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Date: Tue, 07 Oct 2008 16:21:12 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] [5323] Implement an fd pool to get real AIO with
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To: qemu-devel@nongnu.org

Gerd Hoffmann wrote:
> Anthony Liguori wrote:
>   
>>> Are there plans to support vectored block requests with the thread pool
>>> implementation?
>>>   
>>>       
>> Yes, that's the primary reason for doing a new thread pool
>> implementation.
>>     
>
> Cool.
>
>   
>> Of course, we need a zero-copy DMA API before such a
>> thing would make sense.
>>     
>
> Hmm, quick check of the IDE code indeed shows a copy happening there.
> Why is it needed?  My xen disk backend doesn't copy data.  Does that
> mean might have done something wrong?  Does the virtio backend copy data
> too?
>   

It does now, because the cost of splitting up the AIO request for each 
element of the scatter/gather list was considerably higher than the cost 
of copying the data to a linear buffer.

Look back in qemu-devel for a thread about a DMA API.

You can only avoid doing a copy if you do something like phys_ram_base + 
PA.  From an architectural perspective, this is not ideal since it 
doesn't allow for things like IOMMU emulation.  What we need, is a 
zero-copy API at the PCI level.

Regards,

Anthony Liguori

> Disclaimer: I don't know much about ide dma ...
>
> cheers,
>   Gerd
>
>
>