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Date: Wed, 10 Jan 2018 01:40:25 +0100 (CET) From: Thomas Gleixner <tglx@...utronix.de> To: Linus Torvalds <torvalds@...ux-foundation.org> cc: Andi Kleen <andi@...stfloor.org>, the arch/x86 maintainers <x86@...nel.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, David Woodhouse <dwmw@...zon.co.uk>, Paul Turner <pjt@...gle.com>, Andrew Lutomirski <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Tom Lendacky <thomas.lendacky@....com>, Tim Chen <tim.c.chen@...ux.intel.com>, Greg Kroah-Hartman <gregkh@...ux-foundation.org>, Dave Hansen <dave.hansen@...el.com>, Jiri Kosina <jikos@...nel.org>, Andi Kleen <ak@...ux.intel.com> Subject: Re: [PATCH] x86/retpoline: Fix NOSPEC_JMP for tip On Tue, 9 Jan 2018, Linus Torvalds wrote: > On Tue, Jan 9, 2018 at 4:31 PM, Andi Kleen <andi@...stfloor.org> wrote: > > > > The following patch fixes it for me. Something doesn't > > seem to work with ALTERNATIVE_2. It adds only a few bytes > > more code, so seems acceptable. > > Ugh. It's kind of stupid, though. > > Why is the code sequence not simply: > > ALTERNATIVE "", "lfence", X86_FEATURE_RETPOLINE_AMD > ALTERNATIVE __stringify(jmp *\reg), __stringify(RETPOLINE_JMP \reg), > X86_FEATURE_RETPOLINE > > ie make that X86_FEATURE_RETPOLINE_AMD _only_ emit the "lfence", and > simply fall through to what will be the "jmp *\reg" of the > non-RETPOLINE version. > > Then just make sure X86_FEATURE_RETPOLINE_AMD disables X86_FEATURE_RETPOLINE. > > That is both simpler an dsmaller, no? Duh, yes.
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