Re: [Qemu-devel] [PATCH 0/5] Direct memory access for devices

[Blue Swirl]

On 1/18/09, Avi Kivity <address@hidden> wrote:
> One of the deficiencies of the current device layer is that it can only access
>  guest RAM via cpu_physical_memory_rw().  This means that the device emulation
>  code must copy the memory to or from a temporary buffer, even though the host
>  offers APIs which allow direct access to memory.  This reduces efficiency on
>  DMA capable devices, especially disks.
>
>  This patchset introduces a complement to the read/write API,
>  cpu_physical_memory_map() which allows device emulation code to map
>  guest memory directly.  The API bounces memory regions which cannot be
>  mapped (such as mmio regions) using an internal buffer.
>
>  As an example, IDE emulation is converted to use the new API.  This exposes
>  another deficiency: lack of scatter/gather support in the block layer.  To
>  work around this, a vectored block API is introduced, currently emulated
>  by bouncing.  Additional work is needed to convert all block format drivers
>  to use the vectored API.

Great!

I'll try to use the API for ESP and Lance on Sparc32, that may reveal
if there are any problems in the design.