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

[Peter Xu <peterx@redhat.com>]

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.

> > > > > 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.

Thanks,

-- 
Peter Xu