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

[Rusty Russell <rusty@au1.ibm.com>]

"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?

So the advice would be as follows:
Driver authors: 
        If you want simplicity, ignore VIRTIO_BLK_F_WCE.
        If you want performance, you should negotiate VIRTIO_BLK_F_WCE.
                And if you also want to expose a toggle (eg. for testing), you
                can negotiate VIRTIO_BLK_F_CONFIG_WCE.
Device authors:
        If you want simplicity, don't offer VIRTIO_BLK_F_WCE.
        If you want performance, offer VIRTIO_BLK_F_WCE.
                And if you're masochistic, offer VIRTIO_BLK_F_CONFIG_WCE
                so they can toggle.

Cheers,
Rusty.