linux-kernel - Re: [PATCH] x86/retpoline: Also fill return buffer after idle

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Date:   Tue, 9 Jan 2018 15:12:15 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Andi Kleen <andi@...stfloor.org>, pjt@...gle.com,
        linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
        gregkh@...ux-foundation.org, tim.c.chen@...ux.intel.com,
        dave.hansen@...el.com, tglx@...utronix.de, luto@...capital.net,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] x86/retpoline: Also fill return buffer after idle

On Tue, Jan 09, 2018 at 01:58:38PM +0000, David Woodhouse wrote:
> Clever GCC, have biscuit.

Well, we requested this feature exactly because of this. It had better
work.

> However, you are suggesting that we turn the static_cpu_has() trick
> from a "nice to have" optimisation which is all very well when it pans
> out, to something we *rely* on for secure operation of the system.

It must work, we 'rely' on it already. GCC doing something stupid there
is a GCC bug. Any GCC bug is a royal pain, they happen, life goes on.

> It never ends well when we rely on all versions of GCC optimising
> things precisely how we want.
>
> If you can build in a sanity check to ensure that the build will *fail*
> when GCC doesn't do what we want, I suppose we could live with that.
> But we don't have such a sanity check at the moment, do we?

We have STATIC_KEYS_SELFTEST, which might or might not qualify.