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

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

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 <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?

3.17 as guest fails, 3.18 as guest works. Not sure yet why.