RE: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation APIs

Tian, Kevin
2021-04-07T02:23:06Z

> From: Jason Gunthorpe <jgg-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
> Sent: Tuesday, April 6, 2021 8:21 PM
>
> On Tue, Apr 06, 2021 at 01:02:05AM +0000, Tian, Kevin wrote:
> > > From: Jason Gunthorpe <jgg-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
> > > Sent: Tuesday, April 6, 2021 7:40 AM
> > >
> > > On Fri, Apr 02, 2021 at 07:58:02AM +0000, Tian, Kevin wrote:
> > > > > From: Jason Gunthorpe <jgg-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
> > > > > Sent: Thursday, April 1, 2021 9:47 PM
> > > > >
> > > > > On Thu, Apr 01, 2021 at 01:43:36PM +0000, Liu, Yi L wrote:
> > > > > > > From: Jason Gunthorpe <jgg-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
> > > > > > > Sent: Thursday, April 1, 2021 9:16 PM
> > > > > > >
> > > > > > > On Thu, Apr 01, 2021 at 01:10:48PM +0000, Liu, Yi L wrote:
> > > > > > > > > From: Jason Gunthorpe <jgg-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
> > > > > > > > > Sent: Thursday, April 1, 2021 7:47 PM
> > > > > > > > [...]
> > > > > > > > > I'm worried Intel views the only use of PASID in a guest is with
> > > > > > > > > ENQCMD, but that is not consistent with the industry. We need
> to
> > > see
> > > > > > > > > normal nested PASID support with assigned PCI VFs.
> > > > > > > >
> > > > > > > > I'm not quire flow here. Intel also allows PASID usage in guest
> > > without
> > > > > > > > ENQCMD. e.g. Passthru a PF to guest, and use PASID on it
> without
> > > > > > > ENQCMD.
> > > > > > >
> > > > > > > Then you need all the parts, the hypervisor calls from the vIOMMU,
> > > and
> > > > > > > you can't really use a vPASID.
> > > > > >
> > > > > > This is a diagram shows the vSVA setup.
> > > > >
> > > > > I'm not talking only about vSVA. Generic PASID support with arbitary
> > > > > mappings.
> > > > >
> > > > > And how do you deal with the vPASID vs pPASID issue if the system
> has
> > > > > a mix of physical devices and mdevs?
> > > > >
> > > >
> > > > We plan to support two schemes. One is vPASID identity-mapped to
> > > > pPASID then the mixed scenario just works, with the limitation of
> > > > lacking of live migration support. The other is non-identity-mapped
> > > > scheme, where live migration is supported but physical devices and
> > > > mdevs should not be mixed in one VM if both expose SVA capability
> > > > (requires some filtering check in Qemu).
> > >
> > > That just becomes "block vPASID support if any device that
> > > doesn't use ENQCMD is plugged into the guest"
> >
> > The limitation is only for physical device. and in reality it is not that
> > bad. To support live migration with physical device we anyway need
> > additional work to migrate the device state (e.g. based on Max's work),
> > then it's not unreasonable to also mediate guest programming of
> > device specific PASID register to enable vPASID (need to translate in
> > the whole VM lifespan but likely is not a hot path).
>
> IMHO that is pretty unreasonable.. More likely we end up with vPASID
> tables in each migratable device like KVM has.

just like mdev needs to maintain allowed PASID list, this extends it to
all migratable devices.

>
> > > Which needs a special VFIO capability of some kind so qemu knows to
> > > block it. This really needs to all be layed out together so someone
> > > can understand it :(
> >
> > Or could simply based on whether the VFIO device supports live migration.
>
> You need to define affirmative caps that indicate that vPASID will be
> supported by the VFIO device.

Yes, this is required as I acked in another mail.

>
> > > Why doesn't the siov cookbook explaining this stuff??
> > >
> > > > We hope the /dev/ioasid can support both schemes, with the minimal
> > > > requirement of allowing userspace to tag a vPASID to a pPASID and
> > > > allowing mdev to translate vPASID into pPASID, i.e. not assuming that
> > > > the guest will always use pPASID.
> > >
> > > What I'm a unclear of is if /dev/ioasid even needs to care about
> > > vPASID or if vPASID is just a hidden artifact of the KVM connection to
> > > setup the translation table and the vIOMMU driver in qemu.
> >
> > Not just for KVM. Also required by mdev, which needs to translate
> > vPASID into pPASID when ENQCMD is not used.
>
> Do we have any mdev's that will do this?

definitely. Actually any mdev which doesn't do ENQCMD needs to do this.
In normal case, the PASID is programmed to a MMIO register (or in-memory
context) associate with the backend resource of the mdev. The value
programmed from the guest is vPASID, thus must be translated into pPASID
before updating the physical register.

>
> > should only care about the operations related to pPASID. VFIO could
> > carry vPASID information to mdev.
>
> It depends how common this is, I suppose
>

based on above I think it's a common case.

Thanks
Kevin