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

[Jan Kiszka <jan.kiszka@siemens.com>]

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).

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

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux