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

[Amit Daniel Kachhap <amit.kachhap@arm.com>]

Hi,

On 4/17/19 2:05 PM, Marc Zyngier wrote:
> 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@arm.com>
>> Cc: Mark Rutland <mark.rutland@arm.com>
>> Cc: Marc Zyngier <marc.zyngier@arm.com>
>> Cc: Christoffer Dall <christoffer.dall@arm.com>
>> Cc: kvmarm@lists.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))

In the subsequent patches, vcpu->arch.flags is only set to 
KVM_ARM64_GUEST_HAS_PTRAUTH when all host capability check conditions 
matches such as system_supports_address_auth(), 
system_supports_generic_auth() so doing them again is repetitive in my view.

Thanks,
Amit D

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