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

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Message-ID: <CALMp9eSZX_fEy6=wWr=HY_6kDULE6-8_16cRGgfjoVhGguF7AQ@mail.gmail.com>
Date: Mon, 7 Oct 2024 10:38:01 -0700
From: Jim Mattson <jmattson@...gle.com>
To: Borislav Petkov <bp@...en8.de>
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 Mon, Oct 7, 2024 at 7:30 AM Borislav Petkov <bp@...en8.de> wrote:
>
> 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?

Bad git hygiene. This should have been a 4 patch set, not a 3 patch
set. Sigh. I'll send out v5.