Re: [Qemu-devel] [PATCH] KVM: Fix GSI number space limit

[Alexander Graf]

On 06.06.14 15:23, Cornelia Huck wrote:
> On Fri, 06 Jun 2014 15:15:54 +0200
> Alexander Graf <address@hidden> wrote:
>
> > On 06.06.14 15:12, Cornelia Huck wrote:
> > > On Fri,  6 Jun 2014 14:46:05 +0200
> > > Alexander Graf <address@hidden> wrote:
> > >
> > > > KVM tells us the number of GSIs it can handle inside the kernel. That value is
> > > > basically KVM_MAX_IRQ_ROUTES. However when we try to set the GSI mapping table,
> > > > it checks for
> > > >
> > > >       r = -EINVAL;
> > > >       if (routing.nr >= KVM_MAX_IRQ_ROUTES)
> > > >           goto out;
> > > >
> > > > erroring out even when we're only using all of the GSIs. To make sure we never
> > > > hit that limit, let's reduce the number of GSIs we get from KVM by one.
> > > >
> > > > Signed-off-by: Alexander Graf <address@hidden>
> > > > ---
> > > >    kvm-all.c | 2 +-
> > > >    1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/kvm-all.c b/kvm-all.c
> > > > index 4e19eff..56a251b 100644
> > > > --- a/kvm-all.c
> > > > +++ b/kvm-all.c
> > > > @@ -938,7 +938,7 @@ void kvm_init_irq_routing(KVMState *s)
> > > >    {
> > > >        int gsi_count, i;
> > > >
> > > > -    gsi_count = kvm_check_extension(s, KVM_CAP_IRQ_ROUTING);
> > > > +    gsi_count = kvm_check_extension(s, KVM_CAP_IRQ_ROUTING) - 1;
> > > >        if (gsi_count > 0) {
> > > >            unsigned int gsi_bits, i;
> > > But gsi_count is already marked as used further down in this function,
> > > isn't it? Confused.
> >     gsi_bits = ALIGN(gsi_count, 32);
> > [...]
> >           for (i = gsi_count; i < gsi_bits; i++) {
> >               set_gsi(s, i);
> >           }
> >
> > So if you take gsi_count = 1024, what happens?
> >
> >     gsi_count = 1024;
> >     gsi_bits = 1024;
> >     for (i = 1024; i < 1024; i++) {
> >               set_gsi(s, i);
> >     }
> >
> > At least in my world of C that loop never runs, no?
> But then kvm_irqchip_get_virq() should never return 1024, shouldn't it?

Right, because it returns the virq number which starts at 0. However, to 
describe all virqs from [0..1023] we need 1024 entries which the kernel 
errors out on.

> And:
>
> void kvm_irqchip_add_irq_route(KVMState *s, int irq, int irqchip, int pin)
> {
> [...]
>      assert(pin < s->gsi_count);
>
> would trigger too early with your change, wouldn't it?

Not really - with my change we only support 1023 virqs. So the biggest 
virq number is 1022 which is < 1023 :).


Sorry for describing this with actual numbers - I find it easier to 
grasp when I think in concrete numbers here - this stuff is just really 
spinning my head :).

Alex