linux-kernel - Re: virtio_blk: fix defaults for max_hw_sectors and max_segment

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Message-ID: <20141126230055.GA32363@redhat.com>
Date:	Wed, 26 Nov 2014 18:00:55 -0500
From:	Mike Snitzer <snitzer@...hat.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	martin.petersen@...cle.com, mst@...hat.com, rusty@...tcorp.com.au,
	qemu-devel@...gnu.org, linux-kernel@...r.kernel.org,
	Christoph Hellwig <hch@...radead.org>, dm-devel@...hat.com,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: virtio_blk: fix defaults for max_hw_sectors and max_segment_size

On Wed, Nov 26 2014 at  4:53pm -0500,
Jens Axboe <axboe@...nel.dk> wrote:

> On 11/26/2014 02:51 PM, Mike Snitzer wrote:
> > 
> > But while you're here, I wouldn't mind getting your take on virtio-blk
> > setting max_hw_sectors to -1U.
> > 
> > As I said in my original reply to mst: it only makes sense to set a
> > really high initial upper bound like that in a driver if that driver
> > goes on to stack an underlying device's limit.
> 
> -1U should just work, IMHO, there's no reason we should need to cap it
> at some synthetic value.  That said, it seems it should be one of
> those parameters that should be negotiated up and set appropriately. 

I'm saying set it to the underlying device's value for max_hw_sectors --
not some synthetic value.  So I think we're saying the same thing.

But it isn't immediately clear (to me) how that benefits virtio-blk
users (obviously they are getting by today).  So until that is pinned
down I imagine nobody will care to extend the virtio-blk protocol to
allow stacking max_hw_sectors and max_sectors up.
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