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From: Lu Baolu <baolu.lu@linux.intel.com>
To: David Stevens <stevensd@chromium.org>,
	Joerg Roedel <joro@8bytes.org>, Will Deacon <will@kernel.org>
Cc: baolu.lu@linux.intel.com,
	Sergey Senozhatsky <senozhatsky@chromium.org>,
	iommu@lists.linux-foundation.org,
	David Stevens <stevensd@google.com>,
	Christoph Hellwig <hch@lst.de>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/4] Add dynamic iommu backed bounce buffers
Date: Thu, 8 Jul 2021 21:38:34 +0800	[thread overview]
Message-ID: <f6541f4e-648e-d9a0-eda7-b2a117978ebb@linux.intel.com> (raw)
In-Reply-To: <20210707075505.2896824-1-stevensd@google.com>

Hi David,

I like this idea. Thanks for proposing this.

On 2021/7/7 15:55, 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.

Have you ever considered leveraging the per-device swiotlb memory pool
added by below series?

https://lore.kernel.org/linux-iommu/20210625123004.GA3170@willie-the-truck/

> 
> 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.

If so, why not making it more scalable by adding a callback into vendor
iommu drivers? The vendor iommu drivers have enough information to tell
whether the bounce buffer is feasible for a specific domain.

> 
> Bounce buffers serve as an optimization in situations where interactions
> with the iommu are very costly. For example, virtio-iommu operations in

The simulated IOMMU does the same thing.

It's also an optimization for bare metal in cases where the strict mode
of cache invalidation is used. CPU moving data is faster than IOMMU
cache invalidation if the buffer is small.

Best regards,
baolu

WARNING: multiple messages have this Message-ID
From: Lu Baolu <baolu.lu@linux.intel.com>
To: David Stevens <stevensd@chromium.org>,
	Joerg Roedel <joro@8bytes.org>, Will Deacon <will@kernel.org>
Cc: David Stevens <stevensd@google.com>,
	linux-kernel@vger.kernel.org,
	Sergey Senozhatsky <senozhatsky@chromium.org>,
	iommu@lists.linux-foundation.org, Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH 0/4] Add dynamic iommu backed bounce buffers
Date: Thu, 8 Jul 2021 21:38:34 +0800	[thread overview]
Message-ID: <f6541f4e-648e-d9a0-eda7-b2a117978ebb@linux.intel.com> (raw)
In-Reply-To: <20210707075505.2896824-1-stevensd@google.com>

Hi David,

I like this idea. Thanks for proposing this.

On 2021/7/7 15:55, 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.

Have you ever considered leveraging the per-device swiotlb memory pool
added by below series?

https://lore.kernel.org/linux-iommu/20210625123004.GA3170@willie-the-truck/

> 
> 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.

If so, why not making it more scalable by adding a callback into vendor
iommu drivers? The vendor iommu drivers have enough information to tell
whether the bounce buffer is feasible for a specific domain.

> 
> Bounce buffers serve as an optimization in situations where interactions
> with the iommu are very costly. For example, virtio-iommu operations in

The simulated IOMMU does the same thing.

It's also an optimization for bare metal in cases where the strict mode
of cache invalidation is used. CPU moving data is faster than IOMMU
cache invalidation if the buffer is small.

Best regards,
baolu
_______________________________________________
iommu mailing list
iommu@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/iommu

  parent reply	other threads:[~2021-07-08 13:38 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
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 [this message]
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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