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Subject: Re: [virtio-dev] Re: [PATCH v5 5/7] iommu: Add virtio-iommu driver

Hi Robin

On 12/13/18 1:37 PM, Robin Murphy wrote:
> On 2018-12-12 3:27 pm, Auger Eric wrote:
>> Hi,
>> On 12/12/18 3:56 PM, Michael S. Tsirkin wrote:
>>> On Fri, Dec 07, 2018 at 06:52:31PM +0000, Jean-Philippe Brucker wrote:
>>>> Sorry for the delay, I wanted to do a little more performance analysis
>>>> before continuing.
>>>> On 27/11/2018 18:10, Michael S. Tsirkin wrote:
>>>>> On Tue, Nov 27, 2018 at 05:55:20PM +0000, Jean-Philippe Brucker wrote:
>>>>>>>> +ÂÂÂ if (!virtio_has_feature(vdev, VIRTIO_F_VERSION_1) ||
>>>>>>>> +ÂÂÂÂÂÂÂ !virtio_has_feature(vdev, VIRTIO_IOMMU_F_MAP_UNMAP))
>>>>>>> Why bother with a feature bit for this then btw?
>>>>>> We'll need a new feature bit for sharing page tables with the
>>>>>> hardware,
>>>>>> because they require different requests (attach_table/invalidate
>>>>>> instead
>>>>>> of map/unmap.) A future device supporting page table sharing won't
>>>>>> necessarily need to support map/unmap.
>>>>> I don't see virtio iommu being extended to support ARM specific
>>>>> requests. This just won't scale, too many different
>>>>> descriptor formats out there.
>>>> They aren't really ARM specific requests. The two new requests are
>>>> ATTACH_TABLE and INVALIDATE, which would be used by x86 IOMMUs as well.
>>>> Sharing CPU address space with the HW IOMMU (SVM) has been in the scope
>>>> of virtio-iommu since the first RFC, and I've been working with that
>>>> extension in mind since the beginning. As an example you can have a
>>>> look
>>>> at my current draft for this [1], which is inspired from the VFIO work
>>>> we've been doing with Intel.
>>>> The negotiation phase inevitably requires vendor-specific fields in the
>>>> descriptors - host tells which formats are supported, guest chooses a
>>>> format and attaches page tables. But invalidation and fault reporting
>>>> descriptors are fairly generic.
>>> We need to tread carefully here. People expect it that if user does
>>> lspci and sees a virtio device then it's reasonably portable.
>>>>> If you want to go that way down the road, you should avoid
>>>>> virtio iommu, instead emulate and share code with the ARM SMMU
>>>>> (probably
>>>>> with a different vendor id so you can implement the
>>>>> report on map for devices without PRI).
>>>> vSMMU has to stay in userspace though. The main reason we're proposing
>>>> virtio-iommu is that emulating every possible vIOMMU model in the
>>>> kernel
>>>> would be unmaintainable. With virtio-iommu we can process the fast path
>>>> in the host kernel, through vhost-iommu, and do the heavy lifting in
>>>> userspace.
>>> Interesting.
>>>> As said above, I'm trying to keep the fast path for
>>>> virtio-iommu generic.
>>>> More notes on what I consider to be the fast path, and comparison with
>>>> vSMMU:
>>>> (1) The primary use-case we have in mind for vIOMMU is something like
>>>> DPDK in the guest, assigning a hardware device to guest userspace. DPDK
>>>> maps a large amount of memory statically, to be used by a pass-through
>>>> device. For this case I don't think we care about vIOMMU performance.
>>>> Setup and teardown need to be reasonably fast, sure, but the MAP/UNMAP
>>>> requests don't have to be optimal.
>>>> (2) If the assigned device is owned by the guest kernel, then mappings
>>>> are dynamic and require dma_map/unmap() to be fast, but there generally
>>>> is no need for a vIOMMU, since device and drivers are trusted by the
>>>> guest kernel. Even when the user does enable a vIOMMU for this case
>>>> (allowing to over-commit guest memory, which needs to be pinned
>>>> otherwise),
>>> BTW that's in theory in practice it doesn't really work.
>>>> we generally play tricks like lazy TLBI (non-strict mode) to
>>>> make it faster.
>>> Simple lazy TLB for guest/userspace drivers would be a big no no.
>>> You need something smarter.
>>>> Here device and drivers are trusted, therefore the
>>>> vulnerability window of lazy mode isn't a concern.
>>>> If the reason to enable the vIOMMU is over-comitting guest memory
>>>> however, you can't use nested translation because it requires pinning
>>>> the second-level tables. For this case performance matters a bit,
>>>> because your invalidate-on-map needs to be fast, even if you enable
>>>> lazy
>>>> mode and only receive inval-on-unmap every 10ms. It won't ever be as
>>>> fast as nested translation, though. For this case I think vSMMU+Caching
>>>> Mode and userspace virtio-iommu with MAP/UNMAP would perform similarly
>>>> (given page-sized payloads), because the pagetable walk doesn't add a
>>>> lot of overhead compared to the context switch. But given the results
>>>> below, vhost-iommu would be faster than vSMMU+CM.
>>>> (3) Then there is SVM. For SVM, any destructive change to the process
>>>> address space requires a synchronous invalidation command to the
