vfio-pci: Allow mmap of MSIX BAR

At the moment we unconditionally avoid mapping MSIX data of a BAR and
emulate MSIX table in QEMU. However it is 1) not always necessary as
a platform may provide a paravirt interface for MSIX configuration;
2) can affect the speed of MMIO access by emulating them in QEMU when
frequently accessed registers share same system page with MSIX data,
this is particularly a problem for systems with the page size bigger
than 4KB.

A new capability - VFIO_REGION_INFO_CAP_MSIX_MAPPABLE - has been added
to the kernel [1] which tells the userspace that mapping of the MSIX data
is possible now. This makes use of it so from now on QEMU tries mapping
the entire BAR as a whole and emulate MSIX on top of that.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a32295c612c57990d17fb0f41e7134394b2f35f6

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
index b9d2c12b82..02974f4eb9 100644
--- a/hw/vfio/pci.c
+++ b/hw/vfio/pci.c
@@ -1295,6 +1295,15 @@ static void vfio_pci_fixup_msix_region(VFIOPCIDevice *vdev)
     VFIORegion *region = &vdev->bars[vdev->msix->table_bar].region;
 
     /*
+     * If the host driver allows mapping of a MSIX data, we are going to
+     * do map the entire BAR and emulate MSIX table on top of that.
+     */
+    if (vfio_has_region_cap(&vdev->vbasedev, region->nr,
+                            VFIO_REGION_INFO_CAP_MSIX_MAPPABLE)) {
+        return;
+    }
+
+    /*
      * We expect to find a single mmap covering the whole BAR, anything else
      * means it's either unsupported or already setup.
      */