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

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

On 11/4/25 3:37 PM, Shameer Kolothum wrote:
> Hi Eric,
>
>> -----Original Message-----
>> From: Eric Auger <eric.auger@redhat.com>
>> Sent: 04 November 2025 14:12
>> 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
>>
>> External email: Use caution opening links or attachments
>>
>>
>> Hi Shameer, Nicolin,
>>
>> On 10/31/25 11:49 AM, Shameer Kolothum wrote:
>>> On ARM, devices behind an IOMMU have their MSI doorbell addresses
>>> translated by the IOMMU. In nested mode, this translation happens in
>>> two stages (gIOVA → gPA → ITS page).
>>>
>>> In accelerated SMMUv3 mode, both stages are handled by hardware, so
>>> get_address_space() returns the system address space so that VFIO
>>> can setup stage-2 mappings for system address space.
>> Sorry but I still don't catch the above. Can you explain (most probably
>> again) why this is a requirement to return the system as so that VFIO
>> can setup stage-2 mappings for system address space. I am sorry for
>> insisting (at the risk of being stubborn or dumb) but I fail to
>> understand the requirement. As far as I remember the way I integrated it
>> at the old times did not require that change:
>> https://lore.kernel.org/all/20210411120912.15770-1-
>> eric.auger@redhat.com/
>> I used a vfio_prereg_listener to force the S2 mapping.
> Yes I remember that.
>
>> What has changed that forces us now to have this gym
> This approach achieves the same outcome, but through a 
> different mechanism. Returning the system address space
> here ensures that VFIO sets up the Stage-2 mappings for 
> devices behind the accelerated SMMUv3.
>
> I think, this makes sense because, in the accelerated case, the
> device is no longer managed by QEMU’s SMMUv3 model. The
On the other hand, as we discussed on v4 by returning system as you
pretend there is no translation in place which is not true. Now we use
an alias for it but it has not really removed its usage. Also it forces
use to hack around the MSI mapping and introduce new PCIIOMMUOps. Have
you assessed the feasability of using vfio_prereg_listener to force the
S2 mapping. Is it simply not relevant anymore or could it be used also
with the iommufd be integration? Eric
> guest owns the Stage-1 context, and the host (VFIO) is responsible
> for establishing the Stage-2 mappings accordingly. 
>
> Do you see any issues with this approach?
>
> Thanks,
> Shameer