Re: [PATCH v7 9/10] KVM: arm64: docs: document KVM support of pointer authentication

[Julien Thierry]

Hi Amit,

On 19/03/2019 08:30, Amit Daniel Kachhap wrote:
> This adds sections for KVM API extension for pointer authentication.
> A brief description about usage of pointer authentication for KVM guests
> is added in the arm64 documentations.
> 
> Signed-off-by: Amit Daniel Kachhap <amit.kachhap@xxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Christoffer Dall <christoffer.dall@xxxxxxx>
> Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
> Cc: kvmarm@xxxxxxxxxxxxxxxxxxxxx
> ---
>  Documentation/arm64/pointer-authentication.txt | 15 +++++++++++----
>  Documentation/virtual/kvm/api.txt              |  6 ++++++
>  2 files changed, 17 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/arm64/pointer-authentication.txt b/Documentation/arm64/pointer-authentication.txt
> index 5baca42..4b769e6 100644
> --- a/Documentation/arm64/pointer-authentication.txt
> +++ b/Documentation/arm64/pointer-authentication.txt
> @@ -87,7 +87,14 @@ used to get and set the keys for a thread.
>  Virtualization
>  --------------
>  
> -Pointer authentication is not currently supported in KVM guests. KVM
> -will mask the feature bits from ID_AA64ISAR1_EL1, and attempted use of
> -the feature will result in an UNDEFINED exception being injected into
> -the guest.
> +Pointer authentication is enabled in KVM guest when each virtual cpu is
> +initialised by passing flags KVM_ARM_VCPU_PTRAUTH_[ADDRESS/GENERIC] and
> +requesting this feature to be enabled. Without this flag, pointer

"Without these flags"*

> +authentication is not enabled in KVM guests and attempted use of the
> +feature will result in an UNDEFINED exception being injected into the
> +guest.
> +
> +Additionally, when these vcpu feature flags are not set then KVM will
> +filter out the Pointer Authentication system key registers from
> +KVM_GET/SET_REG_* ioctls and mask those features from cpufeature ID
> +register.
> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> index 7de9eee..b5c66bc 100644
> --- a/Documentation/virtual/kvm/api.txt
> +++ b/Documentation/virtual/kvm/api.txt
> @@ -2659,6 +2659,12 @@ Possible features:
>  	  Depends on KVM_CAP_ARM_PSCI_0_2.
>  	- KVM_ARM_VCPU_PMU_V3: Emulate PMUv3 for the CPU.
>  	  Depends on KVM_CAP_ARM_PMU_V3.
> +	- KVM_ARM_VCPU_PTRAUTH_ADDRESS:
> +	- KVM_ARM_VCPU_PTRAUTH_GENERIC:
> +	  Enables Pointer authentication for the CPU.
> +	  Depends on KVM_CAP_ARM_PTRAUTH and only on arm64 architecture. If
> +	  set, then the KVM guest allows the execution of pointer authentication
> +	  instructions. Otherwise, KVM treats these instructions as undefined.
>  

Overall I feel one could easily get confused to whether
PTRAUTH_ADDRESS/GENERIC are two individual features, whether one is a
superset of the other, if the names are just an alias of one another, etc...

I think the doc should at least stress out that *both* flags are
required to enable ptrauth in a guest. However it raises the question,
if we don't plan to support the features individually (because we
can't), should we really expose two feature flags? I seems odd to
introduce two flags that only do something if used together...

Cheers,

-- 
Julien Thierry