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

[Michael S. Tsirkin]

On Wed, Apr 29, 2015 at 04:43:19PM +0200, Christian Borntraeger wrote:
> Am 29.04.2015 um 12:32 schrieb Michael S. Tsirkin:
> > On Wed, Apr 29, 2015 at 10:52:15AM +0200, Cornelia Huck wrote:
> >> On Wed, 29 Apr 2015 10:17:55 +0200
> >> Christian Borntraeger <address@hidden> 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 <address@hidden> wrote:
> >>>>>>>
> >>>>>>>> On 28 April 2015 at 14:13, Michael S. Tsirkin <address@hidden> 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?
> 
> 3.17 as guest fails, 3.18 as guest works. Not sure yet why.
>  

Fascinating. block core changes? bisect will tell.

-- 
MST