[Xen-devel] Re: More virtio users

[Jens Axboe <jens.axboe@xxxxxxxxxx>]

On Sun, Jun 10 2007, Rusty Russell wrote:
> On Sun, 2007-06-10 at 11:16 +0300, Avi Kivity wrote:
> > Rusty Russell wrote:
> > > Lguest doesn't have a framebuffer, so maybe this is a good thing for me
> > > to hack on, but I promised myself I'd finish NAPI for the net device,
> > > and tag for block device first.
> > >   
> > 
> > If you're touching the block device, passing a request's io priority to 
> > the host can be useful.
> 
> OK, here's the interdiff.  I still don't handle non-fs requests, but I
> haven't seen any yet.  I should probably BUG_ON() there and wait for
> Jens to scream...

Ehm no, that would certainly cause screaming :-)

Checking for blk_fs_request() and terminating the request if you don't
know how to handle it is the correct thing to do, a BUG() would
definitely not be.

Patch looks good to me, though:

> +     blk_queue_prep_rq(vblk->disk->queue, blk_queue_start_tag);
> +

is quite a novel way, I actually had to look that code up to check
whether it was correct. I'd much prefer a little wrapper around that,
ala:

static int virtio_block_prep(request_queue_t *q, struct request *rq)
{
        if (!blk_queue_start_tag(q, rq))
                return BLKPREP_OK;

        return BLKPREP_DEFER;
}

That is much easier to read, and as a bonus it wont eat your request
just because you run out of tags! The fact that blk_queue_start_tag()
just happens to share the prep_rq_fn definition is by coincidence only.

-- 
Jens Axboe


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