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

Jason Wang jasowang at redhat.com
Thu Jul 9 05:58:33 UTC 2020

On 2020/7/8 下午10:16, Peter Xu wrote:
> On Wed, Jul 08, 2020 at 01:42:30PM +0800, Jason Wang wrote:
>>>> So it should be functional equivalent to vtd_as_has_notifier().
>>> For example: in vtd_iommu_replay() we'll skip the replay if vhost has
>>> registered the iommu notifier because vtd_as_has_map_notifier() will return
>>> false.
>> Two questions:
>> - Do we care the performance here? If not, vhost may just ignore the MAP
>> event?
> I think we care, because vtd_page_walk() can be expensive.


>> - 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?



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