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

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(a)redhat.com&gt; --- 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