Re: [PATCH v3 2/7] arm64/kvm: context-switch ptrauth registers

[Christoffer Dall]

On Mon, Nov 12, 2018 at 10:32:12PM +0000, Catalin Marinas wrote:
> On Fri, Nov 02, 2018 at 09:37:25AM +0100, Christoffer Dall wrote:
> > On Wed, Oct 17, 2018 at 04:17:55PM +0530, Amit Daniel Kachhap wrote:
> > > From: Mark Rutland <mark.rutland@xxxxxxx>
> > > 
> > > When pointer authentication is supported, a guest may wish to use it.
> > > This patch adds the necessary KVM infrastructure for this to work.
> > > 
> > > When we schedule a vcpu, we enable guest usage of pointer
> > > authentication instructions and accesses to the keys. After these are
> > > enabled, we allow context-switching the keys.
> > > 
> > > Pointer authentication consists of address authentication and generic
> > > authentication, and CPUs in a system might have varied support for
> > > either. Where support for either feature is not uniform, it is hidden
> > > from guests via ID register emulation, as a result of the cpufeature
> > > framework in the host.
> > > 
> > > Unfortunately, address authentication and generic authentication cannot
> > > be trapped separately, as the architecture provides a single EL2 trap
> > > covering both. If we wish to expose one without the other, we cannot
> > > prevent a (badly-written) guest from intermittently using a feature
> > > which is not uniformly supported (when scheduled on a physical CPU which
> > > supports the relevant feature). When the guest is scheduled on a
> > > physical CPU lacking the feature, these attempts will result in an UNDEF
> > > being taken by the guest.
> > > 
> > > Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
> > > Signed-off-by: Amit Daniel Kachhap <amit.kachhap@xxxxxxx>
> > > Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
> > > Cc: Christoffer Dall <christoffer.dall@xxxxxxx>
> > > Cc: kvmarm@xxxxxxxxxxxxxxxxxxxxx
> [...] 
> > Two questions:
> > 
> >  - Can we limit all ptrauth functionality to VHE systems so that we
> >    don't need to touch the non-VHE path and so that we don't need any of
> >    the __hyp_text stuff?
> 
> I would say yes. ARMv8.3 implies v8.1, so can enable ptrauth only when
> VHE is built into the kernel and present in the CPU implementation.
> 

Sounds good.

> >  - Can we move all the save/restore logic to vcpu load/put as long as
> >    the host kernel itself isn't using ptrauth, and if the host kernel at
> >    some point begins to use ptrauth, can we have a hook to save/restore
> >    at that time (similar to what we do for FPSIMD) to avoid this
> >    overhead on every switch?
> 
> We will probably enable ptrauth for the kernel as well fairly soon, so I
> don't think we should base the KVM assumption on the no ptrauth in
> kernel use-case.
> 

I assume in this case ptrauth will be used for all of the kernel,
including most of the KVM code?

In that case, I wonder if we always need to context-switch ptrauth
configruation state or if we can be lazy until the guest actually uses
the feature?


Thanks,

    Christoffer