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

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Message-ID: <20141127201641.GB383@potion.brq.redhat.com>
Date:	Thu, 27 Nov 2014 21:16:42 +0100
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Nadav Amit <nadav.amit@...il.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Paolo Bonzini <pbonzini@...hat.com>,
	Gleb Natapov <gleb@...nel.org>,
	Nadav Amit <namit@...technion.ac.il>
Subject: Re: [PATCH 3/4] KVM: x86: allow 256 logical x2APICs again

2014-11-27 21:53+0200, Nadav Amit:
> Radim Krčmář <rkrcmar@...hat.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.
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