[Xen-devel] Re: hvm_domain_use_pirq() breaks device passthru on xen-4.1

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 4 Aug 2011, Wei Huang wrote:
> Hi Stefano,
> 
> The recent patch you put into xen-4.1-testing.hg broke device passthru 
> on Windows. I bisected the changesets and it turned out to be 23092. The 
> NIC can't get the IP address correctly. Changset 23091 worked fine.
> 
> Regarding your patch, do we need to differentiate pv-on-hvm guests from 
> others? The Windows VM I used is unmodified. I think 
> hvm_domain_use_pirq() returned an incorrect answer for such guests.

The problem is that when qemu enables MSIs for a pt device on behalf of
the guest physdev_map_pirq is called and pirq_emuirq is set to IRQ_PT.
>From that point on hvm_domain_use_pirq is going to return true
incorrectly.
Before 23092 we would also check for the event channel to be bound
before returning true in hvm_domain_use_pirq, but that turns out to be a
problem if a PV on HVM guest disables and unbinds the event channel.

I think that reverting 23092 is the simplest thing to do here, then we
need to find a different way to distinguish MSIs that have to be
injected using the emulated path and the ones that have to be injected
using the PV path.

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