RE: [PATCH] intel_iommu: Optimize out some unnecessary UNMAP calls

[Liu, Yi L]

> From: Jason Wang <jasowang@redhat.com>
> Sent: Friday, May 26, 2023 2:28 PM
> 
> On Fri, May 26, 2023 at 2:22 PM Duan, Zhenzhong
> <zhenzhong.duan@intel.com> wrote:
> >
> >
> > >-----Original Message-----
> > >From: Peter Xu <peterx@redhat.com>
> > >Sent: Thursday, May 25, 2023 9:54 PM
> > >Subject: Re: [PATCH] intel_iommu: Optimize out some unnecessary UNMAP
> > >calls
> > >
> > >On Thu, May 25, 2023 at 11:29:34AM +0000, Duan, Zhenzhong wrote:
> > >> Hi Peter,
> > >>
> > >> See inline.
> > >> >-----Original Message-----
> > >> >From: Peter Xu <peterx@redhat.com>
> > >> >Sent: Thursday, May 25, 2023 12:59 AM
> > >> >Subject: Re: [PATCH] intel_iommu: Optimize out some unnecessary UNMAP
> > >> >calls
> > >> >
> > >> >Hi, Zhenzhong,
> > >> >
> > >> >On Tue, May 23, 2023 at 04:07:02PM +0800, Zhenzhong Duan wrote:
> > >> >> Commit 63b88968f1 ("intel-iommu: rework the page walk logic") adds
> > >> >> logic to record mapped IOVA ranges so we only need to send MAP or
> > >> >> UNMAP when necessary. But there are still a few corner cases of
> > >> >unnecessary UNMAP.
> > >> >>
> > >> >> One is address space switch. During switching to iommu address
> > >> >> space, all the original mappings have been dropped by VFIO memory
> > >> >> listener, we don't need to unmap again in replay. The other is
> > >> >> invalidation, we only need to unmap when there are recorded mapped
> > >> >> IOVA ranges, presuming most of OSes allocating IOVA range
> > >> >> continuously, ex. on x86, linux sets up mapping from 0xffffffff
> > >downwards.
> > >> >>
> > >> >> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
> > >> >> ---
> > >> >> Tested on x86 with a net card passed or hotpluged to kvm guest,
> > >> >> ping/ssh pass.
> > >> >
> > >> >Since this is a performance related patch, do you have any number to
> > >> >show the effect?
> > >>
> > >> I straced the time of UNMAP ioctl, its time is 0.000014us and we have
> > >> 28 ioctl() due to the two notifiers in x86 are split into power of 2 
> > >> pieces.
> > >>
> > >> ioctl(48, VFIO_DEVICE_QUERY_GFX_PLANE or VFIO_IOMMU_UNMAP_DMA,
> > >> 0x7ffffd5c42f0) = 0 <0.000014>
> > >
> > >Could you add some information like this into the commit message when
> > >repost?  E.g. UNMAP was xxx sec before, and this patch reduces it to yyy.
> > Sure, will do.
> >
> > >
> > >>
> > >> >
> > >> >>
> > >> >>  hw/i386/intel_iommu.c | 31 ++++++++++++++-----------------
> > >> >>  1 file changed, 14 insertions(+), 17 deletions(-)
> > >> >>
> > >> >> diff --git a/hw/i386/intel_iommu.c b/hw/i386/intel_iommu.c index
> > >> >> 94d52f4205d2..6afd6428aaaa 100644
> > >> >> --- a/hw/i386/intel_iommu.c
> > >> >> +++ b/hw/i386/intel_iommu.c
> > >> >> @@ -3743,6 +3743,7 @@ static void
> > >> >vtd_address_space_unmap(VTDAddressSpace *as, IOMMUNotifier *n)
> > >> >>      hwaddr start = n->start;
> > >> >>      hwaddr end = n->end;
> > >> >>      IntelIOMMUState *s = as->iommu_state;
> > >> >> +    IOMMUTLBEvent event;
> > >> >>      DMAMap map;
> > >> >>
> > >> >>      /*
> > >> >> @@ -3762,22 +3763,25 @@ static void
> > >> >vtd_address_space_unmap(VTDAddressSpace *as, IOMMUNotifier *n)
> > >> >>      assert(start <= end);
> > >> >>      size = remain = end - start + 1;
> > >> >>
> > >> >> +    event.type = IOMMU_NOTIFIER_UNMAP;
> > >> >> +    event.entry.target_as = &address_space_memory;
> > >> >> +    event.entry.perm = IOMMU_NONE;
> > >> >> +    /* This field is meaningless for unmap */
> > >> >> +    event.entry.translated_addr = 0;
> > >> >> +
> > >> >>      while (remain >= VTD_PAGE_SIZE) {
> > >> >> -        IOMMUTLBEvent event;
> > >> >>          uint64_t mask = dma_aligned_pow2_mask(start, end, 
> > >> >> s->aw_bits);
> > >> >>          uint64_t size = mask + 1;
> > >> >>
> > >> >>          assert(size);
> > >> >>
> > >> >> -        event.type = IOMMU_NOTIFIER_UNMAP;
> > >> >> -        event.entry.iova = start;
> > >> >> -        event.entry.addr_mask = mask;
> > >> >> -        event.entry.target_as = &address_space_memory;
> > >> >> -        event.entry.perm = IOMMU_NONE;
> > >> >> -        /* This field is meaningless for unmap */
> > >> >> -        event.entry.translated_addr = 0;
> > >> >> -
> > >> >> -        memory_region_notify_iommu_one(n, &event);
> > >> >> +        map.iova = start;
> > >> >> +        map.size = size;
> > >> >> +        if (iova_tree_find(as->iova_tree, &map)) {
> > >> >> +            event.entry.iova = start;
> > >> >> +            event.entry.addr_mask = mask;
> > >> >> +            memory_region_notify_iommu_one(n, &event);
> > >> >> +        }
> > >> >
> > >> >This one looks fine to me, but I'm not sure how much benefit we'll
> > >> >get here either as this path should be rare afaiu.
> > >>
> > >> Yes, I only see such UNMAP call at cold bootup/shutdown, hot plug and
> > >unplug.
> > >>
> > >> In fact, the other purpose of this patch is to eliminate noisy error
> > >> log when we work with IOMMUFD. It looks the duplicate UNMAP call will
> > >> fail with IOMMUFD while always succeed with legacy container. This
> > >> behavior difference lead to below error log for IOMMUFD:
> 
> A dumb question, should IOMMUFD stick the same behaviour with legacy 
> container?

