linux-kernel - Re: [PATCH] x86/speculation: Use AMD specific retpoline for inline asm on AMD

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Date:   Tue, 18 Sep 2018 15:00:58 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Zhenzhong Duan <zhenzhong.duan@...cle.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...hat.com,
        konrad.wilk@...cle.com, x86@...nel.org, dwmw@...zon.co.uk,
        tglx@...utronix.de, Srinivas REDDY Eeda <srinivas.eeda@...cle.com>,
        bp@...e.de, hpa@...or.com, dhaval.giani@...cle.com
Subject: Re: [PATCH] x86/speculation: Use AMD specific retpoline for inline
 asm on AMD

On Tue, Sep 18, 2018 at 08:04:44PM +0800, Zhenzhong Duan wrote:
> On 2018/9/18 18:59, Peter Zijlstra wrote:
> > On Tue, Sep 18, 2018 at 06:31:07PM +0800, Zhenzhong Duan wrote:
> > > On 2018/9/18 17:50, Peter Zijlstra wrote:
> > > > On Mon, Sep 17, 2018 at 10:17:30PM -0700, Zhenzhong Duan wrote:
> > > > > -#elif defined(CONFIG_X86_32) && defined(CONFIG_RETPOLINE)
> > > > > +#elif defined(CONFIG_RETPOLINE)
> > > > 
> > > > This doesn't make any sense..
> > > This change is used for x86_64 to have minimal Retpoline support when
> > > CONFIG_RETPOLINE is defined but RETPOLINE isn't defined, or I missed
> > > something?
> > 
> > No it doesn't.
> > 
> > #if defined(X86_64) && defined(RETPOLINE)
> > 
> >   /* x86_64 retpoline goes here */
> > 
> > #elif defined(RETPOLINE)
> > 
> >   /* !x86_64 retpoline goes here */
> > 
> > #else
> > 
> >   /* !retpoline goes here
> > 
> > #endif
> 
> Sorry, but I am confused.
> So where is 'if defined(x86_64) && !defined(RETPOLINE) &&
> defined(CONFIG_RETPOLINE)' go?

Argh, CONFIG_RETPOLINE vs RETPOLINE :/

The thing is, the one you modify has a comment on that explains why it
is i386 only. CET and retpolines don't like one another much.

And the x86_64 version uses %V which requires new GCC.

So I'm all for fixing the RETPOLINE_AMD thing, but at this point nobody
should use the minimal stuff, that's just delusional.

> In original code, it will go to "call *%[thunk_target]\n" while
> we have set SPECTRE_V2_RETPOLINE_MINIMAL or
> SPECTRE_V2_RETPOLINE_MINIMAL_AMD. Is this expected?

Yes, that is exactly right -- it does that with or without your change
though.