Re: [PATCH v6 17/33] hw/arm/smmuv3: Add support for providing a direct MSI doorbell GPA

[Nicolin Chen <nicolinc@nvidia.com>]

On Fri, Nov 21, 2025 at 01:57:37AM -0800, Shameer Kolothum wrote:
> > On Thu, Nov 20, 2025 at 01:21:57PM +0000, Shameer Kolothum wrote:
> > > Accelerated SMMUv3 instances rely on the physical SMMUv3 for nested
> > > translation (Guest Stage-1, Host Stage-2). In this mode the guest’s
> > > Stage-1 tables are programmed directly into hardware, and QEMU should
> > > not attempt to walk them for translation since doing so is not reliably
> > > safe. For vfio-pci endpoints behind such a vSMMU, the only translation
> > > QEMU is responsible for is the MSI doorbell used during KVM MSI setup.
> > >
> > > Add a device property to carry the MSI doorbell GPA from the virt
> > > machine, and expose it through a new get_msi_direct_gpa PCIIOMMUOp.
> > > kvm_arch_fixup_msi_route() can then use this GPA directly instead of
> > > attempting a software walk of guest translation tables.
> > >
> > > This enables correct MSI routing with accelerated SMMUv3 while avoiding
> > > unsafe accesses to page tables.
> > >
> > > For meaningful use of vfio-pci devices with accelerated SMMUv3, both KVM
> > > and a kernel irqchip are required. Enforce this requirement when accel=on
> > > is selected.
> > >
> > > Signed-off-by: Shameer Kolothum <skolothumtho@nvidia.com>
> > 
> > Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
> > 
> > Nits:
> > 
> > > +++ b/hw/arm/virt.c
> > > @@ -3052,6 +3052,14 @@ static void
> > virt_machine_device_pre_plug_cb(HotplugHandler *hotplug_dev,
> > >              /* The new SMMUv3 device is specific to the PCI bus */
> > >              object_property_set_bool(OBJECT(dev), "smmu_per_bus", true,
> > NULL);
> > >          }
> > > +        if (object_property_find(OBJECT(dev), "accel") &&
> > > +            object_property_get_bool(OBJECT(dev), "accel", &error_abort)) {
> > 
> > Do we need object_property_find()? A later patch seems to drop it.
> > Perhaps we shouldn't add it in the first place?
> 
> We need that at this stage as we haven't added the "accel" property yet
> and that will cause "make check" tests to fail without that.
>
> We remove it once we introduce "accel" property later.

Hmm, I assume object_property_get_bool() would return false when
"accel" is not available yet? No?

Nicolin