>>>> hardware (at least when using PCI ATS). Given that SVM is based on page
>>>> faults, fault reporting from host to guest also needs to be fast, as
>>>> well as fault response from guest to host.
>>>> I think this is where performance matters the most. To get a feel of
>>>> the
>>>> advantage we get with virtio-iommu, I compared the vSMMU page-table
>>>> sharing implementation [2] and vhost-iommu + VFIO with page table
>>>> sharing (based on Tomasz Nowicki's vhost-iommu prototype). That's on a
>>>> ThunderX2 with a 10Gb NIC assigned to the guest kernel, which
>>>> corresponds to case (2) above, with nesting page tables and without the
>>>> lazy mode. The host's only job is forwarding invalidation to the HW
>>>> SMMU.
>>>> vhost-iommu performed on average 1.8x and 5.5x better than vSMMU on
>>>> netperf TCP_STREAM and TCP_MAERTS respectively (~200 samples). I think
>>>> this can be further optimized (that was still polling under the vq
>>>> lock), and unlike vSMMU, virtio-iommu offers the possibility of
>>>> multi-queue for improved scalability. In addition, the guest will need
>>>> to send both TLB and ATC invalidations with vSMMU, but virtio-iommu
>>>> allows to multiplex those, and to invalidate ranges. Similarly for
>>>> fault
>>>> injection, having the ability to report page faults to the guest from
>>>> the host kernel should be significantly faster than having to go to
>>>> userspace and back to the kernel.
>>> Fascinating. Any data about host CPU utilization?
>>> Eric what do you think?
>>> Is it true that SMMUv3 is fundmentally slow at the architecture level
>>> and so a PV interface will always scale better until
>>> a new hardware interface is designed?
>> As far as I understand the figures above correspond to vhost-iommu
>> against vsmmuv3. In the 2 cases the guest owns stage1 tables so the
>> difference comes from the IOTLB invalidation handling. With vhost we
>> avoid a kernel <-> userspace round trip which may mostly explain the
>> difference.
>> About SMMUv3 issues I already reported one big limitation with respect
>> to hugepage invalidation. See [RFC v2 4/4] iommu/arm-smmu-v3: add
>> CMD_TLBI_NH_VA_AM command for iova range invalidation
>> (https://lkml.org/lkml/2017/8/11/428).
>> At smmuv3 guest driver level, arm_smmu_tlb_inv_range_nosync(), when
>> called with a hugepage size, invalidates each 4K/64K page of the region
>> and not the whole region at once. Each of them are trapped by the SMMUv3
>> device which forwards them to the host. This stalls the guest. This
>> issue can be observed in DPDK case - not the use case benchmarked
>> above - .
>> I raised this point again in recent discussions and it is unclear
>> whether this is an SMMUv3 driver limitation or an architecture
>> limitation. Seems a single invalidation within the block mapping should
>> invalidate the whole mapping at HW level. In the past I hacked a
>> workaround by defining an implementation defined invalidation command.
>> Robin/Will, could you please explain the rationale behind the
>> arm_smmu_tlb_inv_range_nosync() implementation.
> Fundamentally, TLBI commands only take an address, so invalidations have
> to match the actual leaf PTEs being removed. If iommu_unmap() sees that
> 2MB is a valid block size, it may send a single "unmap this 2MB" request
> to the driver, but nobody knows how that region is actually mapped until
> the pagetable code walks the tables. If it does find a 2MB block PTE,
> then it can simply clear it and generate a single invalidation command -
> if a TLB entry exists for that block mapping then any address within the
> block will match it. If however that 2MB region was actually covered by
> a subtable of 4KB pages, then separate TLB entries may exist for any or
> all of those pages, and a single address can at best only match one of
> them. Thus after the table PTE is cleared, a separate command has to be
> generated for each page to ensure that all possible TLB entries
> associated with that table are invalidated.
> So if you're seeing page-granularity invalidation, it means that the
> thing you're unmapping wasn't actually mapped as that size of hugepage
> in the first place (or something pathological has caused it to be split
> in the meantime - I recall we once had an amusing bug which caused VFIO
> to do that on teardown). There is one suboptimal case if we're taking
> out potentially multiple levels of table at once (e.g. a 1GB region),
> where we don't bother recursing down the removed table to see whether
> anything was mapped with blocks at the intermediate level, and just
> invalidate the whole lot at page granularity to cover the worst-case
> scenario. I think we assumed that sort of unmap would be unlikely enough
> that it wasn't worth optimising for at the time, but there's certainly
> scope to improve it if unmapping 1GB worth of 2MB blocks turns out to be
> a common thing.

