Re: [Qemu-devel] [PATCH for-1.2] qed: refuse unaligned zero writes with a backing file

[Stefan Hajnoczi]

On Tue, Aug 28, 2012 at 04:23:31PM +0200, Paolo Bonzini wrote:
> Il 28/08/2012 15:37, Stefan Hajnoczi ha scritto:
> >> > The "right fix" would not be much more complex though, something like 
> >> > this, right?
> >> > (untested).
> > Yes but it's more complicated.  To do a really good job we should
> > slice off the first/last clusters if they are unaligned, handle them
> > like regular allocating writes, and handle the middle of the request
> > as a zero write.
> > 
> > I decided to do the simplest implementation since this scenario only
> > occurs in test cases, not real guests.
> 
> Yes, I was curious because it reminded me of the patch I did to write
> zeroes when I was playing with discard to avoid the large bounce buffer
> in qed_aio_write_inplace.  That patch takes care of processing clusters
> one by one (though that means one L2 write for each and every cluster,
> not just the first and last).
> 
> It probably causes a performance hit, but anyway I attach it for
> completeness.

Thanks for sharing.  I think this patch could be used as the basis for
something that handles the first and last clusters one-at-a-time and
does the middle clusters in a single L2 update.

I'm not going to implement that right now because I prefer the simple
solution unless this code path becomes more used.

Stefan