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

["Roger Pau Monné" <roger.pau@citrix.com>]

Adding Jan in case he has an opinion on my reply below.

On Fri, Nov 23, 2018 at 12:04:51AM -0500, Zhao Yan wrote:
> On Thu, Nov 22, 2018 at 03:18:05PM +0100, Roger Pau Monné wrote:
> > 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.
> hi roger
> thanks for your sugguestion. it's right that "s->real_device.irq" is needed to be checked before mapping, like if it's 0.
> but on the other hand, maybe xc_physdev_map_pirq() itself can serve as a checking of "s->real_device.irq" ?
> like in our case, it will fail and return -EPERM.
> then error hanling is still conducted ==>set INTX_DISABLE flag, eventhrough the error is not fatal.
> 
>     machine_irq = s->real_device.irq;
>     rc = xc_physdev_map_pirq(xen_xc, xen_domid, machine_irq, &pirq);
>     if (rc < 0) {
>         error_setg_errno(errp, errno, "Mapping machine irq %u to"
>                          " pirq %i failed", machine_irq, pirq);
> 
>         /* Disable PCI intx assertion (turn on bit10 of devctl) */
>         cmd |= PCI_COMMAND_INTX_DISABLE;
>         machine_irq = 0;
>         s->machine_irq = 0;
> So, do you think it's all right just converting fatal error to non-fatal?

As I said above, I think it would be better to leave the error as
fatal and avoid attempting a xc_physdev_map_pirq with a machine_irq ==
0, which will fail.

If we really want to go down the route of making the error non-fatal,
I think you will also have to report PCI_INTERRUPT_PIN as 0 to the
guest, so that it's clear to the guest that the device doesn't have
legacy interrupt support.

Exposing a device with PCI_INTERRUPT_PIN != 0 but then not allowing
the guest to clear PCI_COMMAND_INTX_DISABLE is likely bogus.

Thanks, Roger.