linux-kernel - Re: [PATCH v4 09/12] arm64: Use firmware to detect CPUs that are not affected by Spectre-v2

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Message-ID: <20190131175502.12bed0f0@donnerap.cambridge.arm.com>
Date:   Thu, 31 Jan 2019 17:55:02 +0000
From:   Andre Przywara <andre.przywara@....com>
To:     Jeremy Linton <jeremy.linton@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, stefan.wahren@...e.com,
        mlangsdo@...hat.com, suzuki.poulose@....com, marc.zyngier@....com,
        catalin.marinas@....com, julien.thierry@....com,
        will.deacon@....com, linux-kernel@...r.kernel.org,
        steven.price@....com, ykaukab@...e.de, dave.martin@....com,
        shankerd@...eaurora.org
Subject: Re: [PATCH v4 09/12] arm64: Use firmware to detect CPUs that are
 not affected by Spectre-v2

On Fri, 25 Jan 2019 12:07:08 -0600
Jeremy Linton <jeremy.linton@....com> wrote:

> From: Marc Zyngier <marc.zyngier@....com>
> 
> The SMCCC ARCH_WORKAROUND_1 service can indicate that although the
> firmware knows about the Spectre-v2 mitigation, this particular
> CPU is not vulnerable, and it is thus not necessary to call
> the firmware on this CPU.
> 
> Let's use this information to our benefit.
> 
> Signed-off-by: Marc Zyngier <marc.zyngier@....com>
> Signed-off-by: Jeremy Linton <jeremy.linton@....com>

Yes, I stumbled over this in the firmware spec as well:

Reviewed-by: Andre Przywara <andre.przywara@....com>

Cheers,
Andre.

> ---
>  arch/arm64/kernel/cpu_errata.c | 32 +++++++++++++++++++++++---------
>  1 file changed, 23 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
> index 4d23b4d4cfa8..024c83ffff99 100644
> --- a/arch/arm64/kernel/cpu_errata.c
> +++ b/arch/arm64/kernel/cpu_errata.c
> @@ -217,22 +217,36 @@ static int detect_harden_bp_fw(void)
>  	case PSCI_CONDUIT_HVC:
>  		arm_smccc_1_1_hvc(ARM_SMCCC_ARCH_FEATURES_FUNC_ID,
>  				  ARM_SMCCC_ARCH_WORKAROUND_1, &res);
> -		if ((int)res.a0 < 0)
> +		switch ((int)res.a0) {
> +		case 1:
> +			/* Firmware says we're just fine */
> +			return 0;
> +		case 0:
> +			cb = call_hvc_arch_workaround_1;
> +			/* This is a guest, no need to patch KVM vectors */
> +			smccc_start = NULL;
> +			smccc_end = NULL;
> +			break;
> +		default:
>  			return -1;
> -		cb = call_hvc_arch_workaround_1;
> -		/* This is a guest, no need to patch KVM vectors */
> -		smccc_start = NULL;
> -		smccc_end = NULL;
> +		}
>  		break;
>  
>  	case PSCI_CONDUIT_SMC:
>  		arm_smccc_1_1_smc(ARM_SMCCC_ARCH_FEATURES_FUNC_ID,
>  				  ARM_SMCCC_ARCH_WORKAROUND_1, &res);
> -		if ((int)res.a0 < 0)
> +		switch ((int)res.a0) {
> +		case 1:
> +			/* Firmware says we're just fine */
> +			return 0;
> +		case 0:
> +			cb = call_smc_arch_workaround_1;
> +			smccc_start = __smccc_workaround_1_smc_start;
> +			smccc_end = __smccc_workaround_1_smc_end;
> +			break;
> +		default:
>  			return -1;
> -		cb = call_smc_arch_workaround_1;
> -		smccc_start = __smccc_workaround_1_smc_start;
> -		smccc_end = __smccc_workaround_1_smc_end;
> +		}
>  		break;
>  
>  	default: