Re: [PATCH v5 21/21] KVM: selftests: Add xAPIC support in IRQ test

[Sean Christopherson <seanjc@google.com>]

On Thu, Jun 04, 2026, sashiko-bot@kernel.org wrote:
> > diff --git a/tools/testing/selftests/kvm/irq_test.c b/tools/testing/selftests/kvm/irq_test.c
> > index 41c50caa928e2..7e421cb7bc69b 100644
> > --- a/tools/testing/selftests/kvm/irq_test.c
> > +++ b/tools/testing/selftests/kvm/irq_test.c
> [ ... ]
> > @@ -31,14 +32,20 @@ static bool done;
> >  
> >  static u32 guest_get_vcpu_id(void)
> >  {
> > -	return x2apic_read_reg(APIC_ID);
> > +	if (x2apic)
> > +		return x2apic_read_reg(APIC_ID);
> > +	else
> > +		return xapic_read_reg(APIC_ID) >> 24;
> >  }
> 
> [Severity: Medium]
> Does this truncation to 8 bits cause the test to silently hang if run with
> 256 or more vCPUs in xAPIC mode?

Yep.

> For vCPU IDs 256 and above, the 8-bit xAPIC ID will wrap around.

But not because of wraparound (it's amusing how bad AI is at basic math, when
it's so good with advanced stuff).  The test will use IDs [0-255], not [1-256].
The limit with xAPIC is specifically 255 vCPUs, not 256, because 0xff (i.e. ID
255) is a special broadcast value.