[PATCH] KVM: arm64: pkvm: Fixup boot mode to reflect that the kernel resumes from EL1

[Oliver Upton]

On Tue, Oct 11, 2022 at 05:54:00PM +0100, Marc Zyngier wrote:
> The kernel has an awfully complicated boot sequence in order to cope
> with the various EL2 configurations, including those that "enhanced"
> the architecture. We go from EL2 to EL1, then back to EL2, staying
> at EL2 if VHE capable and otherwise go back to EL1.
> 
> Here's a paracetamol tablet for you.

Heh, still have a bit of a headache from this :)

I'm having a hard time following where we skip the EL2 promotion based
on __boot_cpu_mode.

On the cpu_resume() path it looks like we take the return of
init_kernel_el() and pass that along to finalise_el2(). As we are in EL1
at this point, it seems like we'd go init_kernel_el() -> init_el1().

What am I missing?

--
Thanks,
Oliver

> The cpu_resume path follows the same logic, because coming up with
> two versions of a square wheel is hard.
> 
> However, things aren't this straightforward with pKVM, as the host
> resume path is always proxied by the hypervisor, which means that
> the kernel is always entered at EL1. Which contradicts what the
> __boot_cpu_mode[] array contains (it obviously says EL2).
> 
> This thus triggers a HVC call from EL1 to EL2 in a vain attempt
> to upgrade from EL1 to EL2 VHE, which we are, funnily enough,
> reluctant to grant to the host kernel. This is also completely
> unexpected, and puzzles your average EL2 hacker.
> 
> Address it by fixing up the boot mode at the point the host gets
> deprivileged. is_hyp_mode_available() and co already have a static
> branch to deal with this, making it pretty safe.
> 
> Reported-by: Vincent Donnefort <vdonnefort at google.com>
> Signed-off-by: Marc Zyngier <maz at kernel.org>
> ---
>  arch/arm64/kvm/arm.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index b6c9bfa8492f..cf075c9b9ab1 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -2107,6 +2107,17 @@ static int pkvm_drop_host_privileges(void)
>  	 * once the host stage 2 is installed.
>  	 */
>  	static_branch_enable(&kvm_protected_mode_initialized);
> +
> +	/*
> +	 * Fixup the boot mode so that we don't take spurious round
> +	 * trips via EL2 on cpu_resume. Flush to the PoC for a good
> +	 * measure, so that it can be observed by a CPU coming out of
> +	 * suspend with the MMU off.
> +	 */
> +	__boot_cpu_mode[0] = __boot_cpu_mode[1] = BOOT_CPU_MODE_EL1;
> +	dcache_clean_poc((unsigned long)__boot_cpu_mode,
> +			 (unsigned long)(__boot_cpu_mode + 2));
> +
>  	on_each_cpu(_kvm_host_prot_finalize, &ret, 1);
>  	return ret;
>  }
> -- 
> 2.34.1
>