Re: [PATCH v1 17/22] intel_iommu: do not pass down pasid bind for PASID #0

Peter Xu
2020-03-25T15:12:31Z

On Wed, Mar 25, 2020 at 10:42:25AM +0000, Liu, Yi L wrote:
> > From: Peter Xu < peterx@redhat.com>
> > Sent: Wednesday, March 25, 2020 2:13 AM
> > To: Liu, Yi L <yi.l.liu@intel.com>
> > Subject: Re: [PATCH v1 17/22] intel_iommu: do not pass down pasid bind for PASID
> > #0
> >
> > On Sun, Mar 22, 2020 at 05:36:14AM -0700, Liu Yi L wrote:
> > > RID_PASID field was introduced in VT-d 3.0 spec, it is used for DMA
> > > requests w/o PASID in scalable mode VT-d. It is also known as IOVA.
> > > And in VT-d 3.1 spec, there is definition on it:
> > >
> > > "Implementations not supporting RID_PASID capability (ECAP_REG.RPS is
> > > 0b), use a PASID value of 0 to perform address translation for
> > > requests without PASID."
> > >
> > > This patch adds a check against the PASIDs which are going to be bound
> > > to device. For PASID #0, it is not necessary to pass down pasid bind
> > > request for it since PASID #0 is used as RID_PASID for DMA requests
> > > without pasid. Further reason is current Intel vIOMMU supports gIOVA
> > > by shadowing guest 2nd level page table. However, in future, if guest
> > > IOMMU driver uses 1st level page table to store IOVA mappings, then
> > > guest IOVA support will also be done via nested translation. When
> > > gIOVA is over FLPT, then vIOMMU should pass down the pasid bind
> > > request for PASID #0 to host, host needs to bind the guest IOVA page
> > > table to a proper PASID. e.g PASID value in RID_PASID field for PF/VF
> > > if ECAP_REG.RPS is clear or default PASID for ADI (Assignable Device
> > > Interface in Scalable IOV solution).
> > >
> > > IOVA over FLPT support on Intel VT-d:
> > > https://lkml.org/lkml/2019/9/23/297
> > >
> > > Cc: Kevin Tian <kevin.tian@intel.com>
> > > Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
> > > Cc: Peter Xu <peterx@redhat.com>
> > > Cc: Yi Sun <yi.y.sun@linux.intel.com>
> > > Cc: Paolo Bonzini <pbonzini@redhat.com>
> > > Cc: Richard Henderson <rth@twiddle.net>
> > > Cc: Eduardo Habkost <ehabkost@redhat.com>
> > > Signed-off-by: Liu Yi L <yi.l.liu@intel.com>
> > > ---
> > > hw/i386/intel_iommu.c | 10 ++++++++++
> > > 1 file changed, 10 insertions(+)
> > >
> > > diff --git a/hw/i386/intel_iommu.c b/hw/i386/intel_iommu.c index
> > > 1e0ccde..b007715 100644
> > > --- a/hw/i386/intel_iommu.c
> > > +++ b/hw/i386/intel_iommu.c
> > > @@ -1886,6 +1886,16 @@ static int vtd_bind_guest_pasid(IntelIOMMUState *s,
> > VTDBus *vtd_bus,
> > > struct iommu_gpasid_bind_data *g_bind_data;
> > > int ret = -1;
> > >
> > > + if (pasid < VTD_MIN_HPASID) {
> > > + /*
> > > + * If pasid < VTD_HPASID_MIN, this pasid is not allocated
> >
> > s/VTD_HPASID_MIN/VTD_MIN_HPASID/.
>
> Got it.
>
> >
> > > + * from host. No need to pass down the changes on it to host.
> > > + * TODO: when IOVA over FLPT is ready, this switch should be
> > > + * refined.
> >
> > What will happen if without this patch? Is it a must?
>
> Before gIOVA is supported by nested translation, it is a must. This requires
> IOVA over 1st level page table is ready in guest kernel, also requires the
> QEMU/VFIO supports to bind the guest IOVA page table to host.
> Currently, guest kernel side is ready. However, QEMU and VFIO side is
> not.

OK:

Reviewed-by: Peter Xu <peterx@redhat.com>

--
Peter Xu