Re: [PATCH] virtio-blk: allow toggling host cache between writeback and writethrough

From: "Michael S. Tsirkin" <mst@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: linux-kernel@vger.kernel.org, rusty@rustcorp.com.au,
	kvm@vger.kernel.org, virtualization@lists.linux-foundation.org
Subject: Re: [PATCH] virtio-blk: allow toggling host cache between writeback and writethrough
Date: Thu, 5 Jul 2012 00:30:37 +0300	[thread overview]
Message-ID: <20120704213037.GA27713@redhat.com> (raw)
In-Reply-To: <4FF46A92.3020009@redhat.com>

On Wed, Jul 04, 2012 at 06:08:50PM +0200, Paolo Bonzini wrote:
> Il 04/07/2012 18:02, Michael S. Tsirkin ha scritto:
> > On Wed, Jul 04, 2012 at 05:54:16PM +0200, Paolo Bonzini wrote:
> >> Il 04/07/2012 17:42, Michael S. Tsirkin ha scritto:
> >>> On Tue, Jul 03, 2012 at 03:19:37PM +0200, Paolo Bonzini wrote:
> >>>> This patch adds support for the new VIRTIO_BLK_F_CONFIG_WCE feature,
> >>>> which exposes the cache mode in the configuration space and lets the
> >>>> driver modify it.  The cache mode is exposed via sysfs.
> >>>>
> >>>> Even if the host does not support the new feature, the cache mode is
> >>>> visible (thanks to the existing VIRTIO_BLK_F_WCE), but not modifiable.
> >>>>
> >>>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> >>>
> >>> I note this has been applied but I think the userspace
> >>> API is a bit painful to use. Let's fix it before
> >>> it gets set in stone?
> >>
> >> I'm trying to mimic the existing userspace API for SCSI disks.  FWIW I
> >> would totally agree with you.
> > 
> > Hmm. Want to try fixing scsi? Need to be compatible but it could
> > maybe ignore spaces.
> 
> Honestly I'm not sure it's really worthwhile...  And you also have the
> same problem when printing.  You cannot remove the spaces, because
> clients will look for the "old" string, with the spaces.

Right. Oh well.

> >>>> +static int virtblk_get_cache_mode(struct virtio_device *vdev)
> >>>
> >>> Why are you converting u8 to int here?
> >>
> >> The fact that it is a u8 is really an internal detail.  Perhaps the bug
> >> is using u8 in the callers.
> > 
> > Make it bool then?
> > 
> > You are using u8 in the config. So you could get any value
> > besides 0 and 1, and you interpret that as 1.
> > Is 1 always a safe value? If not maybe it's better to set
> > to a safe value if it is not 0 or 1, that is we don't know how to interpret it.
> 
> That would be a host bug; the spec only gives meaning to 0 and 1.

Yes but if the other side does not validate values implementations
*will* have bugs. Why not declare bits 1-7 reserved?
Then we can reuse other bits later.

>  In
> any case, "have a cache" means "needs to flush", so it's always safe.
> If you interpret another value as 0, you risk omitting flushes.
> 
> Paolo

OK, that's good.

-- 
MST