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Re: [Xen-devel] [Qemu-devel] [PATCH] Xen PCI passthrough: fix passthrough failure when irq map failure

On Thu, Nov 22, 2018 at 08:11:20AM -0500, Zhao Yan wrote:
> On Thu, Oct 18, 2018 at 03:56:36PM +0100, Roger Pau Monné wrote:
> > On Thu, Oct 18, 2018 at 08:22:41AM +0000, Zhao, Yan Y wrote:
> > > Hi
> > > The background for this patch is that: for some pci device, even it's 
> > > PCI_INTERRUPT_PIN is not 0, it actually does not support INTx mode, so we 
> > > should just report error, disable INTx mode and continue the passthrough.
> > > However, the commit 5a11d0f7 regards this as error condition and let qemu 
> > > quit passthrough, which is too rigorous.
> > > 
> > > Error message is below:
> > > libxl: error: libxl_qmp.c:287:qmp_handle_error_response: Domain 
> > > 2:received an error message from QMP server: Mapping machine irq 0 to 
> > > pirq -1 failed: Operation not permitted
> > 
> > I'm having issues figuring out what's happening here.
> > s->real_device.irq is 0, yet the PCI config space read of
> > PCI_INTERRUPT_PIN returns something different than 0.
> > 
> > AFAICT this is due to some kind of error in Linux, so that even when
> > the device is supposed to have a valid IRQ the sysfs node it is set to
> > 0, do you know the actual underlying cause of this?
> > 
> > Thanks, Roger.
> Hi Roger
> Sorry for the later reply, I just missed this mail...
> On my side, it's because the hardware actually does not support INTx mode,
> but its configuration space does not report PCI_INTERRUPT_PIN to 0. It's a
> hardware bug, but previous version of qemu can tolerate it, switch to MSI
> and make passthrough work.

Then I think it would be better to check both PCI_INTERRUPT_PIN and
s->real_device.irq before attempting to map the IRQ.

Making the error non-fatal would mean that a device with a valid IRQ
could fail to be setup correctly but the guest will still be created,
and things won't go as expected when the guest attempts to use it.

Thanks, Roger.

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