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

[Jason Wang <jasowang@redhat.com>]

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)
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?

Thanks


>
> Thanks,
>