Re: [PATCH v9 4/5] KVM: arm64: Add capability to advertise ptrauth for guest

[Dave Martin <Dave.Martin@arm.com>]

On Fri, Apr 12, 2019 at 08:50:35AM +0530, Amit Daniel Kachhap wrote:
> This patch advertises the capability of two cpu feature called address
> pointer authentication and generic pointer authentication. These
> capabilities depend upon system support for pointer authentication and
> VHE mode.
> 
> The current arm64 KVM partially implements pointer authentication and
> support of address/generic authentication are tied together. However,
> separate ABI requirements for both of them is added so that any future
> isolated implementation will not require any ABI changes.
> 
> 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:
> *  Keep the capability check same for the 2 vcpu ptrauth features. [Dave Martin]
> 
>  Documentation/virtual/kvm/api.txt | 2 ++
>  arch/arm64/kvm/reset.c            | 5 +++++
>  include/uapi/linux/kvm.h          | 2 ++
>  3 files changed, 9 insertions(+)
> 
> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> index 9d202f4..56021d0 100644
> --- a/Documentation/virtual/kvm/api.txt
> +++ b/Documentation/virtual/kvm/api.txt
> @@ -2756,9 +2756,11 @@ Possible features:
>  	- KVM_ARM_VCPU_PTRAUTH_ADDRESS: Enables Address Pointer authentication
>  	  for the CPU and supported only on arm64 architecture.
>  	  Must be requested if KVM_ARM_VCPU_PTRAUTH_GENERIC is also requested.
> +	  Depends on KVM_CAP_ARM_PTRAUTH_ADDRESS.

What if KVM_CAP_ARM_PTRAUTH_ADDRESS is absent and
KVM_ARM_VCPU_PTRAUTH_GENERIC is requested?  By these rules, we have a
contradiction: userspace both must request and must not request
KVM_ARM_VCPU_PTRAUTH_ADDRESS.

We could qualify as follows:

	Depends on KVM_CAP_ARM_PTRAUTH_ADDRESS.
	Must be requested if KVM_CAP_ARM_PTRAUTH_ADDRESS is present and
	KVM_ARM_VCPU_PTRAUTH_GENERIC is also requested.

>  	- KVM_ARM_VCPU_PTRAUTH_GENERIC: Enables Generic Pointer authentication
>  	  for the CPU and supported only on arm64 architecture.
>  	  Must be requested if KVM_ARM_VCPU_PTRAUTH_ADDRESS is also requested.
> +	  Depends on KVM_CAP_ARM_PTRAUTH_GENERIC.

Similarly.

Or, we go back to having a single cap and a single feature, and add
more caps/features later on if we decide it's possible to support
address/generic auth separately later on.

Otherwise, we end up with complex rules that can't be tested.  This is a
high price to pay for forwards compatibility: userspace's conformance to
the rules can't be fully tested, so there's a fair chance it won't work
properly anyway when hardware/KVM with just one auth type appears.

[...]

Thoughts?

Cheers
---Dave