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To: Jim Mattson <jmattson@google.com>, Ingo Molnar <mingo@redhat.com>,
 Borislav Petkov <bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>,
 "H. Peter Anvin" <hpa@zytor.com>, Sean Christopherson <seanjc@google.com>,
 Paolo Bonzini <pbonzini@redhat.com>, Pawan Gupta
 <pawan.kumar.gupta@linux.intel.com>, Josh Poimboeuf <jpoimboe@kernel.org>,
 Jim Mattson <jmattson@google.com>, Sandipan Das <sandipan.das@amd.com>,
 Kai Huang <kai.huang@intel.com>, x86@kernel.org,
 linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Subject: Re: [PATCH v3 0/4] Distinguish between variants of IBPB
In-Reply-To: <20240823185323.2563194-1-jmattson@google.com>
References: <20240823185323.2563194-1-jmattson@google.com>
Date: Sun, 25 Aug 2024 14:17:04 +0200
Message-ID: <875xrog5kv.ffs@tglx>
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On Fri, Aug 23 2024 at 11:53, 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]

Assuming this goes through the KVM tree:

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>