thank you for your reply. This last situation looks like the one I
encountered. I will test with DPDK again and let you know.


> FWIW SMMUv3.2 has introduced actual range-invalidation commands -
> reworking all the TLBI logic in the driver to make worthwhile use of
> those is on the to-do list, but until real 3.2 hardware starts coming to
> light I've not been prioritising it.
> Robin.
>> Thanks
>> Eric
>>>> (4) Virtio and vhost endpoints weren't really a priority for the base
>>>> virtio-iommu device, we were looking mainly at device pass-through. I
>>>> have optimizations in mind for this, although a lot of them are
>>>> based on
>>>> page tables, not MAP/UNMAP requests. But just getting the vIOMMU closer
>>>> to vhost devices, avoiding the trip to userspace through vhost-tlb,
>>>> should already improve things.
>>>> The important difference when DMA is done by software is that you don't
>>>> need to mirror all mappings into the HW IOMMU - you don't need
>>>> inval-on-map. The endpoint can ask the vIOMMU for mappings when it
>>>> needs
>>>> them, like vhost-iotlb does for example. So the MAP/UNMAP interface of
>>>> virtio-iommu performs poorly for emulated/PV endpoints compared to an
>>>> emulated IOMMU, since it requires three context switches for DMA
>>>> (MAP/DMA/UNMAP) between host and guest, rather than two (DMA/INVAL).
>>>> There is a feature I call "posted MAP", that avoids the kick on MAP and
>>>> instead lets the device fetch the MAP request on TLB miss, but I
>>>> haven't
>>>> spent enough time experimenting with this.
>>>>> Others on the TC might feel differently.
>>>>> If someone's looking into adding virtio iommu support in hardware,
>>>>> that's a different matter. Which is it?
>>>> I'm not aware of anything like that, and suspect that no one would
>>>> consider it until virtio-iommu is more widely adopted.
>>>> Thanks,
>>>> Jean
>>>> [1] Diff between current spec and page table sharing draft
>>>> ÂÂÂÂ (Very rough, missing page fault support and I'd like to rework the
>>>> ÂÂÂÂÂ PASID model a bit, but table descriptors p.24-26 for both Arm
>>>> ÂÂÂÂÂ SMMUv2 and SMMUv3.)
>>>> http://jpbrucker.net/virtio-iommu/spec-table/diffs/virtio-iommu-pdf-diff-v0.9-v0.10.dev03.pdf
>>>> [2] [RFC v2 00/28] vSMMUv3/pSMMUv3 2 stage VFIO integration
>>>> ÂÂÂÂ https://www.mail-archive.com/qemu-devel@nongnu.org/msg562369.html

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