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Date: Wed, 08 Apr 2009 13:53:52 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] buffer alignment for block backends
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To: qemu-devel@nongnu.org

Stefano Stabellini wrote:
> Hi all,
> I am having an issue with the alignment of the buffers given to the
> block backends. In particular, at the moment we are allocating buffers
> 512 bytes aligned while the backend I am working on (block-vbd) needs
> page alignment.
>   

There is no requirement for this.  The block block-raw-posix backend 
deals with bouncing the buffers if it has to.  A few places do 
qemu_memalign() allocs as an optimization to avoid the bouncing but it's 
not strictly required.

> If you do not want to do that, another possible solution is to create a
> new function called "qemu_blockalign" that would be implemented as
> qemu_memalign(512, size);

This is fine, but this is purely an optimization, it cannot be relied 
upon in the general case.

>  so we don't have to write 512 bytes everywhere
> but only in one place, thus making life easier to people like me that
> have to change the value for a special case.
>
> Thanks in advance for your sympathy :)
>   

Why does your backend requirement page alignment and who's notion of page?

Regards,

Anthony Liguori

> Stefano Stabellini
>
>
>