[Xen-devel] Re: More virtio users

[Jens Axboe <jens.axboe@xxxxxxxxxx>]

On Tue, Jun 12 2007, Rusty Russell wrote:
> On Mon, 2007-06-11 at 09:33 +0200, Jens Axboe wrote:
> > On Mon, Jun 11 2007, Rusty Russell wrote:
> > > So the problem is that I'd like to handle all of them, but I'm not clear
> > > what requests my device can get.  I can't see a source of any other type
> > > of request.
> > 
> > The other main request type is blk_pc_request(). In the data setup it's
> > indentical to blk_fs_request(), there's a bio chain off ->bio. It's a
> > byte granularity entity though, so you should check ->data_len for the
> > size of it. ->cmd[] holds a SCSI cdb, which is the command you are
> > supposed to handle.
> 
> SCSI?  I'm even more lost now.
> 
> Q: So what *are* the commands?

They are SCSI commands!

> Q: Who puts them in my queue?

If you want to support SG_IO for instance, you'd have to deal with SCSI
commands.

> I have *never* seen anything but an fs request come through to my
> driver.

You probably only did basic IO to it.

> > Perhaps you are misunderstanding what the tag is? The tag is a unique
> > identifier for a pending request, so you will by definition never have
> > requests sharing a tag value.
> 
> Yes, I started to suspect that.
> 
> > But the tag is not to be considered as
> > ordered, unless it has the barrier flag set as well. So you only need to
> > serialize on the device side when blk_barrier_rq() is true.
> 
> blk_barrier_rq(req) is never true.  I put a BUG_ON(blk_barrier_rq(req))
> in my code and booted.  This is using the device as a root ext3
> filesystem.
>
> I reverted all the tag changes, still no barriers.  I added
> "blk_queue_ordered(vblk->disk->queue, QUEUE_ORDERED_DRAIN, NULL);",
> still no barriers.

-o barrier=1 for ext3, it doesn't use barriers by default. Or
barrier=flush for reiserfs. XFS defaults to barriers on.

-- 
Jens Axboe


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