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

[Jan Kiszka]

On 2012-03-16 09:38, Wen Congyang wrote:
> 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 <address@hidden>
>>> 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 <address@hidden>
>>>     Signed-off-by: Jan Kiszka <address@hidden>
>>>     Signed-off-by: Marcelo Tosatti <address@hidden>
>>>
>>> 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?

QEMU does a feature check, but as a user you simply have to know which
kernel version includes it (will be 3.4 or 3.5). Of course, that's not
really handy.

Jan

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