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From: Robin Murphy <robin.murphy@arm.com>
To: Joerg Roedel <joro@8bytes.org>, David Stevens <stevensd@chromium.org>
Cc: Will Deacon <will@kernel.org>, Christoph Hellwig <hch@lst.de>,
	Sergey Senozhatsky <senozhatsky@chromium.org>,
	iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org,
	David Stevens <stevensd@google.com>
Subject: Re: [PATCH 0/4] Add dynamic iommu backed bounce buffers
Date: Thu, 8 Jul 2021 18:14:26 +0100	[thread overview]
Message-ID: <a20a8f33-a047-cd89-0a2b-85e4f19c8ffa@arm.com> (raw)
In-Reply-To: <YObFbqEk1nGVuHLF@8bytes.org>

On 2021-07-08 10:29, Joerg Roedel wrote:
> Adding Robin too.
> 
> On Wed, Jul 07, 2021 at 04:55:01PM +0900, David Stevens wrote:
>> Add 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. Allocating buffers dynamically
>> instead of using swiotlb carveouts makes per-domain pools more amenable
>> on systems with large numbers of devices or where devices are unknown.

But isn't that just as true for the currently-supported case? All you 
need is a large enough Thunderbolt enclosure and you could suddenly plug 
in a dozen untrusted GPUs all wanting to map hundreds of megabytes of 
memory. If there's a real concern worth addressing, surely it's worth 
addressing properly for everyone.

>> When enabled, all non-direct streaming mappings below a configurable
>> size will go through bounce buffers. Note that this means drivers which
>> don't properly use the DMA API (e.g. i915) cannot use an iommu when this
>> feature is enabled. However, all drivers which work with swiotlb=force
>> should work.
>>
>> Bounce buffers serve as an optimization in situations where interactions
>> with the iommu are very costly. 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.

Yup, back when the bounce-buffering stuff first came up I know 
networking folks were interested in terms of latency for small packets - 
virtualised IOMMUs are indeed another interesting case I hadn't thought 
of. It's definitely been on the radar as another use-case we'd like to 
accommodate with the bounce-buffering scheme. However, that's the thing: 
bouncing is bouncing and however you look at it it still overlaps so 
much with the untrusted case - there's no reason that couldn't use 
pre-mapped bounce buffers too, for instance - that the only necessary 
difference is really the policy decision of when to bounce. iommu-dma 
has already grown complicated enough, and having *three* different ways 
of doing things internally just seems bonkers and untenable. Pre-map the 
bounce buffers? Absolutely. Dynamically grow them on demand? Yes please! 
Do it all as a special thing in its own NIH module and leave the 
existing mess to rot? Sorry, but no.

Thanks,
Robin.

>> As a performance comparison, on a device with an i5-10210U, I ran fio
>> with a VFIO passthrough NVMe drive 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.
>>
>> This patch set is based on v5.13-rc7 plus the patches at [1].
>>
>> David Stevens (4):
>>    dma-iommu: add kalloc gfp flag to alloc helper
>>    dma-iommu: replace device arguments
>>    dma-iommu: expose a few helper functions to module
>>    dma-iommu: Add iommu bounce buffers to dma-iommu api
>>
>>   drivers/iommu/Kconfig          |  10 +
>>   drivers/iommu/Makefile         |   1 +
>>   drivers/iommu/dma-iommu.c      | 119 ++++--
>>   drivers/iommu/io-buffer-pool.c | 656 +++++++++++++++++++++++++++++++++
>>   drivers/iommu/io-buffer-pool.h |  91 +++++
>>   include/linux/dma-iommu.h      |  12 +
>>   6 files changed, 861 insertions(+), 28 deletions(-)
>>   create mode 100644 drivers/iommu/io-buffer-pool.c
>>   create mode 100644 drivers/iommu/io-buffer-pool.h
>>
>> -- 
>> 2.32.0.93.g670b81a890-goog

WARNING: multiple messages have this Message-ID
From: Robin Murphy <robin.murphy@arm.com>
To: Joerg Roedel <joro@8bytes.org>, David Stevens <stevensd@chromium.org>
Cc: David Stevens <stevensd@google.com>,
	linux-kernel@vger.kernel.org,
	Sergey Senozhatsky <senozhatsky@chromium.org>,
	iommu@lists.linux-foundation.org, Will Deacon <will@kernel.org>,
	Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH 0/4] Add dynamic iommu backed bounce buffers
Date: Thu, 8 Jul 2021 18:14:26 +0100	[thread overview]
Message-ID: <a20a8f33-a047-cd89-0a2b-85e4f19c8ffa@arm.com> (raw)
In-Reply-To: <YObFbqEk1nGVuHLF@8bytes.org>

On 2021-07-08 10:29, Joerg Roedel wrote:
> Adding Robin too.
> 
> On Wed, Jul 07, 2021 at 04:55:01PM +0900, David Stevens wrote:
>> Add 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. Allocating buffers dynamically
>> instead of using swiotlb carveouts makes per-domain pools more amenable
>> on systems with large numbers of devices or where devices are unknown.

