From: Borislav Petkov <bp@alien8.de>
To: Peter Zijlstra <peterz@infradead.org>
Cc: x86@kernel.org, tony.luck@intel.com, pjt@google.com,
linux-kernel@vger.kernel.org, r.marek@assembler.cz,
jpoimboe@redhat.com, jikos@kernel.org,
Dave Hansen <dave.hansen@intel.com>,
Andrew Cooper <andrew.cooper3@citrix.com>
Subject: Re: [RFC PATCH] x86/retpolines: Prevent speculation after RET
Date: Thu, 18 Feb 2021 20:11:38 +0100 [thread overview]
Message-ID: <20210218191138.GH4214@zn.tnic> (raw)
In-Reply-To: <20210218190231.GA59023@worktop.programming.kicks-ass.net>
On Thu, Feb 18, 2021 at 08:02:31PM +0100, Peter Zijlstra wrote:
> On Thu, Feb 18, 2021 at 07:46:39PM +0100, Borislav Petkov wrote:
> > Both vendors speculate after a near RET in some way:
> >
> > Intel:
> >
> > "Unlike near indirect CALL and near indirect JMP, the processor will not
> > speculatively execute the next sequential instruction after a near RET
> > unless that instruction is also the target of a jump or is a target in a
> > branch predictor."
>
> Right, the way I read that means it's not a problem for us here.
Look at that other thread: the instruction *after* the RET can be
speculatively executed if that instruction is the target of a jump or it
is in a branch predictor.
And yes, the text is confusing and no one from Intel has clarified
definitively yet what that text means exactly.
> Now, if AMD were to say something like: hey, that retpoline is pretty
> awesome, we ought to use that instead of an uconditional LFENCE, then
> sure, but as is, I don't think so.
AMD prefers the LFENCE instead of the ratpoline sequence.
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
next prev parent reply other threads:[~2021-02-18 19:43 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-18 16:59 [RFC][PATCH 0/2] x86/retpoline: Retpoline on a diet Peter Zijlstra
2021-02-18 16:59 ` [RFC][PATCH 1/2] x86/retpoline: Simplify retpolines Peter Zijlstra
2021-02-22 11:36 ` Peter Zijlstra
2021-02-18 16:59 ` [RFC][PATCH 2/2] x86/retpoline: Compress retpolines Peter Zijlstra
2021-02-19 7:14 ` Borislav Petkov
2021-02-22 11:27 ` Peter Zijlstra
2021-02-18 18:46 ` [RFC PATCH] x86/retpolines: Prevent speculation after RET Borislav Petkov
2021-02-18 19:02 ` Peter Zijlstra
2021-02-18 19:11 ` Borislav Petkov [this message]
2021-02-19 8:15 ` Peter Zijlstra
2021-02-19 12:08 ` Andrew Cooper
2021-02-19 9:28 ` David Laight
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