linux-kernel - Re: [PATCH v3 0/4] Distinguish between variants of IBPB

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Message-ID: <bd389021-48d8-26d0-875e-d69cc1d77f1a@amd.com>
Date: Fri, 23 Aug 2024 14:41:32 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: Jim Mattson <jmattson@...gle.com>, 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>,
 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
Subject: Re: [PATCH v3 0/4] Distinguish between variants of IBPB

On 8/23/24 13:53, Jim Mattson 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.
> 
> 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 (4):
>   x86/cpufeatures: Clarify semantics of X86_FEATURE_IBPB
>   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
> 
>  arch/x86/include/asm/cpufeatures.h | 3 ++-
>  arch/x86/kvm/cpuid.c               | 8 ++++++--
>  2 files changed, 8 insertions(+), 3 deletions(-)

For the series:

Reviewed-by: Tom Lendacky <thomas.lendacky@....com>

>