Re: [PATCH 1/3] intel-iommu: fail MAP notifier without caching mode

[Jason Wang]

On Tue, Nov 29, 2022 at 11:35 PM Peter Xu <peterx@redhat.com> wrote:
>
> On Tue, Nov 29, 2022 at 04:10:35PM +0800, Jason Wang wrote:
> > Without caching mode, MAP notifier won't work correctly since guest
> > won't send IOTLB update event when it establishes new mappings in the
> > I/O page tables. Let's fail the IOMMU notifiers early instead of
> > misbehaving silently.
> >
> > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > ---
> >  hw/i386/intel_iommu.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> >
> > diff --git a/hw/i386/intel_iommu.c b/hw/i386/intel_iommu.c
> > index a08ee85edf..9143376677 100644
> > --- a/hw/i386/intel_iommu.c
> > +++ b/hw/i386/intel_iommu.c
> > @@ -3186,6 +3186,13 @@ static int 
> > vtd_iommu_notify_flag_changed(IOMMUMemoryRegion *iommu,
> >                           "Snoop Control with vhost or VFIO is not 
> > supported");
> >          return -ENOTSUP;
> >      }
> > +    if (!s->caching_mode && (new & IOMMU_NOTIFIER_MAP)) {
> > +        error_setg_errno(errp, ENOTSUP,
> > +                         "device %02x.%02x.%x requires caching mode",
> > +                         pci_bus_num(vtd_as->bus), PCI_SLOT(vtd_as->devfn),
> > +                         PCI_FUNC(vtd_as->devfn));
> > +        return -ENOTSUP;
> > +    }
>
> We used to have that but got reverted because it's too late to fail, so we
> moved it over even though not as clean..
>
> https://lore.kernel.org/all/20190916080718.3299-5-peterx@redhat.com/

One of the difference is that the patch doesn't do exit() here. I
think it's better to fail instead of misbehving silently, this is what
other vIOMMU did:

E.g in smmu we had:

    if (new & IOMMU_NOTIFIER_MAP) {
        error_setg(errp,
                   "device %02x.%02x.%x requires iommu MAP notifier which is "
                   "not currently supported", pci_bus_num(sdev->bus),
                   PCI_SLOT(sdev->devfn), PCI_FUNC(sdev->devfn));
        return -EINVAL;
    }

So did for amd iommu.

Thanks

>
> Thanks,
>
> --
> Peter Xu
>