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

[Paolo Bonzini]

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.

Paolo

wz.patch
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