linux-kernel - Re: [PATCH v9 1/5] KVM: arm64: Add a vcpu flag to control ptrauth for guest

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Date:   Wed, 17 Apr 2019 09:35:34 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     Amit Daniel Kachhap <amit.kachhap@....com>,
        linux-arm-kernel@...ts.infradead.org
Cc:     Christoffer Dall <christoffer.dall@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Andrew Jones <drjones@...hat.com>,
        Dave Martin <Dave.Martin@....com>,
        Ramana Radhakrishnan <ramana.radhakrishnan@....com>,
        kvmarm@...ts.cs.columbia.edu,
        Kristina Martsenko <kristina.martsenko@....com>,
        linux-kernel@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry@....com>
Subject: Re: [PATCH v9 1/5] KVM: arm64: Add a vcpu flag to control ptrauth for
 guest

On 12/04/2019 04:20, Amit Daniel Kachhap wrote:
> A per vcpu flag is added to check if pointer authentication is
> enabled for the vcpu or not. This flag may be enabled according to
> the necessary user policies and host capabilities.
> 
> This patch also adds a helper to check the flag.
> 
> Signed-off-by: Amit Daniel Kachhap <amit.kachhap@....com>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Marc Zyngier <marc.zyngier@....com>
> Cc: Christoffer Dall <christoffer.dall@....com>
> Cc: kvmarm@...ts.cs.columbia.edu
> ---
> 
> Changes since v8:
> * Added a new per vcpu flag which will store Pointer Authentication enable
>   status instead of checking them again. [Dave Martin]
> 
>  arch/arm64/include/asm/kvm_host.h | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 9d57cf8..31dbc7c 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -355,10 +355,14 @@ struct kvm_vcpu_arch {
>  #define KVM_ARM64_HOST_SVE_ENABLED	(1 << 4) /* SVE enabled for EL0 */
>  #define KVM_ARM64_GUEST_HAS_SVE		(1 << 5) /* SVE exposed to guest */
>  #define KVM_ARM64_VCPU_SVE_FINALIZED	(1 << 6) /* SVE config completed */
> +#define KVM_ARM64_GUEST_HAS_PTRAUTH	(1 << 7) /* PTRAUTH exposed to guest */
>  
>  #define vcpu_has_sve(vcpu) (system_supports_sve() && \
>  			    ((vcpu)->arch.flags & KVM_ARM64_GUEST_HAS_SVE))
>  
> +#define vcpu_has_ptrauth(vcpu)	\
> +			((vcpu)->arch.flags & KVM_ARM64_GUEST_HAS_PTRAUTH)
> +

Just as for SVE, please first check that the system has PTRAUTH.
Something like:

		(cpus_have_const_cap(ARM64_HAS_GENERIC_AUTH_ARCH) && \
		 ((vcpu)->arch.flags & KVM_ARM64_GUEST_HAS_PTRAUTH))

This will save an extra load on unsuspecting CPUs thanks to the static
key embedded in the capability structure.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...