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Date:   Fri, 26 Jan 2018 09:59:01 -0800
From:   Andi Kleen <ak@linux.intel.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     David Woodhouse <dwmw2@infradead.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Liran Alon <liran.alon@oracle.com>,
        Laura Abbott <labbott@redhat.com>,
        Andrew Lutomirski <luto@kernel.org>,
        Janakarajan Natarajan <Janakarajan.Natarajan@amd.com>,
        Borislav Petkov <bp@suse.de>,
        "Mallick, Asit K" <asit.k.mallick@intel.com>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        KarimAllah Ahmed <karahmed@amazon.de>,
        Peter Anvin <hpa@zytor.com>,
        Jun Nakajima <jun.nakajima@intel.com>,
        Ingo Molnar <mingo@redhat.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        Ashok Raj <ashok.raj@intel.com>,
        "Van De Ven, Arjan" <arjan.van.de.ven@intel.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
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        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
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        Masami Hiramatsu <mhiramat@kernel.org>,
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        Joerg Roedel <joro@8bytes.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        KVM list <kvm@vger.kernel.org>
Subject: Re: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict
 Indirect Branch Speculation
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On Fri, Jan 26, 2018 at 09:19:09AM -0800, Linus Torvalds wrote:
> On Fri, Jan 26, 2018 at 1:11 AM, David Woodhouse <dwmw2@infradead.org> wrote:
> >
> > Do we need to look again at the fact that we've disabled the RSB-
> > stuffing for SMEP?
> 
> Absolutely. SMEP helps make people a lot less worried about things,
> but it doesn't fix the "BTB only contains partial addresses" case.
> 
> But did we do that "disable stuffing with SMEP"? I'm not seeing it. In
> my tree, it's only conditional on X86_FEATURE_RETPOLINE.

For Skylake we need RSB stuffing even with SMEP to avoid falling back to the
BTB on underflow.

It's also always needed with virtualization.

-Andi