linux-kernel - Re: [PATCH v2 3/9] dma-iommu: bounce buffers for untrusted devices

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Date:   Tue, 10 Aug 2021 10:41:08 +0900
From:   David Stevens <stevensd@...omium.org>
To:     "Mi, Dapeng1" <dapeng1.mi@...el.com>
Cc:     Robin Murphy <robin.murphy@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        Will Deacon <will@...nel.org>, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v2 3/9] dma-iommu: bounce buffers for untrusted devices

On Tue, Aug 10, 2021 at 10:19 AM Mi, Dapeng1 <dapeng1.mi@...el.com> wrote:
>
> Hi David,
>
> I like this patch set and this is crucial for reducing the significant vIOMMU performance. It looks you totally rewrite the IOMMU mapping/unmapping part and use the dynamically allocated memory from buddy system as bounce buffer instead of using the legacy SWIOTLB bounce buffer. As I know, some legacy devices' DMA could not access the memory larger than 32-bit memory space and the dynamically allocated memory address could exceed the 32-bit memory space. Is it a problem?

My understanding is that when devices with that sort of limitation sit
behind an IOMMU, the IOVA is what matters, not the physical address.
The bounce bounce buffers use the same limits for IOVA allocation as
the regular dma-iommu path, so compatible IOVAs will be allocated for
the bounce buffers.

-David

> Thx,
> Dapeng Mi