linux-kernel - Re: [PATCH v3 2/6] x86/bugs: Use SBPB in __write

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Message-ID: <1e34033b-3e3a-bd23-af5a-866e68d5a98a@amd.com>
Date: Wed, 2 Apr 2025 15:41:25 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: Josh Poimboeuf <jpoimboe@...nel.org>, x86@...nel.org
Cc: linux-kernel@...r.kernel.org, amit@...nel.org, kvm@...r.kernel.org,
 amit.shah@....com, bp@...en8.de, tglx@...utronix.de, peterz@...radead.org,
 pawan.kumar.gupta@...ux.intel.com, corbet@....net, mingo@...hat.com,
 dave.hansen@...ux.intel.com, hpa@...or.com, seanjc@...gle.com,
 pbonzini@...hat.com, daniel.sneddon@...ux.intel.com, kai.huang@...el.com,
 sandipan.das@....com, boris.ostrovsky@...cle.com, Babu.Moger@....com,
 david.kaplan@....com, dwmw@...zon.co.uk, andrew.cooper3@...rix.com
Subject: Re: [PATCH v3 2/6] x86/bugs: Use SBPB in __write_ibpb() if applicable

On 4/2/25 13:19, Josh Poimboeuf wrote:
> __write_ibpb() does IBPB, which (among other things) flushes branch type
> predictions on AMD.  If the CPU has SRSO_NO, or if the SRSO mitigation
> has been disabled, branch type flushing isn't needed, in which case the
> lighter-weight SBPB can be used.

Maybe add something here that indicates the x86_pred_cmd variable tracks
this optimization so switch to using that variable vs the hardcoded IBPB?

Thanks,
Tom

> 
> Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
> ---
>  arch/x86/entry/entry.S     | 2 +-
>  arch/x86/kernel/cpu/bugs.c | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/entry/entry.S b/arch/x86/entry/entry.S
> index 3a53319988b9..a5b421ec19c0 100644
> --- a/arch/x86/entry/entry.S
> +++ b/arch/x86/entry/entry.S
> @@ -21,7 +21,7 @@
>  SYM_FUNC_START(__write_ibpb)
>  	ANNOTATE_NOENDBR
>  	movl	$MSR_IA32_PRED_CMD, %ecx
> -	movl	$PRED_CMD_IBPB, %eax
> +	movl	_ASM_RIP(x86_pred_cmd), %eax
>  	xorl	%edx, %edx
>  	wrmsr
>  
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 310cb3f7139c..c8b8dc829046 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -58,7 +58,7 @@ EXPORT_SYMBOL_GPL(x86_spec_ctrl_base);
>  DEFINE_PER_CPU(u64, x86_spec_ctrl_current);
>  EXPORT_PER_CPU_SYMBOL_GPL(x86_spec_ctrl_current);
>  
> -u64 x86_pred_cmd __ro_after_init = PRED_CMD_IBPB;
> +u32 x86_pred_cmd __ro_after_init = PRED_CMD_IBPB;
>  EXPORT_SYMBOL_GPL(x86_pred_cmd);
>  
>  static u64 __ro_after_init x86_arch_cap_msr;