Re: [Qemu-devel] buffer alignment for block backends

[Stefano Stabellini <stefano.stabellini@eu.citrix.com>]

Anthony Liguori wrote:

>> my backend (block-vbd) needs page aligned buffers because blkfront needs
>> page aligned buffers. I could allocate a new page aligned buffer every
>> time and the memcpy but it is just a waste.
>>   
> 
> You'll need to check the alignment of the request and bounce it if 
> necessary.  In the case that you have zero-copy requests coming from the 
> guest that aren't page aligned, someone has to bounce the thing to make 
> it page aligned.


Yes, but that is exactly what I was trying to avoid.
I can provide a bounce buffer fallback anyway (I actually should) but I
would really like to avoid using it.

 
> So I presume you're implementing blkfront in userspace?  Does minios 
> provide a userspace interface for grant tables that looks similar to the 
> interfaces on Linux?  Were you planning on submitting this for inclusion 
> in upstream QEMU?

The userspace\kernelspace distinction does not make much sense in
minios, but yes it provides an interface to communicate with blkback in
dom0. This interface is used by block-vbd in qemu instead of
block-raw-posix when run in minios.

> I think it's a reasonable thing to include provided it's relatively 
> self-contained.
> 

good!