Re: [PATCH 3/3] intel-iommu: build iova tree during IOMMU translation

[Peter Xu]

On Wed, Nov 30, 2022 at 02:33:51PM +0800, Jason Wang wrote:
> On Tue, Nov 29, 2022 at 11:57 PM Peter Xu <peterx@redhat.com> wrote:
> >
> > On Tue, Nov 29, 2022 at 04:10:37PM +0800, Jason Wang wrote:
> > > The IOVA tree is only built during page walk this breaks the device
> > > that tries to use UNMAP notifier only. One example is vhost-net, it
> > > tries to use UNMAP notifier when vIOMMU doesn't support DEVIOTLB_UNMAP
> > > notifier (e.g when dt mode is not enabled). The interesting part is
> > > that it doesn't use MAP since it can query the IOMMU translation by
> > > itself upon a IOTLB miss.
> > >
> > > This doesn't work since Qemu doesn't build IOVA tree in IOMMU
> > > translation which means the UNMAP notifier won't be triggered during
> > > the page walk since Qemu think it is never mapped. This could be
> > > noticed when vIOMMU is used with vhost_net but dt is disabled.
> > >
> > > Fixing this by build the iova tree during IOMMU translation, this
> > > makes sure the UNMAP notifier event could be identified during page
> > > walk. And we need to walk page table not only for UNMAP notifier but
> > > for MAP notifier during PSI.
> > >
> > > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > > ---
> > >  hw/i386/intel_iommu.c | 43 ++++++++++++++++++-------------------------
> > >  1 file changed, 18 insertions(+), 25 deletions(-)
> > >
> > > diff --git a/hw/i386/intel_iommu.c b/hw/i386/intel_iommu.c
> > > index d025ef2873..edeb62f4b2 100644
> > > --- a/hw/i386/intel_iommu.c
> > > +++ b/hw/i386/intel_iommu.c
> > > @@ -1834,6 +1834,8 @@ static bool vtd_do_iommu_translate(VTDAddressSpace 
> > > *vtd_as, PCIBus *bus,
> > >      uint8_t access_flags;
> > >      bool rid2pasid = (pasid == PCI_NO_PASID) && s->root_scalable;
> > >      VTDIOTLBEntry *iotlb_entry;
> > > +    const DMAMap *mapped;
> > > +    DMAMap target;
> > >
> > >      /*
> > >       * We have standalone memory region for interrupt addresses, we
> > > @@ -1954,6 +1956,21 @@ out:
> > >      entry->translated_addr = vtd_get_slpte_addr(slpte, s->aw_bits) & 
> > > page_mask;
> > >      entry->addr_mask = ~page_mask;
> > >      entry->perm = access_flags;
> > > +
> > > +    target.iova = entry->iova;
> > > +    target.size = entry->addr_mask;
> > > +    target.translated_addr = entry->translated_addr;
> > > +    target.perm = entry->perm;
> > > +
> > > +    mapped = iova_tree_find(vtd_as->iova_tree, &target);
> > > +    if (!mapped) {
> > > +        /* To make UNMAP notifier work, we need build iova tree here
> > > +         * in order to have the UNMAP iommu notifier to be triggered
> > > +         * during the page walk.
> > > +         */
> > > +        iova_tree_insert(vtd_as->iova_tree, &target);
> > > +    }
> > > +
> > >      return true;
> > >
> > >  error:
> > > @@ -2161,31 +2178,7 @@ static void 
> > > vtd_iotlb_page_invalidate_notify(IntelIOMMUState *s,
> > >          ret = vtd_dev_to_context_entry(s, pci_bus_num(vtd_as->bus),
> > >                                         vtd_as->devfn, &ce);
> > >          if (!ret && domain_id == vtd_get_domain_id(s, &ce, 
> > > vtd_as->pasid)) {
> > > -            if (vtd_as_has_map_notifier(vtd_as)) {
> > > -                /*
> > > -                 * As long as we have MAP notifications registered in
> > > -                 * any of our IOMMU notifiers, we need to sync the
> > > -                 * shadow page table.
> > > -                 */
> > > -                vtd_sync_shadow_page_table_range(vtd_as, &ce, addr, 
> > > size);
> > > -            } else {
> > > -                /*
> > > -                 * For UNMAP-only notifiers, we don't need to walk the
> > > -                 * page tables.  We just deliver the PSI down to
> > > -                 * invalidate caches.
> > > -                 */
> > > -                IOMMUTLBEvent event = {
> > > -                    .type = IOMMU_NOTIFIER_UNMAP,
> > > -                    .entry = {
> > > -                        .target_as = &address_space_memory,
> > > -                        .iova = addr,
> > > -                        .translated_addr = 0,
> > > -                        .addr_mask = size - 1,
> > > -                        .perm = IOMMU_NONE,
> > > -                    },
> > > -                };
> > > -                memory_region_notify_iommu(&vtd_as->iommu, 0, event);
> >
> > Isn't this path the one that will be responsible for pass-through the UNMAP
> > events from guest to vhost when there's no MAP notifier requested?
> 
> Yes, but it doesn't do the iova tree removing. More below.
> 
> >
> > At least that's what I expected when introducing the iova tree, because for
> > unmap-only device hierachy I thought we didn't need the tree at all.
> 
> Then the problem is the UNMAP notifier won't be trigger at all during
> DSI page walk in vtd_page_walk_one() because there's no DMAMap stored
> in the iova tree.:
> 
>         if (!mapped) {
>             /* Skip since we didn't map this range at all */
>             trace_vtd_page_walk_one_skip_unmap(entry->iova, entry->addr_mask);
>             return 0;
>         }
> 
> So I choose to build the iova tree in translate then we won't go
> within the above condition.

That's also why it's weird because IIUC we should never walk a page table
at all if there's no MAP notifier regiestered.

When I'm looking at the walk callers I found that indeed there's one path
missing where can cause it to actually walk the pgtables without !MAP, then
I also noticed commit f7701e2c7983b6, and I'm wondering what we really want
is something like this:

diff --git a/hw/i386/intel_iommu.c b/hw/i386/intel_iommu.c
index a08ee85edf..c46f3db992 100644
--- a/hw/i386/intel_iommu.c
+++ b/hw/i386/intel_iommu.c
@@ -1536,7 +1536,7 @@ static int vtd_sync_shadow_page_table(VTDAddressSpace 
*vtd_as)
     VTDContextEntry ce;
     IOMMUNotifier *n;

-    if (!(vtd_as->iommu.iommu_notify_flags & IOMMU_NOTIFIER_IOTLB_EVENTS)) {
+    if (!vtd_as_has_map_notifier(vtd_as)) {
         return 0;
     }

So I'm not sure whether this patch is the problem resolver; so far I feel
like it's patch 2 who does the real fix.  Then we can have the above
oneliner so we stop any walks when there's no map notifiers.

Thanks,

-- 
Peter Xu