Subject: Re: [PATCH v2 11/18] KVM: arm64: don't trap Statistical Profiling controls to EL2

From: Marc Zyngier <[email-masked]>
Date: Sun, 22 Dec 2019 10:42:05 +0000

On Fri, 20 Dec 2019 14:30:18 +0000,
Andrew Murray <[email-masked]> wrote:
> 
> As we now save/restore the profiler state there is no need to trap
> accesses to the statistical profiling controls. Let's unset the
> _TPMS bit.
> 
> Signed-off-by: Andrew Murray <[email-masked]>
> ---
>  arch/arm64/kvm/debug.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
> index 43487f035385..07ca783e7d9e 100644
> --- a/arch/arm64/kvm/debug.c
> +++ b/arch/arm64/kvm/debug.c
> @@ -88,7 +88,6 @@ void kvm_arm_reset_debug_ptr(struct kvm_vcpu *vcpu)
>   *  - Performance monitors (MDCR_EL2_TPM/MDCR_EL2_TPMCR)
>   *  - Debug ROM Address (MDCR_EL2_TDRA)
>   *  - OS related registers (MDCR_EL2_TDOSA)
> - *  - Statistical profiler (MDCR_EL2_TPMS/MDCR_EL2_E2PB)
>   *
>   * Additionally, KVM only traps guest accesses to the debug registers if
>   * the guest is not actively using them (see the KVM_ARM64_DEBUG_DIRTY
> @@ -111,7 +110,6 @@ void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
>  	 */
>  	vcpu->arch.mdcr_el2 = __this_cpu_read(mdcr_el2) & MDCR_EL2_HPMN_MASK;
>  	vcpu->arch.mdcr_el2 |= (MDCR_EL2_TPM |
> -				MDCR_EL2_TPMS |

No. This is an *optional* feature (the guest could not be presented
with the SPE feature, or the the support simply not be compiled in).

If the guest is not allowed to see the feature, for whichever reason,
the traps *must* be enabled and handled.

	M.

-- 
Jazz is not dead, it just smells funny.

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