xen/block.c: support unaligned writes

[Thomas Schwinge]

Hello!

On Mon, Nov 16, 2009 at 03:09:18AM +0000, Samuel Thibault wrote:
> The branch, master-xen has been updated

> commit c4cbe430a3051382714fb9b38d551c5bba65291e
> Author: Samuel Thibault <samuel.thibault@ens-lyon.org>
> Date:   Mon Nov 16 03:22:44 2009 +0100
> 
>     Support unaligned writes

Great!


One question about that one -- I'm not sure whether I asked you already
on IRC some months ago and you told me that this is not possible or
whether I didn't ask yet: as I understand the new code, in the unaligned
case, you copy the *whole* data to align its beginning to a page
boundary.  What about instead only copying that much from the source
until you hit the next page boundary, enqueue that part (with its size
being less than a page's size) for processing in the backend, and then
enqueue the rest of the data, which now begins on a page boundary,
without having to copy it any further?  Or aren't we allowed to split up
one device_write request into two?


Regards,
 Thomas

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