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Date: Thu, 09 Apr 2009 10:56:27 -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

Samuel Thibault wrote:
> Anthony Liguori, le Thu 09 Apr 2009 08:11:27 -0500, a écrit :
>   
>>> - ide.c:ide_init2
>>>  
>>>       
>> This buffer is only used when not doing DMA.  When doing DMA, we are 
>> able to do zero-copy IO so the alignment of the request depends on how 
>> the guest aligned the request.  I suspect you'll find a lot of guests 
>> that, in practice, do not align requests at 4k boundaries.
>>     
>
> Eeeeerrr, why shoudn't they?  Guests usually work on pages, which are
> already aligned on 4k boundaries.
>   

Like if you do a 512-byte O_DIRECT write (in userspace) to a 512-byte, 
but not 4096-byte aligned buffer in Linux.  This may not happen all the 
time, but if you don't handle this in your backend, you'll get a lot of 
lovely bug reports :-)

Regards,

Anthony Liguori