Re: [PATCH v5 31/32] vfio: Synthesize vPASID capability to VM

[Jonathan Cameron via <qemu-arm@nongnu.org>]

On Fri, 31 Oct 2025 10:50:04 +0000
Shameer Kolothum <skolothumtho@nvidia.com> wrote:

> From: Yi Liu <yi.l.liu@intel.com>
> 
> If user wants to expose PASID capability in vIOMMU, then VFIO would also
> report the PASID cap for this device if the underlying hardware supports
> it as well.
> 
> As a start, this chooses to put the vPASID cap in the last 8 bytes of the
> vconfig space. This is a choice in the good hope of no conflict with any
> existing cap or hidden registers. For the devices that has hidden registers,
> user should figure out a proper offset for the vPASID cap. This may require
> an option for user to config it. Here we leave it as a future extension.
> There are more discussions on the mechanism of finding the proper offset.
> 
> https://lore.kernel.org/kvm/BN9PR11MB5276318969A212AD0649C7BE8CBE2@BN9PR11MB5276.namprd11.prod.outlook.com/
> 
> Since we add a check to ensure the vIOMMU supports PASID, only devices
> under those vIOMMUs can synthesize the vPASID capability. This gives
> users control over which devices expose vPASID.
> 
> Signed-off-by: Yi Liu <yi.l.liu@intel.com>
> Tested-by: Zhangfei Gao <zhangfei.gao@linaro.org>
> Signed-off-by: Shameer Kolothum <skolothumtho@nvidia.com>
Whilst not particularly keen on this hack, I can't see a better solution.
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>