Re: [PATCH v8 8/9] KVM: arm64: Add capability to advertise ptrauth for guest

[Amit Daniel Kachhap]

Hi,

On 4/5/19 4:33 PM, Dave Martin wrote:
> On Tue, Apr 02, 2019 at 07:57:16AM +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 the future
> > isolated implementation will not require any ABI changes.
> >
> > Signed-off-by: Amit Daniel Kachhap <amit.kachhap@xxxxxxx>
> > Cc: Mark Rutland <mark.rutland@xxxxxxx>
> > Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
> > Cc: Christoffer Dall <christoffer.dall@xxxxxxx>
> > Cc: kvmarm@xxxxxxxxxxxxxxxxxxxxx
> > ---
> >
> > Changes since v7:
> > * Created 2 capabilities KVM_CAP_ARM_PTRAUTH_ADDRESS and KVM_CAP_ARM_PTRAUTH_GENERIC
> >    instead of one KVM_CAP_ARM_PTRAUTH [Kristina Martsenko].
> > * Added documentation here itself instead of in a new patch.
> >
> >   Documentation/virtual/kvm/api.txt | 3 +++
> >   arch/arm64/kvm/reset.c            | 6 ++++++
> >   include/uapi/linux/kvm.h          | 2 ++
> >   3 files changed, 11 insertions(+)
> >
> > diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> > index aaa048d..9b56892 100644
> > --- a/Documentation/virtual/kvm/api.txt
> > +++ b/Documentation/virtual/kvm/api.txt
> > @@ -2661,8 +2661,11 @@ Possible features:
> >   	  Depends on KVM_CAP_ARM_PMU_V3.
> >   	- KVM_ARM_VCPU_PTRAUTH_ADDRESS: Enables Address Pointer authentication
> >   	  for the CPU and supported only on arm64 architecture.
> > +	  Depends on KVM_CAP_ARM_PTRAUTH_ADDRESS.
> >   	- KVM_ARM_VCPU_PTRAUTH_GENERIC: Enables Generic Pointer authentication
> >   	  for the CPU and supported only on arm64 architecture.
> > +	  Depends on KVM_CAP_ARM_PTRAUTH_GENERIC.
> >   
> >   
> >   4.83 KVM_ARM_PREFERRED_TARGET
> > diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
> > index 717afed..8aa8982 100644
> > --- a/arch/arm64/kvm/reset.c
> > +++ b/arch/arm64/kvm/reset.c
> > @@ -92,6 +92,12 @@ int kvm_arch_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> >   	case KVM_CAP_ARM_VM_IPA_SIZE:
> >   		r = kvm_ipa_limit;
> >   		break;
> > +	case KVM_CAP_ARM_PTRAUTH_ADDRESS:
> > +		r = has_vhe() && system_supports_address_auth();
> > +		break;
> > +	case KVM_CAP_ARM_PTRAUTH_GENERIC:
> > +		r = has_vhe() && system_supports_generic_auth();
> > +		break;
>
> If some hardware supports just one auth type, we would report just one
> of these caps.  Although we have the rule that userspace is not allowed
> to request these independently in KVM_ARM_VCPU_INIT anyway, I think it
> would be easier for userspace if we suppress both caps if either auth
> type isn't available on the host.  e.g.:
>
> 	case KVM_ARM_ARM_PTRAUTH_ADDRESS:
> 	case KVM_ARM_ARM_PTRAUTH_GENERIC:
> 		r = has_vhe() && system_supports_address_auth() &&
> 			system_supports_generic_auth();
>
> We could revert back to the above code later on, and apply the ABI
> relaxations described in my response to the vcpu features patch, if
> someday we add support to KVM for coping with host hardware that
> supports just one auth type.
These 2 different capabilities are introduced in this iteration so I was 
not clear whether to expose the suppression in capability ioctl level or 
KVM_ARM_VCPU_INIT ioctl level. But agree that this way will be more 
clearer to userspace.
> I'd like Mark to comment on this, since he's more aware of the
> architectural situation than I am.
ok.

Thanks,
Amit D.
> Cheers
> ---Dave