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

[Auger Eric]

Hi Peter,

On 05/18/2018 07:53 AM, Peter Xu wrote:
> On Thu, May 17, 2018 at 03:39:50PM +0200, Auger Eric wrote:
>> 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// ;-)
> 
> Ok.
> 
>>> 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 <address@hidden>
>>> ---
>>>  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 ;-)
> 
> Yeah it is.  But okay, I can switch to that especially it's only used
> in this patch and it's new.
> 
>>> +{
>>> +    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?
> 
> Actually this is not 100% clear too, since all the "MAP notifiers" are
> actually both MAP+UNMAP notifiers...  Maybe:

Can't IOMMU_NOTIFIER_MAP flag value be used without
IOMMU_NOTIFIER_UNMAP? I don't see such restriction in the
memory_region_register_iommu_notifier API.
> 
>   As long as we have MAP notifications registered in any of our IOMMU
>   notifiers, we need to sync the shadow page table.
> 
>>
>> 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?
> 
> Isn't it the same thing? :)
> 
> If to be explicit, here we know we only registered UNMAP
> notifications, it's not really "unmap", it's really cache
> invalidations only.
yes you're right I meant We just invalidate the range in cache. The
sentence "We just deliver the PSI down to invalidate caches." was not
crystal clear to me at first reading.

Thanks

Eric
> 
>>> +                 */
>>> +                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!
> 
> I hope so. :) Thanks,
>