Re: [Qemu-devel] Re: [PATCH] vhost: fix features ack

[Anthony Liguori <anthony@codemonkey.ws>]

On 03/31/2010 02:07 PM, Michael S. Tsirkin wrote:
> On Wed, Mar 31, 2010 at 03:38:05PM -0300, Luiz Capitulino wrote:
>    
>> On Wed, 31 Mar 2010 13:26:23 -0500
>> Anthony Liguori<anthony@codemonkey.ws>  wrote:
>>
>>      
>>> On 03/31/2010 01:20 PM, Michael S. Tsirkin wrote:
>>>        
>>>> From: David L Stevens<dlstevens@us.ibm.com>
>>>>
>>>> vhost driver in qemu didn't ack features, and this happens
>>>> to work because we don't really require any features. However,
>>>> it's better not to rely on this. This patch passes features to
>>>> vhost as guest acks them.
>>>>
>>>> Signed-off-by: David L Stevens<dlstevens@us.ibm.com>
>>>> Signed-off-by: Michael S. Tsirkin<mst@redhat.com>
>>>> ---
>>>>
>>>> Anthony, here's a fixup patch to address an issue in vhost
>>>> patches. Incidentially, what's the status of the vhost patchset?
>>>>
>>>>          
>>> http://repo.or.cz/w/qemu/aliguori-queue.git vhost
>>>
>>> Is what I'm currently testing.  With vhost disabled,  the following seg
>>> faults:
>>>
>>> qemu-system-x86_64 -hda ~/images/linux.img -net tap -net
>>> nic,model=virtio -enable-kvm
>>>
>>> But not when using TCG.  I'm not sure that it's your patches at fault
>>> and I'm attempting to bisect now to figure that out.
>>>        
>>   Probably this is the same segfault I'm getting right now in master,
>> bisect says it's:
>>
>> """
>> commit ad96090a01d848df67d70c5259ed8aa321fa8716
>> Author: Blue Swirl<blauwirbel@gmail.com>
>> Date:   Mon Mar 29 19:23:52 2010 +0000
>>
>>      Refactor target specific handling, compile vl.c only once
>> """
>>      
> Why are the compile once patches helpful? They seem to introduce
> churn and bugs, they actively make it harder to extend qemu as you can't use
> target-specific code in code that is compiled once, they might have
> performance penalty - and what do we gain? Any given user is unlikely to
> need to build on more than one target, distros have enough computing
> power to build in parallel.
>
> Maybe it makes sense to revert the compile once patches, and discuss
> these issues before re-commit?
>    

Compiling objects once is certainly useful.  Long term, I think most of 
us want to see a single qemu executable that works for all architectures 
and compiling once is an important step in that direction.

With respect to regressions, it might make sense to slow down these 
refactorings a bit and increase the amount of regression testing that is 
happening during them.

Regards,

Anthony Liguori