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Subject: Re: [RFC,05/10] x86/speculation: Add basic IBRS support
 infrastructure
To:     Eduardo Habkost <ehabkost@redhat.com>,
        David Woodhouse <dwmw2@infradead.org>
Cc:     KarimAllah Ahmed <karahmed@amazon.de>,
        linux-kernel@vger.kernel.org, Andi Kleen <ak@linux.intel.com>,
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        Ashok Raj <ashok.raj@intel.com>,
        Asit Mallick <asit.k.mallick@intel.com>,
        Borislav Petkov <bp@suse.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "H . Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        Janakarajan Natarajan <Janakarajan.Natarajan@amd.com>,
        Joerg Roedel <joro@8bytes.org>,
        Jun Nakajima <jun.nakajima@intel.com>,
        Laura Abbott <labbott@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
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        Thomas Gleixner <tglx@linutronix.de>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Tom Lendacky <thomas.lendacky@amd.com>, kvm@vger.kernel.org,
        x86@kernel.org, "Dr. David Alan Gilbert" <dgilbert@redhat.com>
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From:   Arjan van de Ven <arjan@linux.intel.com>
Message-ID: <31415b7f-9c76-c102-86cd-6bf4e23e3aee@linux.intel.com>
Date:   Mon, 29 Jan 2018 12:44:16 -0800
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On 1/29/2018 12:42 PM, Eduardo Habkost wrote:
> The question is how the hypervisor could tell that to the guest.
> If Intel doesn't give us a CPUID bit that can be used to tell
> that retpolines are enough, maybe we should use a hypervisor
> CPUID bit for that?

the objective is to have retpoline be safe everywhere and never use IBRS
(Linus was also pretty clear about that) so I'm confused by your question