Re: [PATCH] virtio-spec: document block CMD and FLUSH

On Tue, May 04, 2010 at 09:56:18PM +0300, Michael S. Tsirkin wrote:

On Tue, May 04, 2010 at 08:54:59PM +0200, Christoph Hellwig wrote:

quoted

On Fri, Feb 19, 2010 at 12:22:20AM +0200, Michael S. Tsirkin wrote:

quoted

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.

quoted

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.

quoted

+\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.

lguest seems to still use this.

Sorry, it doesn't. No idea why I thought it does.

I guess if you have a reliable host, VIRTIO_BLK_T_BARRIER is enough?

quoted

Btw, did I mention that .lyx is a a really horrible format to review
diffs for?  Plain latex would be a lot better..