Re: [RFC v2 1/1] memory: Delete assertion in memory_region_unregister_iommu_notifier

[Jason Wang <jasowang@redhat.com>]

On 2020/7/10 下午9:30, Peter Xu wrote:
> On Fri, Jul 10, 2020 at 02:34:11PM +0800, Jason Wang wrote:
>> On 2020/7/9 下午10:10, Peter Xu wrote:
>>> On Thu, Jul 09, 2020 at 01:58:33PM +0800, Jason Wang wrote:
>>>>>> - If we care the performance, it's better to implement the MAP event for
>>>>>> vhost, otherwise it could be a lot of IOTLB miss
>>>>> I feel like these are two things.
>>>>>
>>>>> So far what we are talking about is whether vt-d should have knowledge about
>>>>> what kind of events one iommu notifier is interested in.  I still think we
>>>>> should keep this as answered in question 1.
>>>>>
>>>>> The other question is whether we want to switch vhost from UNMAP to MAP/UNMAP
>>>>> events even without vDMA, so that vhost can establish the mapping even before
>>>>> IO starts.  IMHO it's doable, but only if the guest runs DPDK workloads.  When
>>>>> the guest is using dynamic iommu page mappings, I feel like that can be even
>>>>> slower, because then the worst case is for each IO we'll need to vmexit twice:
>>>>>
>>>>>      - The first vmexit caused by an invalidation to MAP the page tables, so vhost
>>>>>        will setup the page table before IO starts
>>>>>
>>>>>      - IO/DMA triggers and completes
>>>>>
>>>>>      - The second vmexit caused by another invalidation to UNMAP the page tables
>>>>>
>>>>> So it seems to be worse than when vhost only uses UNMAP like right now.  At
>>>>> least we only have one vmexit (when UNMAP).  We'll have a vhost translate()
>>>>> request from kernel to userspace, but IMHO that's cheaper than the vmexit.
>>>> Right but then I would still prefer to have another notifier.
>>>>
>>>> Since vtd_page_walk has nothing to do with device IOTLB. IOMMU have a
>>>> dedicated command for flushing device IOTLB. But the check for
>>>> vtd_as_has_map_notifier is used to skip the device which can do demand
>>>> paging via ATS or device specific way. If we have two different notifiers,
>>>> vhost will be on the device iotlb notifier so we don't need it at all?
>>> But we can still have iommu notifier that only registers to UNMAP even after we
>>> introduce dev-iotlb notifier?  We don't want to do page walk for them as well.
>>> TCG should be the only one so far, but I don't know.. maybe there can still be
>>> new ones?
>>
>> I think you're right. But looking at the codes, it looks like the check of
>> vtd_as_has_map_notifier() was only used in:
>>
>> 1) vtd_iommu_replay()
>> 2) vtd_iotlb_page_invalidate_notify() (PSI)
>>
>> For the replay, it's expensive anyhow. For PSI, I think it's just about one
>> or few mappings, not sure it will have obvious performance impact.
>>
>> And I had two questions:
>>
>> 1) The codes doesn't check map for DSI or GI, does this match what spec
>> said? (It looks to me the spec is unclear in this part)
> Both DSI/GI should cover maps too?  E.g. vtd_sync_shadow_page_table() in
> vtd_iotlb_domain_invalidate().


I meant the code doesn't check whether there's an MAP notifier :)


>
>> 2) for the replay() I don't see other implementations (either spapr or
>> generic one) that did unmap (actually they skip unmap explicitly), any
>> reason for doing this in intel IOMMU?
> I could be wrong, but I'd guess it's because vt-d implemented the caching mode
> by leveraging the same invalidation strucuture, so it's harder to make all
> things right (IOW, we can't clearly identify MAP with UNMAP when we receive an
> invalidation request, because MAP/UNMAP requests look the same).
>
> I didn't check others, but I believe spapr is doing it differently by using
> some hypercalls to deliver IOMMU map/unmap requests, which seems a bit close to
> what virtio-iommu is doing.  Anyway, the point is if we have explicit MAP/UNMAP
> from the guest, logically the replay indeed does not need to do any unmap
> because we don't need to call replay() on an already existing device but only
> for e.g. hot plug.


But this looks conflict with what memory_region_iommu_replay( ) did, for 
IOMMU that doesn't have a replay method, it skips UNMAP request:

     for (addr = 0; addr < memory_region_size(mr); addr += granularity) {
         iotlb = imrc->translate(iommu_mr, addr, IOMMU_NONE, n->iommu_idx);
         if (iotlb.perm != IOMMU_NONE) {
             n->notify(n, &iotlb);
         }

I guess there's no knowledge of whether guest have an explicit MAP/UMAP 
for this generic code. Or replay implies that guest doesn't have 
explicit MAP/UNMAP?

(btw, the code shortcut the memory_region_notify_one(), not sure the reason)


>   VT-d does not have that clear interface, so VT-d needs to
> maintain its own mapping structures, and also vt-d is using the same replay &
> page_walk operations to sync all these structures, which complicated the vt-d
> replay a bit.  With that, we assume replay() can be called anytime on a device,
> and we won't notify duplicated MAPs to lower layer like vfio if it is mapped
> before.  At the meantime, since we'll compare the latest mapping with the one
> we cached in the iova tree, UNMAP becomes possible too.


AFAIK vtd_iommu_replay() did a completely UNMAP:

     /*
      * The replay can be triggered by either a invalidation or a newly
      * created entry. No matter what, we release existing mappings
      * (it means flushing caches for UNMAP-only registers).
      */
     vtd_address_space_unmap(vtd_as, n);

Since it doesn't do any comparison with iova tree. Will this cause 
unnecessary UNMAP to be sent to VFIO?

Thanks


>
> Thanks,
>