Re: [PATCH v2] hw/arm/virt: KVM: Enable PAuth when supported by the host

[Richard Henderson <richard.henderson@linaro.org>]

On 1/6/22 1:16 AM, Marc Zyngier wrote:
>>> +static bool kvm_arm_pauth_supported(void)
>>> +{
>>> +    return (kvm_check_extension(kvm_state, KVM_CAP_ARM_PTRAUTH_ADDRESS) &&
>>> +            kvm_check_extension(kvm_state, KVM_CAP_ARM_PTRAUTH_GENERIC));
>>> +}
>>
>> Do we really need to have them both set to play the game?  Given that
>> the only thing that happens is that we disable whatever host support
>> exists, can we have "pauth enabled" mean whatever subset the host has?
> 
> The host will always expose either both features or none, and that's
> part of the ABI. From the bit of kernel documentation located in
> Documentation/virt/kvm/api.rst:
> 
> <quote>
> 4.82 KVM_ARM_VCPU_INIT
> ----------------------
> [...]
>          - KVM_ARM_VCPU_PTRAUTH_ADDRESS: Enables Address Pointer authentication
>            for arm64 only.
>            Depends on KVM_CAP_ARM_PTRAUTH_ADDRESS.
>            If KVM_CAP_ARM_PTRAUTH_ADDRESS and KVM_CAP_ARM_PTRAUTH_GENERIC are
>            both present, then both KVM_ARM_VCPU_PTRAUTH_ADDRESS and
>            KVM_ARM_VCPU_PTRAUTH_GENERIC must be requested or neither must be
>            requested.
> 
>          - KVM_ARM_VCPU_PTRAUTH_GENERIC: Enables Generic Pointer authentication
>            for arm64 only.
>            Depends on KVM_CAP_ARM_PTRAUTH_GENERIC.
>            If KVM_CAP_ARM_PTRAUTH_ADDRESS and KVM_CAP_ARM_PTRAUTH_GENERIC are
>            both present, then both KVM_ARM_VCPU_PTRAUTH_ADDRESS and
>            KVM_ARM_VCPU_PTRAUTH_GENERIC must be requested or neither must be
>            requested.
> </quote>
> 
> KVM will reject the initialisation if only one of the features is
> requested, so checking and enabling both makes sense to me.

Well, no, that's not what that says.  It says that *if* both host flags are set, then both 
guest flags must be set or both unset.

It's probably all academic anyway, because I can't actually imagine a vendor implementing 
ADDR and not GENERIC, but in theory we ought to be able to support a host with only ADDR.


r~