table, emulation of the MSI-X vector table also
> > prevents direct mapping of device MMIO spaces overlapping this page.
> > Thanks to the fact that PCI devices have a standard self discovery
> > mechanism, we can try to resolve this by relocating the MSI-X data
> > structures, either by creating a new PCI BAR or extending an existing
> > BAR and updating the MSI-X capability for the new location.  There's
> > even a very slim chance that this could benefit devices which do not
> > adhere to the PCI spec alignment guidelines on x86_64 systems.
> > 
> > This new x-msix-relocation option accepts the following choices:
> > 
> >   off: Disable MSI-X relocation, use native device config (default)
> >   auto: Use a known good combination for the platform/device (none yet)
> >   bar0..bar5: Specify the target BAR for MSI-X data structures
> > 
> > If compatible, the target BAR will either be created or extended and
> > the new portion will be used for MSI-X emulation.
> > 
> > The first obvious user question with this option is how to determine
> > whether a given platform and device might benefit from this option.
> > In most cases, the answer is that it won't, especially on x86_64.
> > Devices often dedicate an entire BAR to MSI-X and therefore no
> > performance sensitive registers overlap the MSI-X area.  Take for
> > example:
> > 
> > # lspci -vvvs 0a:00.0
> > 0a:00.0 Ethernet controller: Intel Corporation I350 Gigabit Network Connection
> > 	...
> > 	Region 0: Memory at db680000 (32-bit, non-prefetchable) [size=512K]
> > 	Region 3: Memory at db7f8000 (32-bit, non-prefetchable) [size=16K]
> > 	...
> > 	Capabilities: [70] MSI-X: Enable+ Count=10 Masked-
> > 		Vector table: BAR=3 offset=00000000
> > 		PBA: BAR=3 offset=00002000
> > 
> > This device uses the 16K bar3 for MSI-X with the vector table at
> > offset zero and the pending bits arrary at offset 8K, fully honoring  
> array
> > the PCI spec alignment guidance.  The data sheet specifically refers
> > to this as an MSI-X BAR.  This device would not see a benefit from
> > MSI-X relocation regardless of the platform, regardless of the page
> > size.
> > 
> > However, here's another example:
> > 
> > # lspci -vvvs 02:00.0
> > 02:00.0 Serial Attached SCSI controller: xxxxxxxx
> > 	...
> > 	Region 0: I/O ports at c000 [size=256]
> > 	Region 1: Memory at ef640000 (64-bit, non-prefetchable) [size=64K]
> > 	Region 3: Memory at ef600000 (64-bit, non-prefetchable) [size=256K]
> > 	...
> > 	Capabilities: [c0] MSI-X: Enable+ Count=16 Masked-
> > 		Vector table: BAR=1 offset=0000e000
> > 		PBA: BAR=1 offset=0000f000
> > 
> > Here the MSI-X data structures are placed on separate 4K pages at the
> > end of a 64KB BAR.  If our host page size is 4K, we're likely fine,
> > but at 64KB page size, MSI-X emulation at that location prevents the
> > entire BAR from being directly mapped into the VM address space.
> > Overlapping performance sensitive registers then starts to be a very
> > likely scenario on such a platform.  At this point, the user could
> > enable tracing on vfio_region_read and vfio_region_write to determine
> > more conclusively if device accesses are being trapped through QEMU.
> > 
> > Upon finding a device and platform in need of MSI-X relocation, the
> > next problem is how to choose target PCI BAR to host the MSI-X data
> > structures.  A few key rules to keep in mind for this selection
> > include:
> > 
> >  * There are only 6 BAR slots, bar0..bar5
