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[Qemu-devel] Re: [PATCH] virtio-spec: document block CMD and FLUSH
From: |
Rusty Russell |
Subject: |
[Qemu-devel] Re: [PATCH] virtio-spec: document block CMD and FLUSH |
Date: |
Wed, 5 May 2010 14:30:17 +0930 |
User-agent: |
KMail/1.13.2 (Linux/2.6.32-21-generic; KDE/4.4.2; i686; ; ) |
On Wed, 5 May 2010 04:24:59 am Christoph Hellwig wrote:
> On Fri, Feb 19, 2010 at 12:22:20AM +0200, Michael S. Tsirkin wrote:
> > I took a stub at documenting CMD and FLUSH request types in virtio
> > block. Christoph, could you look over this please?
> >
> > I note that the interface seems full of warts to me,
> > this might be a first step to cleaning them.
>
> The whole virtio-blk interface is full of warts. It has been
> extended rather ad-hoc, so that is rather expected.
>
> > One issue I struggled with especially is how type
> > field mixes bits and non-bit values. I ended up
> > simply defining all legal values, so that we have
> > CMD = 2, CMD_OUT = 3 and so on.
>
> It's basically a complete mess without much logic behind it.
>
> > +\change_unchanged
> > +the high bit
> > +\change_inserted 0 1266497301
> > + (VIRTIO_BLK_T_BARRIER)
> > +\change_unchanged
> > + indicates that this request acts as a barrier and that all preceeding
> > requests
> > + must be complete before this one, and all following requests must not be
> > + started until this is complete.
> > +
> > +\change_inserted 0 1266504385
> > + Note that a barrier does not flush caches in the underlying backend device
> > + in host, and thus does not serve as data consistency guarantee.
> > + Driver must use FLUSH request to flush the host cache.
> > +\change_unchanged
>
> I'm not sure it's even worth documenting it. I can't see any way to
> actually implement safe behaviour with the VIRTIO_BLK_T_BARRIER-style
> barriers.
>
>
> Btw, did I mention that .lyx is a a really horrible format to review
> diffs for? Plain latex would be a lot better..
Yeah, or export as text post that diff for content review. I do like the
patches to the lyx source though (I check all versions into revision control,
before and after merging changes, which makes it easy to produce annotated
versions).
Cheers,
Rusty.
>
[Qemu-devel] Re: [PATCH] virtio-spec: document block CMD and FLUSH, Christoph Hellwig, 2010/05/04
Re: [Qemu-devel] Re: [PATCH] virtio-spec: document block CMD and FLUSH, Jamie Lokier, 2010/05/04
[Qemu-devel] Re: [PATCH] virtio-spec: document block CMD and FLUSH, Christoph Hellwig, 2010/05/04