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

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Message-ID: <20191220180815.GE25258@lakrids.cambridge.arm.com>
Date:   Fri, 20 Dec 2019 18:08:15 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Andrew Murray <andrew.murray@....com>
Cc:     Marc Zyngier <marc.zyngier@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 11/18] KVM: arm64: don't trap Statistical Profiling
 controls to EL2

On Fri, Dec 20, 2019 at 02:30:18PM +0000, Andrew Murray 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 <andrew.murray@....com>
> ---
>  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 |
>  				MDCR_EL2_TPMCR |
>  				MDCR_EL2_TDRA |
>  				MDCR_EL2_TDOSA);

I think that this should be conditional on some vcpu feature flag.

If nothing else, this could break existing migration cases otherwise.

Thanks,
Mark.