Re: [Qemu-devel] pci-assign can not work

[Wen Congyang <wency@cn.fujitsu.com>]

At 03/16/2012 04:27 PM, Jan Kiszka Wrote:
> On 2012-03-16 03:38, Wen Congyang wrote:
>> At 03/15/2012 06:21 PM, Wen Congyang Wrote:
>>> Hi all
>>>
>>> When I use pci-assign, I meet the following error:
>>>
>>> Failed to assign irq for "hostdev0": Input/output error
>>> Perhaps you are assigning a device that shares an IRQ with another device?
>>>
>>> Is it a bug or I miss something?
>>
>> Hi, Jan
>>
>> This problem is caused by your patch:
>> commit 6919115a8715c34cd80baa08422d90496f11f5d7
>> Author: Jan Kiszka <jan.kiszka@siemens.com>
>> Date:   Thu Mar 8 11:10:27 2012 +0100
>>
>>     pci_assign: Flip defaults of prefer_msi and share_intx
>>     
>>     INTx sharing is a bit more expensive than exclusive host interrupts, but
>>     this channel is not supposed to be used for high-performance scenarios
>>     anyway. Modern devices support MSI/MSI-X and do not depend on using INTx
>>     under critical workload, real old devices do not support INTx sharing
>>     anyway.
>>     
>>     For those in the middle, the user experience is much better if they just
>>     work even when IRQ sharing is required. If there is nothing to share,
>>     share_intx=off can still be applied as tuning parameter.
>>     
>>     With INTx sharing as default, the primary reason for prefer_msi=on is
>>     gone. Make it default off, specifically as it is known to cause troubles
>>     with devices that have incomplete/broken MSI support or otherwise
>>     stumble if host IRQ configuration does not match guest driver
>>     expectation.
>>     
>>     Acked-by: Alex Williamson <alex.williamson@redhat.com>
>>     Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
>>     Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
>>
>> If I revert this commit. qemu can work.
>>
> 
> This should be "solvable" by passing prefer_msi=on to the pci-assign
> device, or likely by updating your host kernel to latest kvm.git (to
> enable INTx sharing).

Is there some way to find out if the kernel supports to enable INTx
sharing?

Thanks
Wen Congyang

> 
> Hmm, unfortunate. We needed a conditional default for the prefer_msi
> property here. If INTx sharing doesn't work for some reason AND the user
> did not ask for disabling the host-side MSI usage, we should fall back
> to it again.
> 
> Markus, is there some easy way to find out if a specific qdev property
> was set due to a command line switch or was defined by the default value?
> 
> Jan
>