On Fri, Aug 01, 2025 at 02:55:28PM +0100, Daniel P. Berrangé wrote:
> On Tue, Jul 01, 2025 at 02:18:15PM +0200, Oliver Steffen wrote:
> > From: Gerd Hoffmann <kra...@redhat.com>
> >
> > Add virtio-mmio slots to the q35 machine model, intended to be used by
> > an SVSM.
> >
> > Disabled by default, enable using '-machine q35,x-svsm-virtio-mmio=on'.
> >
> > When enabled it is possible to plug up to 4 virtio devices into the
> > slots virtio-mmio using '-device virtio-${kind}-device'.
> >
> > The devices can be found at base address 0xfef00000, each slot on a
> > separate page. No IRQ is wired up, the SVSM has to drive the devices
> > in polling mode.
> >
> > The base addresses are communicated to the SVSM via the etc/hardware-info
> > fw_cfg file.
>
> This feels a somewhat uncomfortable in that it ties QEMU to specific
> impl details of SVSM at a point in time.
>
> With the IGVM support for loading guest firmware, we've avoided QEMU
> needing any knowledge of what the firmware actually is. It is just
> an opaque blob that is loaded based on its own IGVM metadata. This
> also made it possible for any hypervisor with IGVM support to be able
> to load any firmware, including SVSM or equiv impls.
Well, there isn't anything svsm-specific in there, it is just that svsm
is the only user right now (and I don't expect that to change anytime
soon). But we could very well rename the option to "virtio-mmio=on".
> Is there any way to extend IGVM to express that it supports these
> 4 virtio-mmio slots, at the given address in polling mode, so that
> hypervisors can auto-discover this facility ?
You mean some igvm directive requesting that qemu creates virtio-mmio
slots?
Should be possible. Not sure how much of a win that would actually be
though. You still have to create the devices and the corresponding
backends on the qemu command line.
> Failing that, can we make it possible to create virtio-mmio slots
> on the QEMU command line, instead of hardcoding this SVSM-specific
> setup in QEMU code.
That would work too.
take care,
Gerd
