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

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Date:   Tue, 23 Mar 2021 09:47:08 +0000
From:   Quentin Perret <qperret@...gle.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     catalin.marinas@....com, james.morse@....com,
        julien.thierry.kdev@...il.com, suzuki.poulose@....com,
        android-kvm@...gle.com, seanjc@...gle.com, mate.toth-pal@....com,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        kernel-team@...roid.com, kvmarm@...ts.cs.columbia.edu,
        tabba@...gle.com, ardb@...nel.org, mark.rutland@....com,
        dbrazdil@...gle.com
Subject: Re: [PATCH 2/3] KVM: arm64: Generate final CTR_EL0 value when
 running in Protected mode

Hi Marc,

On Monday 22 Mar 2021 at 18:37:14 (+0000), Marc Zyngier wrote:
> Can't say I'm keen on the yucky bit, but here's an alternative (ha!)
> for you:
> 
> diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
> index 1a4cee7eb3c9..7582c3bd2f05 100644
> --- a/arch/arm64/include/asm/assembler.h
> +++ b/arch/arm64/include/asm/assembler.h
> @@ -278,6 +278,9 @@ alternative_else
>  	ldr_l	\reg, arm64_ftr_reg_ctrel0 + ARM64_FTR_SYSVAL
>  alternative_endif
>  #else
> +alternative_if_not ARM64_KVM_PROTECTED_MODE
> +	ASM_BUG()
> +alternative_else_nop_endif
>  alternative_cb kvm_compute_final_ctr_el0
>  	movz	\reg, #0
>  	movk	\reg, #0, lsl #16
> 
> Yes, it is one more instruction, but it is cleaner and allows us to
> from the first patch of the series.
> 
> What do you think?

Yes, I think having the ASM_BUG() in this macro is bit nicer and I doubt
the additional nop will make any difference, so this is looking good to
me!

Thanks,
Quentin