> >  * 64-bit BARs occupy two BAR slots, 'lspci -vvv' lists the first slot
> >  * PCI BARs are always a power of 2 in size, extending == doubling
> >  * The maximum size of a 32-bit BAR is 2GB
> >  * MSI-X data structures must reside in an MMIO BAR
> > 
> > Using these rules, we can evaluate each BAR of the second example
> > device above as follows:
> > 
> >  bar0: I/O port BAR, incompatible with MSI-X tables
> >  bar1: BAR could be extended, incurring another 64KB of MMIO
> >  bar2: Unavailable, bar1 is 64-bit, this register is used by bar1
> >  bar3: BAR could be extended, incurring another 256KB of MMIO
> >  bar4: Unavailable, bar3 is 64bit, this register is used by bar3
> >  bar5: Available, empty BAR, minimum additional MMIO
> > 
> > A secondary optimization we might wish to make in relocating MSI-X
> > is to minimize the additional MMIO required for the device, therefore
> > we might test the available choices in order of preference as bar5,
> > bar1, and finally bar3.  The original proposal for this feature
> > included an 'auto' option which would choose bar5 in this case, but
> > various drivers have been found that make assumptions about the
> > properties of the "first" BAR or the size of BARs such that there
> > appears to be no foolproof automatic selection available, requiring
> > known good combinations to be sourced from users.  This patch is
> > pre-enabled for an 'auto' selection making use of a validated lookup
> > table, but no entries are yet identified.
> > 
> > Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> > ---
> >  hw/vfio/pci.c        |  101 ++++++++++++++++++++++++++++++++++++++++++++++++++
> >  hw/vfio/pci.h        |    1 
> >  hw/vfio/trace-events |    2 +
> >  3 files changed, 103 insertions(+), 1 deletion(-)
> > 
> > diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
> > index 20252ea7aeb7..7171ba18213c 100644
> > --- a/hw/vfio/pci.c
> > +++ b/hw/vfio/pci.c
> > @@ -1352,6 +1352,100 @@ static void vfio_pci_fixup_msix_region(VFIOPCIDevice *vdev)
> >      }
> >  }
> >  
> > +static void vfio_pci_relocate_msix(VFIOPCIDevice *vdev, Error **errp)
> > +{
> > +    int target_bar = -1;
> > +    size_t msix_sz;
> > +
> > +    if (!vdev->msix || vdev->msix_relo == OFF_AUTOPCIBAR_OFF) {
> > +        return;
> > +    }
> > +
> > +    /* The actual minimum size of MSI-X structures */
> > +    msix_sz = (vdev->msix->entries * PCI_MSIX_ENTRY_SIZE) +
> > +              (QEMU_ALIGN_UP(vdev->msix->entries, 64) / 8);
> > +    /* Round up to host pages, we don't want to share a page */
> > +    msix_sz = REAL_HOST_PAGE_ALIGN(msix_sz);
> > +    /* PCI BARs must be a power of 2 */
> > +    msix_sz = pow2ceil(msix_sz);
> > +
> > +    if (vdev->msix_relo == OFF_AUTOPCIBAR_AUTO) {
> > +        /*
> > +         * TODO: Lookup table for known devices.
> > +         *
> > +         * Logically we might use an algorithm here to select the BAR adding
> > +         * the least additional MMIO space, but we cannot programatically
> > +         * predict the driver dependency on BAR ordering or sizing, therefore
> > +         * 'auto' becomes a lookup for combinations reported to work.
> > +         */
> > +        if (target_bar < 0) {
> > +            error_setg_errno(errp, EINVAL, "No automatic MSI-X relocation "
> > +                             "available for device %04x:%04x",
> > +                             vdev->vendor_id, vdev->device_id);  
> don't you want error_setg here and below?

