Re: [Xen-devel] [Qemu-devel] [PATCH] xen_disk: implement BLKIF_OP_FLUSH_DISKCACHE, remove BLKIF_OP_WRITE_BARRIER

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Wed, 25 Apr 2012, Christoph Hellwig wrote:
> On Wed, Apr 25, 2012 at 10:02:45AM +0100, Ian Campbell wrote:
> > The blkif spec was recently much improved, you can find it at
> > http://xenbits.xen.org/docs/unstable/hypercall/include,public,io,blkif.h.html
> > 
> > TBH I'm not sure it actually answers your questions wrt
> > BLKIF_OP_FLUSH_DISKCACHE, if not please let us know and we can see about
> > improving it.
> 
> That description in there is overly simple, and does not match any of the
> implementations known to me on either end.

That is true, in fact I couldn't figure out what I had to implement just
reading the comment. So I went through the blkback code and tried to
understand what I had to do, but I got it wrong.

Reading the code again it seems to me that BLKIF_OP_FLUSH_DISKCACHE
is supposed to have the same semantics as REQ_FLUSH, that implies a
preflush if nr_segments > 0, not a postflush like I did.

Konrad, can you please confirm this?

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