Re: [virtio-dev] Re: [virtio-comment] Problems with VIRTIO-4 and writeback only disks

[James Bottomley <jbottomley@parallels.com>]

On Mon, 2013-09-16 at 16:50 +0930, Rusty Russell wrote:
> "Michael S. Tsirkin" <mst@redhat.com> writes:
> > Redirecting discussion to virtio-dev@lists.oasis-open.org
> > as per oasis rules.
> >
> > On Tue, Sep 10, 2013 at 05:27:26PM +0000, James Bottomley wrote:
> >> [resending to virtio-comment; it looks like I'm not subscribed to
> >> virtio-dev ... how do you subscribe?]
> >> 
> >> Sorry, I don't have a copy of the original email to reply to:
> >> 
> >> https://lists.oasis-open.org/archives/virtio-comment/201308/msg00078.html
> >> 
> >> The part that concerns me is this:
> >> 
> >> > +5. The cache mode should be read from the writeback field of the configuration
> >> > +  if the VIRTIO_BLK_F_CONFIG_WCE feature if available; the driver can also
> >> > +  write to the field in order to toggle the cache between writethrough (0)
> >> > +  and writeback (1) mode.
> >> > +  If the feature is not available, the driver can instead look at the result
> >> > +  of negotiating VIRTIO_BLK_F_WCE: the cache will be in writeback mode after
> >> > +  reset if and only if VIRTIO_BLK_F_WCE is negotiated[30]
> >> 
> >> The questions are twofold and have to do with Write Back only disks (to
> >> date we've seen quite a few ATA devices like this and a huge number of
> >> USB devices):
> >> 
> >>      1. If the guest doesn't negotiate WCE, what do you do on the host
> >>         (flush on every write is one possible option; run unsafe and
> >>         hope the host doesn't crash is another).
> >>      2. If the guest asks to toggle the device from writeback (1) to
> >>         writethrough (0) mode, what do you do?  Refuse the toggle would
> >>         be reasonable or flip back into whatever mode you were using to
> >>         handle 1. is also possible.
> >> 
> >> James
> >
> >
> > Basically if I understand what you are saying, we should
> > just require devices to make WCE state always present in config space,
> > and ask drivers to always read the value from config space
> > instead of assuming tricky logic ""the cache will be in writeback mode
> > after reset if and only if VIRTIO_BLK_F_WCE is negotiated"
> 
> I think we should do the reverse:
> 1) If you negotiate VIRTIO_BLK_F_WCE, you need to flush (ie. it's
>    writeback).  Otherwise it's write-through.
> 2) If (and only if) you negotiate VIRTIO_BLK_F_WCE, you may also negotiate
>    VIRTIO_BLK_F_CONFIG_WCE, in which case you can toggle it.
> 
> But I'm not convinced that #2 should exist at all.  Does someone have a
> real use case?

There is no use case in Linux.  We provide knobs to toggle the cache
type in the SCSI disk class, but the kernel doesn't do it by default, it
just runs with the cache type because the toggle can be unreliable.

James