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

[Aurelien Jarno]

On Wed, Mar 31, 2010 at 02:24:51PM -0500, Anthony Liguori wrote:
> 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<address@hidden>  wrote:
>>>
>>>      
>>>> On 03/31/2010 01:20 PM, Michael S. Tsirkin wrote:
>>>>        
>>>>> From: David L Stevens<address@hidden>
>>>>>
>>>>> 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<address@hidden>
>>>>> Signed-off-by: Michael S. Tsirkin<address@hidden>
>>>>> ---
>>>>>
>>>>> 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<address@hidden>
>>> 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.
>

While it probably make sense to achieve this goal, it doesn't mean it
should be done the dirty way. 

For example it is known for a lot of time that the solution for the 
bswap in the device code is to add a bus model doing the byteswapping.
Removing the #ifdef by deciding "this device will only be big/little
endian" doesn't seem to go in the right direction.

-- 
Aurelien Jarno                          GPG: 1024D/F1BCDB73
address@hidden                 http://www.aurel32.net