Re: [PATCH v5 15/32] hw/pci/pci: Introduce optional get_msi_address_space() callback

[Eric Auger <eric.auger@redhat.com>]

On 11/5/25 7:58 PM, Jason Gunthorpe wrote:
> On Wed, Nov 05, 2025 at 10:33:08AM -0800, Nicolin Chen wrote:
>> On Wed, Nov 05, 2025 at 02:10:49PM -0400, Jason Gunthorpe wrote:
>>> On Wed, Nov 05, 2025 at 06:25:05PM +0100, Eric Auger wrote:
>>>> if the guest doorbell address is wrong because not properly translated,
>>>> vgic_msi_to_its() will fail to identify the ITS to inject the MSI in.
>>>> See kernel kvm/vgic/vgic-its.c vgic_msi_to_its and
>>>> vgic_its_inject_msi
>>> Which has been exactly my point to Nicolin. There is no way to
>>> "properly translate" the vMSI address in a HW accelerated SMMU
>>> emulation.
>> Hmm, I still can't connect the dots here. QEMU knows where the
>> guest CD table is to get the stage-1 translation table to walk
>> through. We could choose to not let it walk through. Yet, why?
> You cannot walk any tables in guest memory without fully trapping all
> invalidation on all command queues. Like real HW qemu needs to fence
> its walks with any concurrent invalidate & sync to ensure it doesn't
> walk into a UAF situation.
But at the moment we do trap IOTLB invalidates so logically we can still
do the translate in that config. The problem you describe will show up
with vCMDQ which is not part of this series.
>
> Since we can't trap or mediate vCMDQ the walking simply cannot be
> done.
>
> Thus, the general principle of the HW accelerated vSMMU is that it
> NEVER walks any of these guest tables for any reason.
>
> Thus, we cannot do anything with vMSI address beyond program it
> directly into a real PCI device so it undergoes real HW translation.
But anyway you need to provide KVM a valid info about the guest doorbell
for this latter to setup irqfd gsi routing and also program ITS
translation tables. At the moment we have a single vITS in qemu so maybe
we can cheat.

Eric
>
> Jason
>