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

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Date:	Thu, 27 Nov 2014 22:39:45 +0200
From:	Nadav Amit <nadav.amit@...il.com>
To:	Radim Krčmář <rkrcmar@...hat.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

Radim Krčmář <rkrcmar@...hat.com> wrote:

> 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.
To clarify my concern - I am worried that some of the CPUs are still in
xAPIC mode with LDR that does not follow x2APIC LDR scheme.

> 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.
>From the SDM, I am not sure you are correct, but your solution is fine.

Nadav

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