Re: [RFC PATCH] spapr/xive: Allocate vCPU IPIs from local context

[Greg Kurz <groug@kaod.org>]

On Fri, 24 Sep 2021 16:58:00 +0200
Cédric Le Goater <clg@kaod.org> wrote:

> [ ... ]
> 
> >> The changes only impact KVM support since we are deferring the IRQ
> >> initialization at the KVM level. What we have to be careful about is not
> >> accessing an ESB page of an interrupt that would not have been initiliazed
> >> in the KVM device, else QEMU gets a sigbus.
> >>
> > 
> > Ok, so this is just needed on a path that leads to xive_esb_rw() or
> > kvmppc_xive_esb_trigger(), right ?
> > 
> > It seems that
> > 
> > h_int_esb()
> >   kvmppc_xive_esb_rw()
> > 
> > can get there with an lisn provided by the guest, and I don't see any
> > such check on the way : h_int_esb() only checks xive_eas_is_valid().
> 
> This call is for LSI interrupts (device only) and not vCPU IPIs. see hcall
> h_int_get_source_info(). I agree a hcall fuzzer could reach it.
> 

Yes we tell the guest to use H_INT_ESB for LSIs only but I don't have
the impression that it is forbidden for the guest to call H_INT_ESB
for arbitrary interrupts.

> An alternative solution would be to claim the vCPU IPIs on demand, like
> we do for the MSIs, and not in spapr_irq_init(). It's a much bigger change
> tough, because the H_INT hcalls consider that the interrupt numbers have
> been claimed.
> 

Maybe it would be simpler to call xive_source_is_initialized() instead of
xive_eas_is_valid() in cases like this, e.g. hcall code since it is shared
between emulation and KVM ?

> C.