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

[Cornelia Huck <cornelia.huck@de.ibm.com>]

On Fri, 06 Jun 2014 15:15:54 +0200
Alexander Graf <agraf@suse.de> wrote:

> 
> On 06.06.14 15:12, Cornelia Huck wrote:
> > On Fri,  6 Jun 2014 14:46:05 +0200
> > Alexander Graf <agraf@suse.de> 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 <agraf@suse.de>
> >> ---
> >>   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?

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?