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 12:41 PM, Shameer Kolothum wrote:
> Hi Eric,
>
>> -----Original Message-----
>> From: Eric Auger <eric.auger@redhat.com>
>> Sent: 05 November 2025 08:57
>> To: Shameer Kolothum <skolothumtho@nvidia.com>; qemu-
>> arm@nongnu.org; qemu-devel@nongnu.org
>> Cc: peter.maydell@linaro.org; Jason Gunthorpe <jgg@nvidia.com>; Nicolin
>> Chen <nicolinc@nvidia.com>; ddutile@redhat.com; berrange@redhat.com;
>> Nathan Chen <nathanc@nvidia.com>; Matt Ochs <mochs@nvidia.com>;
>> smostafa@google.com; wangzhou1@hisilicon.com;
>> jiangkunkun@huawei.com; jonathan.cameron@huawei.com;
>> zhangfei.gao@linaro.org; zhenzhong.duan@intel.com; yi.l.liu@intel.com;
>> Krishnakant Jaju <kjaju@nvidia.com>
>> Subject: Re: [PATCH v5 15/32] hw/pci/pci: Introduce optional
>> get_msi_address_space() callback
> [...]
>>> IIUC, the prereg_listener mechanism just enables us to setup the s2
>>> mappings. For MSI, In your version, I see that smmu_find_add_as()
>>> always returns IOMMU as. How is that supposed to work if the Guest
>>> has s1 bypass mode STE for the device?
>> in kvm_arch_fixup_msi_route(), as we have as != &address_space_memory in
>> my case, we proceed with the actual translation for the doorbell gIOVA
>> using address_space_translate(). I  guess if the S1 is in bypass mode
>> you get the flat translation, no?
> Yes, I noted that and replied as well.
>
> Again, coming back to kvm_arch_fixup_msi_route(), I see that this was introduced
> as part of your " ARM SMMUv3 Emulation Support" here,
> https://lore.kernel.org/qemu-devel/1523518688-26674-12-git-send-email-eric.auger@redhat.com/
>
> The VFIO support was not there at that time. I am trying to understand why
> we need this MSI translation for vfio-pci in this accelerated case. My understanding
> was that this is to setup the KVM MSI routings via KVM_SET_GSI_ROUTING ioctl.

yes that's correct. This was first needed for vhost integration. And
obviously this is also needed for VFIO.

allows vhost irqfd to trigger a gsi that will be routed by KVM to the
actual guest doorbell. On top of that it registers the guest PCI BDF for
GiCv2m or GICv3 MSI translation setup.
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

Eric
>
> Is that right?
>
> Thanks,
> Shameer
>
>
>