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Re: [RFC v2 1/1] memory: Delete assertion in memory_region_unregister_io
From: |
Peter Xu |
Subject: |
Re: [RFC v2 1/1] memory: Delete assertion in memory_region_unregister_iommu_notifier |
Date: |
Wed, 15 Jul 2020 21:00:05 -0400 |
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. :)
But I agree with you that it should be cleaner to introduce the dev-iotlb
notifier type.
>
>
> >
> > > 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().
>
> (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. :)
>
>
> > 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...
Thanks,
--
Peter Xu
- Re: [RFC v2 1/1] memory: Delete assertion in memory_region_unregister_iommu_notifier, (continued)
- Re: [RFC v2 1/1] memory: Delete assertion in memory_region_unregister_iommu_notifier, Peter Xu, 2020/07/03
- Re: [RFC v2 1/1] memory: Delete assertion in memory_region_unregister_iommu_notifier, Jason Wang, 2020/07/07
- Re: [RFC v2 1/1] memory: Delete assertion in memory_region_unregister_iommu_notifier, Peter Xu, 2020/07/07
- Re: [RFC v2 1/1] memory: Delete assertion in memory_region_unregister_iommu_notifier, Jason Wang, 2020/07/08
- Re: [RFC v2 1/1] memory: Delete assertion in memory_region_unregister_iommu_notifier, Peter Xu, 2020/07/08
- Re: [RFC v2 1/1] memory: Delete assertion in memory_region_unregister_iommu_notifier, Jason Wang, 2020/07/09
- Re: [RFC v2 1/1] memory: Delete assertion in memory_region_unregister_iommu_notifier, Peter Xu, 2020/07/09
- Re: [RFC v2 1/1] memory: Delete assertion in memory_region_unregister_iommu_notifier, Jason Wang, 2020/07/10
- Re: [RFC v2 1/1] memory: Delete assertion in memory_region_unregister_iommu_notifier, Peter Xu, 2020/07/10
- Re: [RFC v2 1/1] memory: Delete assertion in memory_region_unregister_iommu_notifier, Jason Wang, 2020/07/13
- Re: [RFC v2 1/1] memory: Delete assertion in memory_region_unregister_iommu_notifier,
Peter Xu <=
- Re: [RFC v2 1/1] memory: Delete assertion in memory_region_unregister_iommu_notifier, Jason Wang, 2020/07/15
- Re: [RFC v2 1/1] memory: Delete assertion in memory_region_unregister_iommu_notifier, Peter Xu, 2020/07/17
- Re: [RFC v2 1/1] memory: Delete assertion in memory_region_unregister_iommu_notifier, Jason Wang, 2020/07/20
- Re: [RFC v2 1/1] memory: Delete assertion in memory_region_unregister_iommu_notifier, Peter Xu, 2020/07/20
- Re: [RFC v2 1/1] memory: Delete assertion in memory_region_unregister_iommu_notifier, Jason Wang, 2020/07/21
- Re: [RFC v2 1/1] memory: Delete assertion in memory_region_unregister_iommu_notifier, Peter Xu, 2020/07/21
Re: [RFC v2 1/1] memory: Delete assertion in memory_region_unregister_iommu_notifier, Jason Wang, 2020/07/01