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Re: [Qemu-devel] [PATCH 2/6] PCI DMA API

From: Paul Brook
Subject: Re: [Qemu-devel] [PATCH 2/6] PCI DMA API
Date: Sun, 30 Mar 2008 10:18:14 +0000
User-agent: KMail/1.9.9

On Saturday 29 March 2008, Anthony Liguori wrote:
> This patch introduces a PCI DMA API and some generic code to support other
> DMA APIs.  Two types are introduced: PhysIOVector and IOVector.  A DMA API
> maps a PhysIOVector, which is composed of target_phys_addr_t, into an
> IOVector, which is composed of void *.

Devices should not be using IOVector. They should either use the DMA copy 
routines to copy from a PhysIOVector into a local buffer, or they should pass 
a PhysIOVector to a block/network read/write routine. The DMA API should 
allow devices to be agnostic about how DMA is implemented. They should not be 
trying to manually implement zero copy.

> This enables zero-copy IO to be preformed without introducing assumptions
> of phys_ram_base.  This API is at the PCI device level to enable support of
> per-device IOMMU remapping.

By my reading it *requires* bridges be zero-copy.  For big-endian targets we 
need to ability to byteswap accesses.

Some description (in the form of source comments) of how it's meant to be used 
would also be helpful.


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