Re: [PATCH v2 0/9] Add dynamic iommu backed bounce buffers

David Stevens
2022-06-06T01:24:41Z

On Fri, Jun 3, 2022 at 11:53 PM Niklas Schnelle <schnelle@linux.ibm.com> wrote:
>
> On Fri, 2022-05-27 at 10:25 +0900, David Stevens wrote:
> > On Tue, May 24, 2022 at 9:27 PM Niklas Schnelle <schnelle@linux.ibm.com> wrote:
> > > On Fri, 2021-08-06 at 19:34 +0900, David Stevens wrote:
> > > > From: David Stevens <stevensd@chromium.org>
> > > >
> > > > This patch series adds support for per-domain dynamic pools of iommu
> > > > bounce buffers to the dma-iommu API. This allows iommu mappings to be
> > > > reused while still maintaining strict iommu protection.
> > > >
> > > > This bounce buffer support is used to add a new config option that, when
> > > > enabled, causes all non-direct streaming mappings below a configurable
> > > > size to go through the bounce buffers. This serves as an optimization on
> > > > systems where manipulating iommu mappings is very expensive. For
> > > > example, virtio-iommu operations in a guest on a linux host require a
> > > > vmexit, involvement the VMM, and a VFIO syscall. For relatively small
> > > > DMA operations, memcpy can be significantly faster.
> > > >
> > > > As a performance comparison, on a device with an i5-10210U, I ran fio
> > > > with a VFIO passthrough NVMe drive and virtio-iommu with '--direct=1
> > > > --rw=read --ioengine=libaio --iodepth=64' and block sizes 4k, 16k, 64k,
> > > > and 128k. Test throughput increased by 2.8x, 4.7x, 3.6x, and 3.6x. Time
> > > > spent in iommu_dma_unmap_(page|sg) per GB processed decreased by 97%,
> > > > 94%, 90%, and 87%. Time spent in iommu_dma_map_(page|sg) decreased
> > > > by >99%, as bounce buffers don't require syncing here in the read case.
> > > > Running with multiple jobs doesn't serve as a useful performance
> > > > comparison because virtio-iommu and vfio_iommu_type1 both have big
> > > > locks that significantly limit mulithreaded DMA performance.
> > > >
> > > > These pooled bounce buffers are also used for subgranule mappings with
> > > > untrusted devices, replacing the single use bounce buffers used
> > > > currently. The biggest difference here is that the new implementation
> > > > maps a whole sglist using a single bounce buffer. The new implementation
> > > > does not support using bounce buffers for only some segments of the
> > > > sglist, so it may require more copying. However, the current
> > > > implementation requires per-segment iommu map/unmap operations for all
> > > > untrusted sglist mappings (fully aligned sglists included). On a
> > > > i5-10210U laptop with the internal NVMe drive made to appear untrusted,
> > > > fio --direct=1 --rw=read --ioengine=libaio --iodepth=64 --bs=64k showed
> > > > a statistically significant decrease in CPU load from 2.28% -> 2.17%
> > > > with the new iommu bounce buffer optimization enabled.
> > > >
> > > > Each domain's buffer pool is split into multiple power-of-2 size
> > > > classes. Each class allocates a fixed number of buffer slot metadata. A
> > > > large iova range is allocated, and each slot is assigned an iova from
> > > > the range. This allows the iova to be easily mapped back to the slot,
> > > > and allows the critical section of most pool operations to be constant
> > > > time. The one exception is finding a cached buffer to reuse. These are
> > > > only separated according to R/W permissions - the use of other
> > > > permissions such as IOMMU_PRIV may require a linear search through the
> > > > cache. However, these other permissions are rare and likely exhibit high
> > > > locality, so the should not be a bottleneck in practice.
> > > >
> > > > Since untrusted devices may require bounce buffers, each domain has a
> > > > fallback rbtree to manage single use buffers. This may be necessary if a
> > > > very large number of DMA operations are simultaneously in-flight, or for
> > > > very large individual DMA operations.
> > > >
> > > > This patch set does not use swiotlb. There are two primary ways in which
> > > > swiotlb isn't compatible with per-domain buffer pools. First, swiotlb
> > > > allocates buffers to be compatible with a single device, whereas
> > > > per-domain buffer pools don't handle that during buffer allocation as a
> > > > single buffer may end up being used by multiple devices. Second, swiotlb
> > > > allocation establishes the original to bounce buffer mapping, which
> > > > again doesn't work if buffers can be reused. Effectively the only code
> > > > that can be shared between the two use cases is allocating slots from
