linux-kernel - Re: [PATCH] x86/srso: Disable the mitigation on unaffected configurations

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Date:   Mon, 14 Aug 2023 09:39:11 +0300
From:   Nikolay Borisov <nik.borisov@...e.com>
To:     Borislav Petkov <bp@...en8.de>, X86 ML <x86@...nel.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/srso: Disable the mitigation on unaffected
 configurations



On 13.08.23 г. 13:45 ч., Borislav Petkov wrote:
> From: "Borislav Petkov (AMD)" <bp@...en8.de>
> 
> Skip the srso cmd line parsing which is not needed on Zen1/2 with SMT
> disabled and with the proper microcode applied (latter should be the
> case anyway) as those are not affected.
> 
> Fixes: 5a15d8348881 ("x86/srso: Tie SBPB bit setting to microcode patch detection")
> Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
> ---
>   arch/x86/kernel/cpu/bugs.c | 7 ++++++-
>   1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index d02f73c5339d..8959a1b9fb80 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -2418,8 +2418,10 @@ static void __init srso_select_mitigation(void)
>   		 * IBPB microcode has been applied.
>   		 */
>   		if ((boot_cpu_data.x86 < 0x19) &&
> -		    (!cpu_smt_possible() || (cpu_smt_control == CPU_SMT_DISABLED)))
> +		    (!cpu_smt_possible() || (cpu_smt_control == CPU_SMT_DISABLED))) {
>   			setup_force_cpu_cap(X86_FEATURE_SRSO_NO);
> +			goto pred_cmd;

Actually can't you simply return, given that zen1/2 never have the SBPB 
flag set? Only zen3/4 with the updated microcode?


> +		}
>   	}
>   
>   	if (retbleed_mitigation == RETBLEED_MITIGATION_IBPB) {
> @@ -2696,6 +2698,9 @@ static ssize_t retbleed_show_state(char *buf)
>   
>   static ssize_t srso_show_state(char *buf)
>   {
> +	if (boot_cpu_has(X86_FEATURE_SRSO_NO))
> +		return sysfs_emit(buf, "Not affected\n");
> +
>   	return sysfs_emit(buf, "%s%s\n",
>   			  srso_strings[srso_mitigation],
>   			  (cpu_has_ibpb_brtype_microcode() ? "" : ", no microcode"));