Re: [RFC] /dev/ioasid uAPI proposal

Jason Gunthorpe
2021-06-04T17:40:09Z

On Fri, Jun 04, 2021 at 10:27:43AM -0700, Jacob Pan wrote:
> Hi Jason,
>
> On Fri, 4 Jun 2021 09:05:55 -0300, Jason Gunthorpe <jgg@nvidia.com> wrote:
>
> > On Fri, Jun 04, 2021 at 12:24:08PM +0200, Jean-Philippe Brucker wrote:
> >
> > > I think once it binds a device to an IOASID fd, QEMU will want to probe
> > > what hardware features are available before going further with the
> > > vIOMMU setup (is there PASID, PRI, which page table formats are
> > > supported,
> >
> > I think David's point was that qemu should be told what vIOMMU it is
> > emulating exactly (right down to what features it has) and then
> > the goal is simply to match what the vIOMMU needs with direct HW
> > support via /dev/ioasid and fall back to SW emulation when not
> > possible.
> >
> > If qemu wants to have some auto-configuration: 'pass host IOMMU
> > capabilities' similar to the CPU flags then qemu should probe the
> > /dev/ioasid - and maybe we should just return some highly rolled up
> > "this is IOMMU HW ID ARM SMMU vXYZ" out of some query to guide qemu in
> > doing this.
> >
> There can be mixed types of physical IOMMUs on the host. So not until a
> device is attached, we would not know if the vIOMMU can match the HW
> support of the device's IOMMU. Perhaps, vIOMMU should check the
> least common denominator features before commit.

qemu has to set the vIOMMU at VM startup time, so if it is running in
some "copy host" mode the only thing it can do is evaluate the VFIO
devices that are present at boot and select a vIOMMU from that list.

Probably would pick the most capable physical IOMMU and software
emulate the reset.

platforms really should avoid creating wildly divergent IOMMUs in the
same system if they want to support virtualization effectively.

Jason