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Date: Fri, 16 Mar 2012 10:38:39 +0800
From: Wen Congyang <wency@cn.fujitsu.com>
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Subject: Re: [Qemu-devel] pci-assign can not work
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To: qemu-devel <qemu-devel@nongnu.org>, kvm list <kvm@vger.kernel.org>, Jan Kiszka <jan.kiszka@siemens.com>

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.

Thanks
Wen Congyang

> 
> I use libvirt to start the guest, and the user/group is root/root(not qemu/qemu).
> 
> If I use the default qemu(provided in RHEL6.2), the guest can be started.
> 
> Thanks
> Wen Congyang
> 
>