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

[Christoph Hellwig]

On Wed, Apr 25, 2012 at 12:21:53PM +0100, Stefano Stabellini wrote:
> 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.

It's worse - blkfront translates both a REQ_FLUSH or a REQ_FUA
into BLKIF_OP_FLUSH_DISKCACHE.

REQ_FLUSH either is a pre flush or a pure flush without a data transfer,
and REQ_FUA is a post flush.  So to get the proper semantics you'll have
to do both, _and_ sequence it so that no operation starts before the
previous one finished.