What's the benefit of one vs the other?  I wasn't sure which to use.

> > +            return;
> > +        }
> > +    } else {
> > +        target_bar = (int)(vdev->msix_relo - OFF_AUTOPCIBAR_BAR0);
> > +    }
> > +
> > +    /* I/O port BARs cannot host MSI-X structures */
> > +    if (vdev->bars[target_bar].ioport) {
> > +        error_setg_errno(errp, EINVAL, "Invalid MSI-X relocation BAR %d, "
> > +                         "I/O port BAR", target_bar);  
> 
> > +        return;
> > +    }
> > +
> > +    /* Cannot use a BAR in the "shadow" of a 64-bit BAR */
> > +    if (!vdev->bars[target_bar].size &&
> > +         target_bar > 0 && vdev->bars[target_bar - 1].mem64) {
> > +        error_setg_errno(errp, EINVAL, "Invalid MSI-X relocation BAR %d, "
> > +                         "consumed by 64-bit BAR %d", target_bar,
> > +                         target_bar - 1);
> > +        return;
> > +    }
> > +
> > +    /* 2GB max size for 32-bit BARs */
> > +    if (vdev->bars[target_bar].size > (1 * 1024 * 1024 * 1024) &&  
> nit: the comment versus the check is a bit misleading. If I understand
> correctly, the x2 size would be gt 2GB.

Right, if the BAR is >1G already (ie. 2G), there's no room to make it
bigger.  I'll add "... cannot double if already > 1G".

> > +        !vdev->bars[target_bar].mem64) {
> > +        error_setg_errno(errp, EINVAL, "Invalid MSI-X relocation BAR %d, "
> > +                         "no space to extend 32-bit BAR", target_bar);
> > +        return;
> > +    }
> > +
> > +    /*
> > +     * If adding a new BAR, test if we can make it 64bit.  We make it
> > +     * prefetchable since QEMU MSI-X emulation has no read side effects
> > +     * and doing so makes mapping more flexible.
> > +     */
> > +    if (!vdev->bars[target_bar].size) {
> > +        if (target_bar < (PCI_ROM_SLOT - 1) &&
> > +            !vdev->bars[target_bar + 1].size) {
> > +            vdev->bars[target_bar].mem64 = true;
> > +            vdev->bars[target_bar].type = PCI_BASE_ADDRESS_MEM_TYPE_64;
> > +        }
> > +        vdev->bars[target_bar].type |= PCI_BASE_ADDRESS_MEM_PREFETCH;
> > +        vdev->bars[target_bar].size = msix_sz;
> > +        vdev->msix->table_offset = 0;
> > +    } else {
> > +        vdev->bars[target_bar].size = MAX(vdev->bars[target_bar].size * 2,
> > +                                          msix_sz * 2);
> > +        /*
> > +         * Due to above size calc, MSI-X always starts halfway into the BAR,
> > +         * which will always be a separate host page.  
> 
> nit: the spec gives this recommendation.
> 
> "If a dedicated Base Address register is not feasible, it is recommended
> that a function isolate the MSI-X structures from the non-MSI-X
> structures with aligned 8 KB ranges rather than
> the mandatory aligned 4 KB ranges."
> 
> In some corner circumstances, with 4kB - which is not our main use case
> - this may not be enforced here.

I think this was an attempt to future proof hardware designs for larger
page sizes, but 8K turned out to be mostly (entirely?) skipped as a
common page size.  We're not building real hardware and I can't think
of any advantage to using a minimum of 8K alignment if the host page
size is 4K, can you?  Thanks,

