linux-kernel - Re: [PATCH v4 1/3] x86/cpufeatures: Define X86_FEATURE_AMD_IBPB

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Message-ID: <20241007143019.GAZwPwe286itXE2Wj2@fat_crate.local>
Date: Mon, 7 Oct 2024 16:30:19 +0200
From: Borislav Petkov <bp@...en8.de>
To: Jim Mattson <jmattson@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Sean Christopherson <seanjc@...gle.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Sandipan Das <sandipan.das@....com>,
	Kai Huang <kai.huang@...el.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Venkatesh Srinivas <venkateshs@...omium.org>
Subject: Re: [PATCH v4 1/3] x86/cpufeatures: Define X86_FEATURE_AMD_IBPB_RET

On Fri, Sep 13, 2024 at 10:32:27AM -0700, Jim Mattson wrote:
> AMD's initial implementation of IBPB did not clear the return address
> predictor. Beginning with Zen4, AMD's IBPB *does* clear the return
> address predictor. This behavior is enumerated by
> CPUID.80000008H:EBX.IBPB_RET[bit 30].
> 
> Define X86_FEATURE_AMD_IBPB_RET for use in KVM_GET_SUPPORTED_CPUID,
> when determining cross-vendor capabilities.
> 
> Suggested-by: Venkatesh Srinivas <venkateshs@...omium.org>
> Signed-off-by: Jim Mattson <jmattson@...gle.com>
> ---
>  arch/x86/include/asm/cpufeatures.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index cabd6b58e8ec..a222a24677d7 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -215,7 +215,7 @@
>  #define X86_FEATURE_SPEC_STORE_BYPASS_DISABLE	( 7*32+23) /* Disable Speculative Store Bypass. */
>  #define X86_FEATURE_LS_CFG_SSBD		( 7*32+24)  /* AMD SSBD implementation via LS_CFG MSR */
>  #define X86_FEATURE_IBRS		( 7*32+25) /* "ibrs" Indirect Branch Restricted Speculation */
> -#define X86_FEATURE_IBPB		( 7*32+26) /* "ibpb" Indirect Branch Prediction Barrier without RSB flush */

I see upstream

#define X86_FEATURE_IBPB		( 7*32+26) /* "ibpb" Indirect Branch Prediction Barrier */

Where does "without RSB flush" come from?

-- 
Regards/Gruss,
    Boris.

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