Re: [PATCH 4/6] KVM: x86: Fix determining flat mode in recalculate_apic_map

[Gleb Natapov <gleb@kernel.org>]

On Tue, Sep 30, 2014 at 08:49:17PM +0300, Nadav Amit wrote:
> Determining flat mode according to cid_mask is wrong, since currently KVM
> supports zero clusters in x2apic mode. Use ldr_bits instead.
As a comment above the 'if' you are fixing says the code assumes all APICs are in
the same mode (if they are not that's an OS bug). In that case the code never gets
here with x2apic mode.

> 
> Since we recalculate the apic map whenever the DFR is changed, the bug appears
> to have no effect, and perhaps the entire check can be removed.
> 
> Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
> ---
>  arch/x86/kvm/lapic.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index b8345dd..cfce429 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -171,7 +171,7 @@ static void recalculate_apic_map(struct kvm *kvm)
>  			new->cid_mask = (1 << KVM_X2APIC_CID_BITS) - 1;
>  			new->lid_mask = 0xffff;
>  		} else if (kvm_apic_sw_enabled(apic) &&
> -				!new->cid_mask /* flat mode */ &&
> +				new->ldr_bits == 8 /* flat mode */ &&
>  				kvm_apic_get_reg(apic, APIC_DFR) == APIC_DFR_CLUSTER) {
>  			new->cid_shift = 4;
>  			new->cid_mask = 0xf;
> -- 
> 1.9.1
> 
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--
			Gleb.