Alex


> > +         */
> > +        vdev->msix->table_offset = vdev->bars[target_bar].size / 2;
> > +    }
> > +
> > +    vdev->msix->table_bar = target_bar;
> > +    vdev->msix->pba_bar = target_bar;
> > +    /* Requires 8-byte alignment, but PCI_MSIX_ENTRY_SIZE guarantees that */
> > +    vdev->msix->pba_offset = vdev->msix->table_offset +
> > +                                  (vdev->msix->entries * PCI_MSIX_ENTRY_SIZE);
> > +
> > +    trace_vfio_msix_relo(vdev->vbasedev.name,
> > +                         vdev->msix->table_bar, vdev->msix->table_offset);
> > +}
> > +
> >  /*
> >   * We don't have any control over how pci_add_capability() inserts
> >   * capabilities into the chain.  In order to setup MSI-X we need a
> > @@ -1430,6 +1524,8 @@ static void vfio_msix_early_setup(VFIOPCIDevice *vdev, Error **errp)
> >      vdev->msix = msix;
> >  
> >      vfio_pci_fixup_msix_region(vdev);
> > +
> > +    vfio_pci_relocate_msix(vdev, errp);
> >  }
> >  
> >  static int vfio_msix_setup(VFIOPCIDevice *vdev, int pos, Error **errp)
> > @@ -2845,13 +2941,14 @@ static void vfio_realize(PCIDevice *pdev, Error **errp)
> >  
> >      vfio_pci_size_rom(vdev);
> >  
> > +    vfio_bars_prepare(vdev);
> > +
> >      vfio_msix_early_setup(vdev, &err);
> >      if (err) {
> >          error_propagate(errp, err);
> >          goto error;
> >      }
> >  
> > -    vfio_bars_prepare(vdev);
> >      vfio_bars_register(vdev);
> >  
> >      ret = vfio_add_capabilities(vdev, errp);
> > @@ -3041,6 +3138,8 @@ static Property vfio_pci_dev_properties[] = {
> >      DEFINE_PROP_UNSIGNED_NODEFAULT("x-nv-gpudirect-clique", VFIOPCIDevice,
> >                                     nv_gpudirect_clique,
> >                                     qdev_prop_nv_gpudirect_clique, uint8_t),
> > +    DEFINE_PROP_OFF_AUTO_PCIBAR("x-msix-relocation", VFIOPCIDevice, msix_relo,
> > +                                OFF_AUTOPCIBAR_OFF),
> >      /*
> >       * TODO - support passed fds... is this necessary?
> >       * DEFINE_PROP_STRING("vfiofd", VFIOPCIDevice, vfiofd_name),
> > diff --git a/hw/vfio/pci.h b/hw/vfio/pci.h
> > index dcdb1a806769..588381f201b4 100644
> > --- a/hw/vfio/pci.h
> > +++ b/hw/vfio/pci.h
> > @@ -135,6 +135,7 @@ typedef struct VFIOPCIDevice {
> >                                  (1 << VFIO_FEATURE_ENABLE_IGD_OPREGION_BIT)
> >      int32_t bootindex;
> >      uint32_t igd_gms;
> > +    OffAutoPCIBAR msix_relo;
> >      uint8_t pm_cap;
> >      uint8_t nv_gpudirect_clique;
> >      bool pci_aer;
> > diff --git a/hw/vfio/trace-events b/hw/vfio/trace-events
> > index fae096c0724f..437ccdd29053 100644
> > --- a/hw/vfio/trace-events
> > +++ b/hw/vfio/trace-events
> > @@ -16,6 +16,8 @@ vfio_msix_pba_disable(const char *name) " (%s)"
> >  vfio_msix_pba_enable(const char *name) " (%s)"
> >  vfio_msix_disable(const char *name) " (%s)"
> >  vfio_msix_fixup(const char *name, int bar, uint64_t start, uint64_t end) " (%s) MSI-X region %d mmap fixup [0x%"PRIx64" - 0x%"PRIx64"]"
> > +vfio_msix_relo_cost(const char *name, int bar, uint64_t cost) " (%s) BAR %d cost 0x%"PRIx64""
> > +vfio_msix_relo(const char *name, int bar, uint64_t offset) " (%s) BAR %d offset 0x%"PRIx64""
> >  vfio_msi_enable(const char *name, int nr_vectors) " (%s) Enabled %d MSI vectors"
> >  vfio_msi_disable(const char *name) " (%s)"
> >  vfio_pci_load_rom(const char *name, unsigned long size, unsigned long offset, unsigned long flags) "Device %s ROM:\n  size: 0x%lx, offset: 0x%lx, flags: 0x%lx"
> >