Re: [PATCH 3/4] KVM: x86: allow 256 logical x2APICs again

Radim Krčmář
2014-11-27T20:16:58Z

2014-11-27 21:53+0200, Nadav Amit:
> Radim Krčmář <rkrcmar@redhat.com> wrote:
> > - new->cid_mask = (1 << KVM_X2APIC_CID_BITS) - 1;
> > - new->lid_mask = 0xffff;
> > + new->cid_mask = new->lid_mask = 0xffff;
> You set cid_mask to 0xffff, while there are only 16 clusters. I think it is
> risky (if you twist my hand would come with a scenario).

Let's see :) APIC id is 8 bit, and we compute cluster part of LDR by
taking four upper bits, so 16 is enough.

It isn't the safest programming practice, but we already fail to check
physical_map bounds and any boost to maximal APIC ID is going to require
a rewrite, thus I didn't bother to do it ...

All uses should be covered with the following hunk, I will add it to v2
after all reviews,

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 6c2b8a5..30e4cc1 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -198,7 +198,7 @@ static void recalculate_apic_map(struct kvm *kvm)
cid = apic_cluster_id(new, ldr);
lid = apic_logical_id(new, ldr);

- if (lid)
+ if (lid && cid < ARRAY_SIZE(map->logical_map))
new->logical_map[cid][ffs(lid) - 1] = apic;
}
out:

> Yet, why not to set
> cid_mask to (ARRAY_SIZE(map->logical_map) - 1) ?

We would incorrectly deliver messages intended for high clusters,
it has to be 0xffff.