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

Jason Wang jasowang at redhat.com
Thu Jul 16 02:54:31 UTC 2020

On 2020/7/16 上午9:00, Peter Xu wrote:
> On Mon, Jul 13, 2020 at 12:04:16PM +0800, Jason Wang wrote:
>> 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 :)
> It's actually checked, because it loops over vtd_as_with_notifiers, and only
> MAP notifiers register to that. :)

I may miss something but I don't find the code to block UNMAP notifiers?



> But I agree with you that it should be cleaner to introduce the dev-iotlb
> notifier type.

Yes, it can solve the issues of duplicated UNMAP issue of vhost.

>>>> 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
> I think it matches exactly with a hot plug case?  Note that when IOMMU_NONE
> could also mean the translation does not exist.  So it's actually trying to map
> everything that can be translated and then notify().

Yes, so the question is what's the assumption before calling 
memory_region_iommu_replay(). If it assumes an empty mapping, there's 
probably no need for unamp.

>> (btw, the code shortcut the memory_region_notify_one(), not sure the reason)
> I think it's simply because memory_region_notify_one() came later. :)

Ok, that explains.

>>>    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?
> I feel like that can be removed now, but needs some testings...

Probably, but need to answer the above question about replay first.


> Thanks,

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