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

[Cédric Le Goater]

On 9/24/21 19:13, Greg Kurz wrote:
> 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 ?

Yes but we need a better check than :

    if (lisn < SPAPR_XIRQ_BASE) {
        return !!xive_get_field64(EAS_END_INDEX, xive->eat[lisn].w);
    }

This is more an heuristic than a precise test on the "validity" of
a source at the KVM level.


Thanks,

C.