Re: [virtio] [PATCH] virtio-blk: restore VIRTIO_BLK_F_FLUSH and VIRTIO_BLK_F_CONFIG_WCE

[Paolo Bonzini <pbonzini@redhat.com>]

On 08/07/2015 12:53, Michael S. Tsirkin wrote:
>>> Below 2 paragraphs tell you why :)
>>> Because it does not always *have* non-volatile storage.
>>
>> So are we still discussing what MUST means?
> 
> I'm discussing what non-volatile means.

Since we're down to just 1 or 2 contentious points, I'm sending v4.
Let's reboot the discussion on top of that version.  Just a couple
comments below:

>> If the guest cannot see the
>> difference, QEMU is not violating anything.
> 
> Of course it can see the difference after reset.

Not after reset, only after a cold restart (there was a
crash---restoring from migration).  A lot more changes could happen
between shutdown (doesn't matter if clean or crashy) and the next cold
restart, and the guest can detect them the same way.

E.g. in one case the guest could detect that the kernel was updated via
libguestfs.  In another the guest could detect that the storage
(formerly in tmpfs, though the guest didn't know that) is now all zero.

>> If they don't, they've already done something not recommended, and a MAY
>> is just fine in this paragraph.  This paragraph only applies to
>> implementation that have already disregarded that other "SHOULD".  It
>> makes little sense to do yet another recommendation that they would
>> disregard.
>>
>> This "MAY" is here to explicitly allow the behavior.
> 
> Well ignoring that SHOULD leads to bad performance, ignoring
> this one leads to data loss. So you need to pause and think
> at both points ...

I see your point.  However, whoever ignores that SHOULD is very likely
not caring about data safety anyway.  This MAY allows them to keep the
data safe at the price of bad performance, in the unlikely case that
they do care about data safety.

Paolo