But isn't that just as true for the currently-supported case? All you 
need is a large enough Thunderbolt enclosure and you could suddenly plug 
in a dozen untrusted GPUs all wanting to map hundreds of megabytes of 
memory. If there's a real concern worth addressing, surely it's worth 
addressing properly for everyone.

>> When enabled, all non-direct streaming mappings below a configurable
>> size will go through bounce buffers. Note that this means drivers which
>> don't properly use the DMA API (e.g. i915) cannot use an iommu when this
>> feature is enabled. However, all drivers which work with swiotlb=force
>> should work.
>>
>> Bounce buffers serve as an optimization in situations where interactions
>> with the iommu are very costly. 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.

Yup, back when the bounce-buffering stuff first came up I know 
networking folks were interested in terms of latency for small packets - 
virtualised IOMMUs are indeed another interesting case I hadn't thought 
of. It's definitely been on the radar as another use-case we'd like to 
accommodate with the bounce-buffering scheme. However, that's the thing: 
bouncing is bouncing and however you look at it it still overlaps so 
much with the untrusted case - there's no reason that couldn't use 
pre-mapped bounce buffers too, for instance - that the only necessary 
difference is really the policy decision of when to bounce. iommu-dma 
has already grown complicated enough, and having *three* different ways 
of doing things internally just seems bonkers and untenable. Pre-map the 
bounce buffers? Absolutely. Dynamically grow them on demand? Yes please! 
Do it all as a special thing in its own NIH module and leave the 
existing mess to rot? Sorry, but no.

Thanks,
Robin.

>> As a performance comparison, on a device with an i5-10210U, I ran fio
>> with a VFIO passthrough NVMe drive 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.
>>
>> This patch set is based on v5.13-rc7 plus the patches at [1].
>>
>> David Stevens (4):
>>    dma-iommu: add kalloc gfp flag to alloc helper
>>    dma-iommu: replace device arguments
>>    dma-iommu: expose a few helper functions to module
>>    dma-iommu: Add iommu bounce buffers to dma-iommu api
>>
>>   drivers/iommu/Kconfig          |  10 +
>>   drivers/iommu/Makefile         |   1 +
>>   drivers/iommu/dma-iommu.c      | 119 ++++--
>>   drivers/iommu/io-buffer-pool.c | 656 +++++++++++++++++++++++++++++++++
>>   drivers/iommu/io-buffer-pool.h |  91 +++++
>>   include/linux/dma-iommu.h      |  12 +
>>   6 files changed, 861 insertions(+), 28 deletions(-)
>>   create mode 100644 drivers/iommu/io-buffer-pool.c
>>   create mode 100644 drivers/iommu/io-buffer-pool.h
>>
>> -- 
>> 2.32.0.93.g670b81a890-goog
_______________________________________________
iommu mailing list
iommu@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/iommu

  reply	other threads:[~2021-07-08 17:14 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-07  7:55 David Stevens
2021-07-07  7:55 ` David Stevens
2021-07-07  7:55 ` [PATCH 1/4] dma-iommu: add kalloc gfp flag to alloc helper David Stevens
2021-07-07  7:55   ` David Stevens
2021-07-08 17:22   ` Robin Murphy
2021-07-08 17:22     ` Robin Murphy
2021-07-07  7:55 ` [PATCH 2/4] dma-iommu: replace device arguments David Stevens
2021-07-07  7:55   ` David Stevens
2021-07-07  7:55 ` [PATCH 3/4] dma-iommu: expose a few helper functions to module David Stevens
2021-07-07  7:55   ` David Stevens
2021-07-07  7:55 ` [PATCH 4/4] dma-iommu: Add iommu bounce buffers to dma-iommu api David Stevens
2021-07-07  7:55   ` David Stevens
2021-07-08  9:29 ` [PATCH 0/4] Add dynamic iommu backed bounce buffers Joerg Roedel
2021-07-08  9:29   ` Joerg Roedel
2021-07-08 17:14   ` Robin Murphy [this message]
2021-07-08 17:14     ` Robin Murphy
2021-07-09  7:25     ` David Stevens
2021-07-09  7:25       ` David Stevens
2021-07-08 13:38 ` Lu Baolu
2021-07-08 13:38   ` Lu Baolu
2021-07-09  6:04   ` David Stevens
2021-07-09  6:04     ` David Stevens

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