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

[Christian Borntraeger <borntraeger@de.ibm.com>]

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