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

[Peter Krempa]

On Fri, Mar 16, 2018 at 13:37:54 +0100, Erik Skultety wrote:
> 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

I'd drop the word IOMMU here.

> 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:

That is true only if the mdev does not exist (since it creates an
iommu/isolation group) and also no other iommu groups are present on the
host.

If there are any other iommu group then qemuHostdevHostSupportsPassthroughVFIO
will return true and you'll still get the proper error that you report
below.

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

In that case this error would be wrong.

> 
> 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"

This is okay as long as you point out that the above case would happen
only if no other iommu groups were present on the host.

> 
> 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) {

ACK with the reworded commit message

> --
> 2.13.6
> 
> --
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