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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>, Andrea Arcangeli <aarcange@redhat.com>,
 Andy Lutomirski <luto@kernel.org>, Ashok Raj <ashok.raj@intel.com>,
 Asit Mallick <asit.k.mallick@intel.com>, Borislav Petkov <bp@suse.de>,
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 "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>, =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?=
 <rkrcmar@redhat.com>, 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