linux-kernel - Re: [PATCH] x86/retpoline: Fill return stack buffer on vmexit

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Message-ID: <1515664721.22302.309.camel@amazon.co.uk>
Date:   Thu, 11 Jan 2018 09:58:41 +0000
From:   "Woodhouse, David" <dwmw@...zon.co.uk>
To:     Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>
CC:     "Thomas.Lendacky@....com" <Thomas.Lendacky@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
        "Ghitulete, Razvan-alin" <rga@...zon.com>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "riel@...hat.com" <riel@...hat.com>,
        "keescook@...gle.com" <keescook@...gle.com>,
        "gnomes@...rguk.ukuu.org.uk" <gnomes@...rguk.ukuu.org.uk>,
        "pjt@...gle.com" <pjt@...gle.com>,
        "dave.hansen@...el.com" <dave.hansen@...el.com>,
        "luto@...capital.net" <luto@...capital.net>,
        "jikos@...nel.org" <jikos@...nel.org>,
        "gregkh@...ux-foundation.org" <gregkh@...ux-foundation.org>
Subject: Re: [PATCH] x86/retpoline: Fill return stack buffer on vmexit

On Thu, 2018-01-11 at 10:47 +0100, Borislav Petkov wrote:
> On Thu, Jan 11, 2018 at 10:32:31AM +0100, Peter Zijlstra wrote:
> > 
> > can't you do lovely things like:
> > 
> > 	volatile asm ("call __fill_rsb_thunk_%1" : : "r" (dummy))
> > 
> > which would still let gcc select the register ?

I've had to do that for the __x86_indirect_thunk_\reg thunks and
provide all variants thereof, and export them to modules.

I'd much rather have this one inline. If I couldn't do that (which I
can, and have), then my next choice would probably have been to just
hard-code it to use %eax/%r12 and emit a simple call to that from the
call sites.

> Calling a function from asm is nasty because you need to pay attention
> to clobbered registers as gcc doesn't see the function.
>
> What one can do, I *think*, is do a non-inlined wrapping function and do
> all the alternative_call() fun inside. There you can do all the fun and
> have callee-clobbered regs which you can use.

That's OK; it's not really a C function and it only clobbers the *one*
register which is easy enough to tell __asm__() about.
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