Re: [Xen-devel] [PATCH qemu-xen-traditional] xen/pt: allow QEMU to request MSI unmasking at bind time

["Paraschiv, Andra-Irina" <andraprs@xxxxxxxxxx>]

> > >Ian, any objections?
>
> Sorry for dropping this.  It's been a while!

No problem, Ian, we have this heads-up now.

>
> > >> > When a MSI interrupt is bound to a guest using
> > >> > xc_domain_update_msi_irq (XEN_DOMCTL_bind_pt_irq) the interrupt
> > >> > is left masked by default.
> > >> >
> > >> > This causes problems with guests that first configure
> > >> > interrupts and clean the per-entry MSIX table mask bit and
> > >> > afterwards enable MSIX globally. In such scenario the Xen
> > >> > internal msixtbl handlers would not detect the unmasking of
> > >> > MSIX entries because vectors are not yet registered since MSIX
> > >> > is not enabled, and vectors would be left masked.
> > >> >
> > >> > Introduce a new flag in the gflags field to signal Xen whether
> > >> > a MSI interrupt should be unmasked after being bound.
> > >> >
> > >> > This also requires to track the mask register for MSI
> > >> > interrupts, so QEMU can also notify to Xen whether the MSI
> > >> > interrupt should be bound masked or unmasked
>
> I searched my email archives but I haven't been able to find a copy of
> the actual patch.  Was it ever sent to me ?
> 
> Also, qemu-trad is quite deeply frozen.  I'd like an explanation of
> the practical impact of this bug, and the use cases where the fix is
> needed, to justify the change.  Sorry to be awkward.

That's the link to the patch for qemu-xen-traditional:
https://lists.xenproject.org/archives/html/xen-devel/2018-06/msg01248.html

(https://lists.xenproject.org/archives/html/xen-devel/2018-06/msg01373.html - 
added you in CC)

The issue was notified here - 
https://lists.xenproject.org/archives/html/xen-devel/2018-05/msg01238.html and 
fixes were added in xen and qemu. QEMU needs to interact with Xen to have guest 
interrupts masked/unmasked.

This patch is a backport from qemu.

One of the use cases where this fix is needed is: guest OS is Windows and the 
host has the latest stable version of xen and qemu-xen-traditional. Using this 
environment, setting up a PCI device (using MSI/MSI-X interrupts) will not 
successfully finish.

Thanks,
Andra

>
> Regards,
> Ian.



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