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

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Date:   Thu, 11 Jan 2018 10:32:31 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Woodhouse, David" <dwmw@...zon.co.uk>
Cc:     "Thomas.Lendacky@....com" <Thomas.Lendacky@....com>,
        "bp@...en8.de" <bp@...en8.de>,
        "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, Jan 11, 2018 at 09:07:09AM +0000, Woodhouse, David wrote:
> On Thu, 2018-01-11 at 09:49 +0100, Boris Petkov wrote:
> > On January 11, 2018 9:42:38 AM GMT+01:00, Peter Zijlstra  wrote:
> > >Or we teach the alternative thing to patch in a jmp to end instead of
> > >NOP padding the entire thing as soon as the jmp (3 bytes) fits ?
> > 
> > Or, even better: use alternative_call() to call functions instead of patching gazillion bytes.
> 
> For this one I kind of wanted to keep it as a macro so we can select
> which register it uses. 

can't you do lovely things like:

	volatile asm ("call __fill_rsb_thunk_%1" : : "r" (dummy))

which would still let gcc select the register ?