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References: <20221104213651.141057-1-kim.phillips@amd.com> <20221104213651.141057-4-kim.phillips@amd.com>
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From:   Jim Mattson <jmattson@google.com>
Date:   Fri, 4 Nov 2022 15:00:45 -0700
Message-ID: <CALMp9eSpKGCYK_1r3o326ui5RVoH73_RR5-LR2Div9Jm5zvk6A@mail.gmail.com>
Subject: Re: [PATCH 3/3] x86/speculation: Support Automatic IBRS under virtualization
To:     Kim Phillips <kim.phillips@amd.com>
Cc:     x86@kernel.org, Borislav Petkov <bp@alien8.de>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        Joao Martins <joao.m.martins@oracle.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        David Woodhouse <dwmw@amazon.co.uk>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Juergen Gross <jgross@suse.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Tony Luck <tony.luck@intel.com>,
        Babu Moger <Babu.Moger@amd.com>,
        Tom Lendacky <thomas.lendacky@amd.com>, kvm@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org
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On Fri, Nov 4, 2022 at 2:38 PM Kim Phillips <kim.phillips@amd.com> wrote:
>
> VM Guests may want to use Auto IBRS, so propagate the CPUID to them.
>
> Co-developed-by: Babu Moger <Babu.Moger@amd.com>
> Signed-off-by: Kim Phillips <kim.phillips@amd.com>

The APM says that, under AutoIBRS, CPL0 processes "have IBRS
protection." I'm taking this to mean only that indirect branches in
CPL0 are not subject to steering from a less privileged predictor
mode. This would imply that indirect branches executed at CPL0 in L1
could potentially be subject to steering by code running at CPL0 in
L2, since L1 and L2 share hardware predictor modes.

Fortunately, there is an IBPB when switching VMCBs in svm_vcpu_load().
But it might be worth noting that this is necessary for AutoIBRS to
work (unless it actually isn't).