vfio/pci: Hide Resizable BAR capability

The resizable BAR capability is currently exposed read-only from the
kernel and we don't yet implement a protocol for virtualizing it to
the VM.  Exposing it to the guest read-only introduces poor behavior
as the guest has no reason to test that a control register write is
accepted by the hardware.  This can lead to cases where the guest OS
assumes the BAR has been resized, but it hasn't.  This has been
observed when assigning AMD Vega GPUs.

Note, this does not preclude future enablement of resizable BARs, but
it's currently incorrect to expose this capability as read-only, so
better to not expose it at all.

Reported-by: James Courtier-Dutton <james.dutton@gmail.com>
Tested-by: James Courtier-Dutton <james.dutton@gmail.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
index 4a04f795162bca682cb9b9d9a8eba9846bddbe1b..48f4e19a02cf78395134064f1496c9c848afe828 100644 (file)
--- a/hw/vfio/pci.c
+++ b/hw/vfio/pci.c
@@ -2119,6 +2119,7 @@ static void vfio_add_ext_cap(VFIOPCIDevice *vdev)
         case 0: /* kernel masked capability */
         case PCI_EXT_CAP_ID_SRIOV: /* Read-only VF BARs confuse OVMF */
         case PCI_EXT_CAP_ID_ARI: /* XXX Needs next function virtualization */
+        case PCI_EXT_CAP_ID_REBAR: /* Can't expose read-only */
             trace_vfio_add_ext_cap_dropped(vdev->vbasedev.name, cap_id, next);
             break;
         default: