Re: [Xen-devel] [PATCH] x86/pt: skip setup of posted format IRTE when gvec is 0

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Apr 30, 2019 at 01:19:19PM +0800, Chao Gao wrote:
> When testing with an UP guest with a pass-thru device with vt-d pi
> enabled in host, we observed that guest couldn't receive interrupts
> from that pass-thru device. Dumping IRTE, we found the corresponding
> IRTE is set to posted format with "vector" field as 0.
> 
> We would fall into this issue when guest used the pirq format of MSI

I think the above sentence is a bit confusing. I would rather say that
the guest is routing interrupts from passthrough devices over event
channels using pirqs.

> (see the comment xen_msi_compose_msg() in linux kernel). As 'dest_id'
> is repurposed, skip migration which is based on 'dest_id'.

Like Jan, I wonder why the device model (I assume QEMU) issues a
XEN_DOMCTL_bind_pt_irq hypercall when the guest is attempting to route
this interrupts over an event channel, attempting to bind it to a
vector seems like a bug in the device model.

I would also consider whether it would make sense to not expose the
XENFEAT_hvm_pirqs feature when doing passthrough if posted interrupts
are supported, performance wise it's better to use posted interrupts
rather than routing them over event channels (which requires Xen
interaction).

Note that I think this whole XENFEAT_hvm_pirqs is a big hack which
abuses a hardware interface, and I would like to see it removed.

Thanks, Roger.

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