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

[Jason Wang <jasowang@redhat.com>]

On 2020/7/17 下午10:18, Peter Xu wrote:
> On Thu, Jul 16, 2020 at 10:54:31AM +0800, Jason Wang wrote:
>> 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?
>>
>> vhost_iommu_region_add()
>>      memory_region_register_iommu_notifier()
>>          memory_region_update_iommu_notify_flags()
>>              imrc->notify_flag_changed()
>>                  vtd_iommu_notify_flag_changed()
>>
>> ?
> Yeah I think you're right.  I might have confused with some previous
> implementations.  Maybe we should also do similar thing for DSI/GI just like
> what we do in PSI.


Ok.


>
>>>>>> 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?
>>> 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.
> The only caller of memory_region_iommu_replay() is vfio_listener_region_add(),
> when there's a new vIOMMU memory region detected.  So IIUC that guarantees the
> previous state should be all empty.


Right, so there's no need to deal with unmap in vtd's replay 
implementation (as what generic one did).

Thanks


>
> Thanks,
>