May need to hear from JasonG. 😊 Should IOMMU_IOAS_UNMAP return error or
success if the iova is not found?

Regards,
Yi Liu

> Thanks
> 
> > >>
> > >> IOMMU_IOAS_UNMAP failed: No such file or directory
> > >> vfio_container_dma_unmap(0x562012d6b6d0, 0x0, 0x80000000) = -2 (No
> > >> such file or directory) IOMMU_IOAS_UNMAP failed: No such file or
> > >> directory vfio_container_dma_unmap(0x562012d6b6d0, 0x80000000,
> > >> 0x40000000) = -2 (No such file or directory)
> > >
> > >I see.  Please also mention this in the commit log, that'll help reviewers
> > >understand the goal of the patch, thanks!
> > Will do.
> >
> > >
> > >>
> > >> >
> > >> >>
> > >> >>          start += size;
> > >> >>          remain -= size;
> > >> >> @@ -3826,13 +3830,6 @@ static void
> > >> >vtd_iommu_replay(IOMMUMemoryRegion *iommu_mr, IOMMUNotifier
> > >*n)
> > >> >>      uint8_t bus_n = pci_bus_num(vtd_as->bus);
> > >> >>      VTDContextEntry ce;
> > >> >>
> > >> >> -    /*
> > >> >> -     * 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);
> > >> >
> > >> >IIUC this is needed to satisfy current replay() semantics:
> > >> >
> > >> >    /**
> > >> >     * @replay:
> > >> >     *
> > >> >     * Called to handle memory_region_iommu_replay().
> > >> >     *
> > >> >     * The default implementation of memory_region_iommu_replay() is to
> > >> >     * call the IOMMU translate method for every page in the address 
> > >> > space
> > >> >     * with flag == IOMMU_NONE and then call the notifier if translate
> > >> >     * returns a valid mapping. If this method is implemented then it
> > >> >     * overrides the default behaviour, and must provide the full 
> > >> > semantics
> > >> >     * of memory_region_iommu_replay(), by calling @notifier for every
> > >> >     * translation present in the IOMMU.
> > >> Above semantics claims calling @notifier for every translation present
> > >> in the IOMMU But it doesn't claim if calling @notifier for non-present
> > >translation.
> > >> I checked other custom replay() callback, ex. virtio_iommu_replay(),
> > >> spapr_tce_replay() it looks only intel_iommu is special by calling 
> > >> unmap_all()
> > >before rebuild mapping.
> > >
> > >Yes, and I'll reply below for this..
> > >
> > >>
> > >> >
> > >> >The problem is vtd_page_walk() currently by default only notifies on
> > >> >page changes, so we'll notify all MAP only if we unmap all of them 
> > >> >first.
> > >> Hmm, I didn't get this point. Checked vtd_page_walk_one(), it will
> > >> rebuild the mapping except the DMAMap is exactly same which it will skip.
> > >See below:
> > >>
> > >>     /* Update local IOVA mapped ranges */
> > >>     if (event->type == IOMMU_NOTIFIER_MAP) {
> > >>         if (mapped) {
> > >>             /* If it's exactly the same translation, skip */
> > >>             if (!memcmp(mapped, &target, sizeof(target))) {
> > >>                 trace_vtd_page_walk_one_skip_map(entry->iova, entry-
> > >>addr_mask,
> > >>                                                  entry->translated_addr);
> > >>                 return 0;
> > >>             } else {
> > >>                 /*
> > >>                  * Translation changed.  Normally this should not
> > >>                  * happen, but it can happen when with buggy guest
> > >
> > >So I haven't touched the vIOMMU code for a few years, but IIRC if we
> > >replay() on an address space that has mapping already, then if without the
> > >unmap_all() at the start we'll just notify nothing, because "mapped" will 
> > >be
> > >true for all the existing mappings, and memcmp() should return 0 too if
> > >nothing changed?
> > Understood, you are right. VFIO migration dirty sync needs to be notified
> > even if mapping is unchanged.
> >
> > >
> > >I think (and agree) it could be a "bug" for vtd only, mostly not affecting
> > >anything at least before vfio migration.
> > >
> > >Do you agree, and perhaps want to fix it altogether?  If so I suppose 
> > >it'll also
> > >fix the issue below on vfio dirty sync.
> > Yes, I'll write an implementation.
> >
> > Thanks
> > Zhenzhong