[libvirt] [PATCH v2 2/2] qemu: hostdev: Fix the error on VM start with an mdev when IOMMU is off

[Erik Skultety]

Commit b4c2ac8d56 made a false assumption that IOMMU support necessary
for an mdev device to be assigned to a VM. Unlike direct PCI assignment,
IOMMU support is not needed for mediated devices, as the physical parent
device provides the IOMMU isolation, therefore, simply checking for VFIO
presence is enough to successfully start a VM.
Luckily, this issue is not serious, since as of yet, libvirt mandates
mdevs to be pre-created prior to a domain's launch - if it is,
everything does work smoothly, because the parent device will ensure the
iommu groups we try to access exist. However, if the mdev didn't exist,
one would see the following error:

"unsupported configuration: Mediated host device assignment requires VFIO
support"

The error msg above is simply wrong and doesn't even reflect the IOMMU
reality, so after applying this patch one would rather hit the following
error:

"device not found: mediated device '<UUID>' not found"

Signed-off-by: Erik Skultety <eskultet at redhat.com>
---
 src/qemu/qemu_hostdev.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/src/qemu/qemu_hostdev.c b/src/qemu/qemu_hostdev.c
index 73d26f4c6..afe445d4e 100644
--- a/src/qemu/qemu_hostdev.c
+++ b/src/qemu/qemu_hostdev.c
@@ -330,9 +330,14 @@ qemuHostdevPrepareMediatedDevices(virQEMUDriverPtr driver,
                                   int nhostdevs)
 {
     virHostdevManagerPtr hostdev_mgr = driver->hostdevMgr;
-    bool supportsVFIO = qemuHostdevHostSupportsPassthroughVFIO();
+    bool supportsVFIO;
     size_t i;

+    /* Checking for VFIO only is fine with mdev, as IOMMU isolation is achieved
+     * by the physical parent device.
+     */
+    supportsVFIO = virFileExists("/dev/vfio/vfio");
+
     for (i = 0; i < nhostdevs; i++) {
         if (hostdevs[i]->mode == VIR_DOMAIN_HOSTDEV_MODE_SUBSYS &&
             hostdevs[i]->source.subsys.type == VIR_DOMAIN_HOSTDEV_SUBSYS_TYPE_MDEV) {
--
2.13.6