Re: [Qemu-devel] [PATCH v2 04/10] intel-iommu: only do page walk for MAP notifiers

[Auger Eric <eric.auger@redhat.com>]

Hi Peter,

On 05/04/2018 05:08 AM, Peter Xu wrote:
> For UNMAP-only IOMMU notifiers, we don't really need to walk the page
s/really// ;-)
> tables.  Fasten that procedure by skipping the page table walk.  That
> should boost performance for UNMAP-only notifiers like vhost.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  include/hw/i386/intel_iommu.h |  2 ++
>  hw/i386/intel_iommu.c         | 43 +++++++++++++++++++++++++++++++----
>  2 files changed, 40 insertions(+), 5 deletions(-)
> 
> diff --git a/include/hw/i386/intel_iommu.h b/include/hw/i386/intel_iommu.h
> index ee517704e7..9e0a6c1c6a 100644
> --- a/include/hw/i386/intel_iommu.h
> +++ b/include/hw/i386/intel_iommu.h
> @@ -93,6 +93,8 @@ struct VTDAddressSpace {
>      IntelIOMMUState *iommu_state;
>      VTDContextCacheEntry context_cache_entry;
>      QLIST_ENTRY(VTDAddressSpace) next;
> +    /* Superset of notifier flags that this address space has */
> +    IOMMUNotifierFlag notifier_flags;
>  };
>  
>  struct VTDBus {
> diff --git a/hw/i386/intel_iommu.c b/hw/i386/intel_iommu.c
> index 112971638d..9a418abfb6 100644
> --- a/hw/i386/intel_iommu.c
> +++ b/hw/i386/intel_iommu.c
> @@ -138,6 +138,12 @@ static inline void vtd_iommu_unlock(IntelIOMMUState *s)
>      qemu_mutex_unlock(&s->iommu_lock);
>  }
>  
> +/* Whether the address space needs to notify new mappings */
> +static inline gboolean vtd_as_notify_mappings(VTDAddressSpace *as)
would suggest vtd_as_has_map_notifier()? But tastes & colours ;-)
> +{
> +    return as->notifier_flags & IOMMU_NOTIFIER_MAP;
> +}
> +
>  /* GHashTable functions */
>  static gboolean vtd_uint64_equal(gconstpointer v1, gconstpointer v2)
>  {
> @@ -1433,14 +1439,35 @@ static void vtd_iotlb_page_invalidate_notify(IntelIOMMUState *s,
>      VTDAddressSpace *vtd_as;
>      VTDContextEntry ce;
>      int ret;
> +    hwaddr size = (1 << am) * VTD_PAGE_SIZE;
>  
>      QLIST_FOREACH(vtd_as, &(s->notifiers_list), next) {
>          ret = vtd_dev_to_context_entry(s, pci_bus_num(vtd_as->bus),
>                                         vtd_as->devfn, &ce);
>          if (!ret && domain_id == VTD_CONTEXT_ENTRY_DID(ce.hi)) {
> -            vtd_page_walk(&ce, addr, addr + (1 << am) * VTD_PAGE_SIZE,
> -                          vtd_page_invalidate_notify_hook,
> -                          (void *)&vtd_as->iommu, true, s->aw_bits);
> +            if (vtd_as_notify_mappings(vtd_as)) {
> +                /*
> +                 * For MAP-inclusive notifiers, we need to walk the
> +                 * page table to sync the shadow page table.
> +                 */
Potentially we may have several notifiers attached to the IOMMU MR ~
vtd_as, each of them having different flags. Those flags are OR'ed in
memory_region_update_iommu_notify_flags and this is the one you now
store in the vtd_as. So maybe your comment may rather state:
as soon as we have at least one MAP notifier, we need to do the PTW?

nit: not related to this patch: vtd_page_walk kerneldoc comments misses
@notify_unmap param comment
side note: the name of the hook is a bit misleading as it suggests we
invalidate the entry, whereas we update any valid entry and invalidate
stale ones (if notify_unmap=true)?
> +                vtd_page_walk(&ce, addr, addr + size,
> +                              vtd_page_invalidate_notify_hook,
> +                              (void *)&vtd_as->iommu, true, s->aw_bits);
> +            } else {
> +                /*
> +                 * For UNMAP-only notifiers, we don't need to walk the
> +                 * page tables.  We just deliver the PSI down to
> +                 * invalidate caches.

We just unmap the range?
> +                 */
> +                IOMMUTLBEntry 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, entry);
> +            }
>          }
>      }
>  }
> @@ -2380,6 +2407,9 @@ static void vtd_iommu_notify_flag_changed(IOMMUMemoryRegion *iommu,
>          exit(1);
>      }
>  
> +    /* Update per-address-space notifier flags */
> +    vtd_as->notifier_flags = new;
> +
>      if (old == IOMMU_NOTIFIER_NONE) {
>          /* Insert new ones */
>          QLIST_INSERT_HEAD(&s->notifiers_list, vtd_as, next);
> @@ -2890,8 +2920,11 @@ static void vtd_iommu_replay(IOMMUMemoryRegion *iommu_mr, IOMMUNotifier *n)
>                                    PCI_FUNC(vtd_as->devfn),
>                                    VTD_CONTEXT_ENTRY_DID(ce.hi),
>                                    ce.hi, ce.lo);
> -        vtd_page_walk(&ce, 0, ~0ULL, vtd_replay_hook, (void *)n, false,
> -                      s->aw_bits);
> +        if (vtd_as_notify_mappings(vtd_as)) {
> +            /* This is required only for MAP typed notifiers */
> +            vtd_page_walk(&ce, 0, ~0ULL, vtd_replay_hook, (void *)n, false,
> +                          s->aw_bits);
> +        }
>      } else {
>          trace_vtd_replay_ce_invalid(bus_n, PCI_SLOT(vtd_as->devfn),
>                                      PCI_FUNC(vtd_as->devfn));
> 
A worthwhile improvement indeed!

Thanks

Eric