> > > > the swiotlb's memory. However, given that we're going to be allocating
> > > > memory for use with an iommu, allocating memory from a block of memory
> > > > explicitly set aside to deal with a lack of iommu seems kind of
> > > > contradictory. At best there might be a small performance improvement if
> > > > wiotlb allocation is faster than regular page allocation, but buffer
> > > > allocation isn't on the hot path anyway.
> > > >
> > > > Not using the swiotlb has the benefit that memory doesn't have to be
> > > > preallocated. Instead, bounce buffers consume memory only for in-flight
> > > > dma transactions (ignoring temporarily cached buffers), which is the
> > > > smallest amount possible. This makes it easier to use bounce buffers as
> > > > an optimization on systems with large numbers of devices or in
> > > > situations where devices are unknown, since it is not necessary to try
> > > > to tune how much memory needs to be set aside to achieve good
> > > > performance without costing too much memory.
> > > >
> > > > Finally, this series adds a new DMA_ATTR_PERSISTENT_STREAMING flag. This
> > > > is meant to address devices which create long lived streaming mappings
> > > > but manage CPU cache coherency without using the dma_sync_* APIs.
> > > > Currently, these devices don't function properly with swiotlb=force. The
> > > > new flag is used to bypass bounce buffers so such devices will function
> > > > when the new bounce buffer optimization is enabled. The flag is added to
> > > > the i915 driver, which creates such mappings. It can also be added to
> > > > various dma-buf implementations as an optimization, although that is not
> > > > done here.
> > > >
> > > > v1 -> v2:
> > > > - Replace existing untrusted bounce buffers with new bounce
> > > > buffer pools. This includes significant rework to account for
> > > > untrusted bounce buffers being required instead of an
> > > > optimization.
> > > > - Add flag for persistent streaming mappings.
> > > >
> > >
> > > Hi David,
> > >
> > > I'm currently looking into converting s390 from our custom IOMMU based
> > > DMA API implementation to using dma-iommu.c. We're always using an
> > > IOMMU for PCI devices even when doing pass-through to guests (under
> > > both the KVM and z/VM hypervisors). In this case I/O TLB flushes, which
> > > we use to do the shadowing of the guest I/O translations, are
> > > relatively expensive I'm thus very interested in your work. I've tried
> > > rebasing it on v5.18 and got it to compile but didn't get DMA to work
> > > though it seems to partially work as I don't get probe failures unlike
> > > with a completely broken DMA API. Since I might have very well screwed
> > > up the rebase and my DMA API conversion is experimental too I was
> > > wondering if you're still working on this and might have a current
> > > version I could experiment with?
> >
> > Unfortunately I don't have anything more recent to share. I've come
> > across some performance issues caused by pathological usage patterns
> > in internal usage, but I haven't seen any correctness issues. I'm
> > hoping that I'll be able to address the performance issues and send a
> > rebased series within the next month or so.
> >
> > It's definitely possible that this series has some bugs. I've tested
> > it on a range of chromebooks and their various hardware and drivers,
> > but that's still all relatively normal x86_64/arm64. If your hardware
> > is more particular about its DMA, this series might be missing
> > something.
> >
> > -David
>
>
> Hi David,
>
> Thanks for the answer. The only unusual thing about our DMA is that we
> only do 64 bit DMA and IOVAs are always >2^32. I don't think I
> triggered a bug in your code though, rather I think I made some mistake
> in the rebase onto 5.18 as some of the APIs changed a bit. I'm out next
> week but may try it again and possibly just test on x86_64 if it
> doesn't work on s390. If you have anything new I'd be interested to
> hear of course. Also could you say anything more about the pathological
> usage patterns?

The problem with this implementation is that if you fall outside the
max number/size for the bounce buffer pools, then DMA performance can
fall off of a cliff. Although those max parameters are tunable, it's
always possible to construct a workload that falls outside of the
expected bounds. I think this can be addressed by adding a fallback
pooling structure. It won't necessarily be as performant as the
primary buffer pools, but I think it should work reasonably well for
usage patterns that fall outside the primary buffer pools.

-David