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

["Michael S. Tsirkin" <mst@redhat.com>]

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"


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