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Date:   Wed, 11 Jan 2023 13:51:03 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     Breno Leitao <leitao@debian.org>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Cc:     tglx@linutronix.de, mingo@redhat.com, dave.hansen@linux.intel.com,
        hpa@zytor.com, jpoimboe@kernel.org, peterz@infradead.org,
        x86@kernel.org, cascardo@canonical.com, leit@meta.com,
        kexec@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3] x86/bugs: Explicitly clear speculative MSR bits
Message-ID: <Y76wtwWN1+XPbnQx@zn.tnic>
References: <20221128153148.1129350-1-leitao@debian.org>
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On Mon, Nov 28, 2022 at 07:31:48AM -0800, Breno Leitao wrote:
> Currently x86_spec_ctrl_base is read at boot time, and speculative bits
> are set if configs are enable, such as MSR[SPEC_CTRL_IBRS] is enabled if
> CONFIG_CPU_IBRS_ENTRY is configured. These MSR bits are not cleared if
> the mitigations are disabled.
> 
> This is a problem when kexec-ing a kernel that has the mitigation
> disabled, from a kernel that has the mitigation enabled. In this case,
> the MSR bits are carried forward and not cleared at the boot of the new
> kernel. This might have some performance degradation that is hard to
> find.
> 
> This problem does not happen if the machine is (hard) rebooted, because
> the bit will be cleared by default.
> 
> Suggested-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
> Signed-off-by: Breno Leitao <leitao@debian.org>
> ---
>  arch/x86/include/asm/msr-index.h |  4 ++++
>  arch/x86/kernel/cpu/bugs.c       | 10 +++++++++-
>  2 files changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
> index 4a2af82553e4..22986a8f18bc 100644
> --- a/arch/x86/include/asm/msr-index.h
> +++ b/arch/x86/include/asm/msr-index.h
> @@ -54,6 +54,10 @@
>  #define SPEC_CTRL_RRSBA_DIS_S_SHIFT	6	   /* Disable RRSBA behavior */
>  #define SPEC_CTRL_RRSBA_DIS_S		BIT(SPEC_CTRL_RRSBA_DIS_S_SHIFT)
>  
> +/* A mask for bits which the kernel toggles when controlling mitigations */
> +#define SPEC_CTRL_MITIGATIONS_MASK	(SPEC_CTRL_IBRS | SPEC_CTRL_STIBP | SPEC_CTRL_SSBD \
> +							| SPEC_CTRL_RRSBA_DIS_S)

SPEC_CTRL_RRSBA_DIS_S is a disable bit and I presume it needs to stay enabled.
Only when spec_ctrl_disable_kernel_rrsba() runs. And I'd say perf-wise it
doesn't cost that much...

Pawan?

> +
>  #define MSR_IA32_PRED_CMD		0x00000049 /* Prediction Command */
>  #define PRED_CMD_IBPB			BIT(0)	   /* Indirect Branch Prediction Barrier */
>  
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 3e3230cccaa7..4030358216c8 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -137,8 +137,16 @@ void __init check_bugs(void)
>  	 * have unknown values. AMD64_LS_CFG MSR is cached in the early AMD
>  	 * init code as it is not enumerated and depends on the family.
>  	 */
> -	if (boot_cpu_has(X86_FEATURE_MSR_SPEC_CTRL))
> +	if (boot_cpu_has(X86_FEATURE_MSR_SPEC_CTRL)) {
>  		rdmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
> +		/*
> +		 * Previously running software, like kexec for example, may
> +		 * have some controls turned ON.
> +		 * Clear them and let the mitigations setup below set them
> +		 * based on configuration.
> +		 */

                /*
                 * Previously running kernel (kexec), may have some controls
                 * turned ON. Clear them and let the mitigations setup below
                 * rediscover them based on configuration.
                 */

There's no "previously running software, like kexec".

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette