Re: [Xen-devel] Re: Still struggling with HVM: tx timeouts on emulated nics

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Tue, 4 Oct 2011, Andrew Cooper wrote:
> On 03/10/11 19:13, Stefano Stabellini wrote:
> > CC'ing Jan, that probably is going to have an opinion on this.
> >
> > Let me add a bit of background: Stefan found out that PV on HVM guests
> > could loose level interrupts coming from emulated devices. Looking
> > through the code I realized that we need to add some logic to inject a
> > pirq in the guest if a level interrupt has been raised while the guest
> > is servicing the first one.
> > While this is all very specific to interrupt remapping and emulated
> > devices, I realized that something similar could happen even with dom0
> > or other PV guests with PCI passthrough:
> >
> > 1) the device raises a level interrupt and xen injects it into the
> > guest;
> >
> > 2) the guest is temporarely stuck: it does not ack it or eoi it;
> >
> > 3) the xen timer kicks in and eois the interrupt;
> >
> > 4) the device thinks it is all fine and sends a second interrupt;
> >
> > 5) Xen fails to inject the second interrupt into the guest because the
> > guest has still the event channel pending bit set;
> >
> > at this point the guest looses the second interrupt notification, that
> > is not supposed to happen with level interrupts and I think it might
> > cause problems with some devices.
> >
> > Jan, do you think we should try to handle this case, or is it too
> > unlikely?
> 
> I am not certain whether this is relevant, but the ICH10 IO-APIC
> documentation indicated that early EOI'ing of a line level interrupt
> should not have this effect.  Specifically, it states that EOI'ing a
> line level interrupt whos line is still asserted will cause the
> interrupt to be "re-raised" from the IO-APIC.  It uses this to assert
> that it is fine to use multiple IO-APIC entries with the same vector,
> with a broadcast of vector number alone to EOI the interrupt.
> 
> In this case, while Xen sees two interrupts, from the devices point of
> view, only I has happened.
> 
> In the case where the device has dropped its line level interrupt of its
> own accord, then I would agree that the current Xen behavior is wrong. 
> I cant offhand think of a good reason why this would occur.

I think this scenario is actually possible. It is certainly happening
with qemu's emulated devices.
This patch would take care of re-injecting the interrupts both in the
case of the device deasserting and reasserting the interrupt while the
guest hasn't cleared the pending bit yet and in case a PV on HVM guest
eois the interrupt too early.


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