On Mon, Dec 11, 2023 at 11:03:45AM -0700, Alex Williamson wrote: > On Sun, 26 Nov 2023 22:39:09 -0800 > Yi Liu <yi.l.liu@xxxxxxxxx> wrote: > > > This reports the PASID capability data to userspace via VFIO_DEVICE_FEATURE, > > hence userspace could probe PASID capability by it. This is a bit different > > with other capabilities which are reported to userspace when the user reads > > the device's PCI configuration space. There are two reasons for this. > > > > - First, Qemu by default exposes all available PCI capabilities in vfio-pci > > config space to the guest as read-only, so adding PASID capability in the > > vfio-pci config space will make it exposed to the guest automatically while > > an old Qemu doesn't really support it. > > Shouldn't we also be working on hiding the PASID capability in QEMU > ASAP? This feature only allows QEMU to know PASID control is actually > available, not the guest. Maybe we're hoping this is really only used > by VFs where there's no capability currently exposed to the guest? Makes sense, yes > > the PF). Creating a virtual PASID capability in vfio-pci config space needs > > to find a hole to place it, but doing so may require device specific > > knowledge to avoid potential conflict with device specific registers like > > hiden bits in VF config space. It's simpler by moving this burden to the > > VMM instead of maintaining a quirk system in the kernel. > > This feels a bit like an incomplete solution though and we might > already posses device specific knowledge in the form of a variant > driver. Should this feature structure include a flag + field that > could serve to generically indicate to the VMM a location for > implementing the PASID capability? The default core implementation > might fill this only for PFs where clearly an emualted PASID capability > can overlap the physical capability. Thanks, In many ways I would perfer to solve this for good by having a way to learn a range of available config space - I liked the suggestion to use a DVSEC to mark empty space. Jason
