Re: [PATCH 0/4] KVM/arm64: Enable PtrAuth on non-VHE KVM

Dave Martin
2020-06-15T12:59:26Z

On Mon, Jun 15, 2020 at 09:19:50AM +0100, Marc Zyngier wrote:
> Not having PtrAuth on non-VHE KVM (for whatever reason VHE is not
> enabled on a v8.3 system) has always looked like an oddity. This
> trivial series remedies it, and allows a non-VHE KVM to offer PtrAuth
> to its guests.

How likely do you think it is that people will use such a configuration?

The only reason I can see for people to build a kernel with CONFIG_VHE=n
is as a workaround for broken hardware, or because the kernel is too old
to support VHE (in which case it doesn't understand ptrauth either, so
it is irrelevant whether ptrauth depends on VHE).

I wonder whether it's therefore better to "encourage" people to turn
VHE on by making subsequent features depend on it where appropriate.
We do want multiplatform kernels to be configured with CONFIG_VHE=y for
example.

I ask this, because SVE suffers the same "oddity". If SVE can be
enabled for non-VHE kernels straightforwardly then there's no reason not
to do so, but I worried in the past that this would duplicate complex
code that would never be tested or used.

If supporting ptrauth with !VHE is as simple as this series suggests,
then it's low-risk. Perhaps SVE isn't much worse. I was chasing nasty
bugs around at the time the SVE KVM support was originally written, and
didn't want to add more unknowns into the mix...

(Note, this is not an offer from me to do the SVE work!)

[...]

Cheers
---Dave