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:

quoted

From: Mark Rutland <mark.rutland@arm.com>

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@arm.com>
Signed-off-by: Amit Daniel Kachhap <redacted>
Cc: Marc Zyngier <redacted>
Cc: Christoffer Dall <redacted>
Cc: kvmarm at lists.cs.columbia.edu

[...] 

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.

 - 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.

-- 
Catalin