Re: [Qemu-devel] [PATCH v4 0/2] virtio: Move host features to backends

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

On Wed, Apr 29, 2015 at 10:52:15AM +0200, Cornelia Huck wrote:
> On Wed, 29 Apr 2015 10:17:55 +0200
> Christian Borntraeger <borntraeger@de.ibm.com> wrote:
> 
> > Am 28.04.2015 um 20:32 schrieb Michael S. Tsirkin:
> > > On Tue, Apr 28, 2015 at 08:14:44PM +0200, Michael S. Tsirkin wrote:
> > >> On Tue, Apr 28, 2015 at 04:35:16PM +0200, Michael S. Tsirkin wrote:
> > >>> On Tue, Apr 28, 2015 at 03:24:19PM +0200, Cornelia Huck wrote:
> > >>>> On Tue, 28 Apr 2015 14:16:40 +0100
> > >>>> Peter Maydell <peter.maydell@linaro.org> wrote:
> > >>>>
> > >>>>> On 28 April 2015 at 14:13, Michael S. Tsirkin <mst@redhat.com> wrote:
> > >>>>>> The patches look correct to me too, but I want s390
> > >>>>>> cleaned up so it does not include COMMON_FEATURES
> > >>>>>> in 100 places, and I prefer merging it all together.
> > >>>>>
> > >>>>> It seems a bit harsh to ask Shannon to do s390 cleanup when
> > >>>>> he doesn't have any access to s390 guests or test cases...
> > >>>>> Making S390 put COMMON_FEATURES in the right places seems
> > >>>>> to me like a separate bit of s390-specific cleanup.
> > >>>>
> > >>>> Yep, see my other reply... I'm not quite sure what's wrong with
> > >>>> event_idx on virtio-blk for s390-virtio, or I would gladly make this
> > >>>> consistent with the other transports. Any hints appreciated :)
> > >>>
> > >>> Is this still happening?
> > >>>
> > >>> It is possible that what was missing was
> > >>> 92045d80badc43c9f95897aad675dc7ef17a3b3f
> > >>> and/or
> > >>> a281ebc11a6917fbc27e1a93bb5772cd14e241fc
> > >>>
> > >>
> > >> Found this:
> > >> http://thread.gmane.org/gmane.comp.emulators.qemu/280334/focus=280357
> > >> so it's unlikely: these commits are from 2012, you saw
> > >> issues in 2014.
> > >>
> > >> We really need to fix it. virtio 1 work will be much easier if
> > >> we can just move features into virtio dev.
> > 
> > Yes, we have to understand why event_idx breaks for the s390-virtio transport.
> > > 
> > > I'm beginning to suspect this is a wrong implementation of barriers.
> > > Questions:
> > >     - which compiler to you use?
> > >     - can you pls disassemble code for smp_wmb smp_rmb and smp_mb?
> > >       They all must do br %r14 I think, and this is what
> > >       s390x-linux-gnu-gcc generated for me:
> > >         s390x-linux-gnu-gcc (GCC) 4.9.1
> > 
> > s390 has strong memory ordering. Reads are in order, writes are in order. 
> > bcr 14,0 or bcr 15,0 then only serialize the reads against the writes.
> > So smp_rmb and smp_wmb can be implemented as no-ops like QEMU.
> > If your change "fixes" the issue then we have a problem somewhere else
> 
> And (surprise, surprise) virtio-blk now works - but it also works when
> I back out the atomic.h change again. No barrier problems :)
> 
> Good news is that we can change s390-virtio to be just like the other
> transports. Although I'd like to understand why it was broken before.
> Maybe a guest change?

Or a compiler change? Try compiling some old release, see what happens.
Anyway, let's move DEFINE_VIRTIO_COMMON_FEATURES into the base class
now.  Can you send a patch pls?

-- 
MST