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

[Amit Daniel Kachhap]

Hi Julien/Kristina,

On 3/21/19 2:26 AM, Kristina Martsenko wrote:
> On 20/03/2019 18:06, Julien Thierry wrote:
> > On 20/03/2019 15:04, Kristina Martsenko wrote:
> > > On 20/03/2019 13:37, Julien Thierry wrote:
> > > > 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.
> > >
> > > [...]
> > >
> > > > > 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...
> > >
> > > Why can't we support the features individually? For example, if we ever
> > > get a system where all CPUs support address authentication and none of
> > > them support generic authentication, then we could still support address
> > > authentication in the guest.
> >
> > That's a good point, I didn't think of that.
> >
> > Although, currently we don't have a way to detect that we are in such a
> > configuration. So as is, both flags are required to enable either
> > feature, and I feel the documentation should be clear on that aspect.
>
> For now we only support enabling both features together, so both flags
> need to be set. I agree that the documentation should be made clear on this.
>
> In the future, if we need to, we can add "negative" cpucaps to detect
> that a feature is absent on all CPUs.
>
> > Another option would be to introduce a flag that enables both for now,
> > and if one day we decide to support the configuration you mentioned we
> > could add "more modular" flags that allow you to control those features
> > individually. While a bit cumbersome, I would find that less awkward
> > than having two flags that only do something if both are present.
>
> That would work too.
>
> I find it more logical to have two flags since there are two features
> (two ID register fields), and KVM enables two features for the guest.
> The fact that KVM does not currently support enabling them separately is
> a KVM implementation choice, and could change in the future.
Kristina, this comments of yours is actually what this patch series is 
trying to do. I should have added more details about the necessity of 
keeping two flags and enhancement of them is actually a future work.

Thanks,
Amit Daniel
> Thanks,
> Kristina