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

[Konrad Rzeszutek Wilk]

On Wed, Apr 25, 2012 at 01:23:35PM +0200, Christoph Hellwig wrote:
> 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.

I think that is what remained of the BARRIER request.
> 
> 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.

If I were to emulate the SCSI SYNC command which one would it be?

I think REQ_FLUSH? In which I would think that the blkfront needs to
get rid of the REQ_FUA part?