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Re: [Qemu-devel] [question] virtio-blk performance degradationhappened w


From: Christian Borntraeger
Subject: Re: [Qemu-devel] [question] virtio-blk performance degradationhappened with virito-serial
Date: Mon, 01 Sep 2014 16:03:57 +0200
User-agent: Mozilla/5.0 (X11; Linux i686; rv:31.0) Gecko/20100101 Thunderbird/31.0

On 01/09/14 15:29, Paolo Bonzini wrote:
> Il 01/09/2014 15:22, Christian Borntraeger ha scritto:
>>>> If virtio-blk and virtio-serial share an IRQ, the guest operating system 
>>>> has to check each virtqueue for activity. Maybe there is some inefficiency 
>>>> doing that.
>>>> AFAIK virtio-serial registers 64 virtqueues (on 31 ports + console) even 
>>>> if everything is unused.
>>>
>>> That could be the case if MSI is disabled.
>>
>> Do the windows virtio drivers enable MSIs, in their inf file?
> 
> It depends on the version of the drivers, but it is a reasonable guess
> at what differs between Linux and Windows.  Haoyu, can you give us the
> output of lspci from a Linux guest?
> 
> Paolo

Zhang Haoyu, which virtio drivers did you use?

I just checked the Fedora virtio driver. The INF file does not contain the MSI 
enablement as described in
http://msdn.microsoft.com/en-us/library/windows/hardware/ff544246%28v=vs.85%29.aspx
That would explain the performance issues - given that the link information is 
still true.



Christian








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