Re: [PATCH 2/3] KVM: arm64: Generate final CTR_EL0 value when running in Protected mode

[Quentin Perret]

Hey Marc,

On Monday 22 Mar 2021 at 16:48:27 (+0000), Marc Zyngier wrote:
> In protected mode, late CPUs are not allowed to boot (enforced by
> the PSCI relay). We can thus specialise the read_ctr macro to
> always return a pre-computed, sanitised value.
> 
> Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
> ---
>  arch/arm64/include/asm/assembler.h | 9 +++++++++
>  arch/arm64/kernel/image-vars.h     | 1 +
>  arch/arm64/kvm/va_layout.c         | 7 +++++++
>  3 files changed, 17 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
> index fb651c1f26e9..1a4cee7eb3c9 100644
> --- a/arch/arm64/include/asm/assembler.h
> +++ b/arch/arm64/include/asm/assembler.h
> @@ -270,12 +270,21 @@ alternative_endif
>   * provide the system wide safe value from arm64_ftr_reg_ctrel0.sys_val
>   */
>  	.macro	read_ctr, reg
> +#ifndef __KVM_NVHE_HYPERVISOR__
>  alternative_if_not ARM64_MISMATCHED_CACHE_TYPE
>  	mrs	\reg, ctr_el0			// read CTR
>  	nop
>  alternative_else
>  	ldr_l	\reg, arm64_ftr_reg_ctrel0 + ARM64_FTR_SYSVAL
>  alternative_endif
> +#else
> +alternative_cb kvm_compute_final_ctr_el0
> +	movz	\reg, #0
> +	movk	\reg, #0, lsl #16
> +	movk	\reg, #0, lsl #32
> +	movk	\reg, #0, lsl #48
> +alternative_cb_end
> +#endif
>  	.endm

So, FWIW, if we wanted to make _this_ macro BUG in non-protected mode
(and drop patch 01), I think we could do something like:

alternative_cb kvm_compute_final_ctr_el0
	movz	\reg, #0
	ASM_BUG()
	nop
	nop
alternative_cb_end

and then make kvm_compute_final_ctr_el0() check that we're in protected
mode before patching. That would be marginally better as that would
cover _all_ users of read_ctr and not just __flush_dcache_area, but that
first movz is a bit yuck (but necessary to keep generate_mov_q() happy I
think?), so I'll leave the decision to you.

No objection from me for the current implementation, and if you decide to
go with it:

Reviewed-by: Quentin Perret <qperret@xxxxxxxxxx>

Thanks,
Quentin