linux-kernel - Re: [PATCH v4 0/3] Distinguish between variants of IBPB

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Message-ID: <ZvrPmnUeNHXaS7_E@google.com>
Date: Mon, 30 Sep 2024 09:19:38 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Jim Mattson <jmattson@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.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
Subject: Re: [PATCH v4 0/3] Distinguish between variants of IBPB

On Fri, Sep 27, 2024, Jim Mattson wrote:
> On Fri, Sep 13, 2024 at 10:32 AM Jim Mattson <jmattson@...gle.com> wrote:
> >
> > Prior to Zen4, AMD's IBPB did not flush the RAS (or, in Intel
> > terminology, the RSB). Hence, the older version of AMD's IBPB was not
> > equivalent to Intel's IBPB. However, KVM has been treating them as
> > equivalent, synthesizing Intel's CPUID.(EAX=7,ECX=0):EDX[bit 26] on any
> > platform that supports the synthetic features X86_FEATURE_IBPB and
> > X86_FEATURE_IBRS.
> >
> > Equivalence also requires a previously ignored feature on the AMD side,
> > CPUID Fn8000_0008_EBX[IBPB_RET], which is enumerated on Zen4.
> >
> > v4: Added "guaranteed" to X86_FEATURE_IBPB comment [Pawan]
> >     Changed logic for deducing AMD IBPB features from Intel IBPB features
> >     in kvm_set_cpu_caps [Tom]
> >     Intel CPUs that suffer from PBRSB can't claim AMD_IBPB_RET [myself]
> >
> > v3: Pass through IBPB_RET from hardware to userspace. [Tom]
> >     Derive AMD_IBPB from X86_FEATURE_SPEC_CTRL rather than
> >     X86_FEATURE_IBPB. [Tom]
> >     Clarify semantics of X86_FEATURE_IBPB.
> >
> > v2: Use IBPB_RET to identify semantic equality. [Venkatesh]
> >
> > Jim Mattson (3):
> >   x86/cpufeatures: Define X86_FEATURE_AMD_IBPB_RET
> >   KVM: x86: Advertise AMD_IBPB_RET to userspace
> >   KVM: x86: AMD's IBPB is not equivalent to Intel's IBPB
> 
> Oops. I forgot to include the v3 responses:
> 
> > For the series:
> >
> > Reviewed-by: Tom Lendacky <thomas.lendacky@....com>
> 
> and
> 
> > Assuming this goes through the KVM tree:
> >
> > Reviewed-by: Thomas Gleixner <tglx@...utronix.de>
> 
> The only substantive change was to patch 3/3.
> 
> Sean: Are you willing to take this through KVM/x86?

Yep, and I can fixup the reviews when applying.