linux-kernel - RE: [PATCH v5] x86: use builtins to read eflags

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Date:   Fri, 18 Mar 2022 22:43:13 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Linus Torvalds' <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>
CC:     Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>, Bill Wendling <morbo@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        "Juergen Gross" <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-toolchains <linux-toolchains@...r.kernel.org>
Subject: RE: [PATCH v5] x86: use builtins to read eflags

From: Linus Torvalds
> Sent: 18 March 2022 18:19
> 
> On Fri, Mar 18, 2022 at 10:59 AM Andy Lutomirski <luto@...nel.org> wrote:
> >
> > I generally agree.  In this particular case, though, I will keep using
> > the builtin in tools/testing/selftests/x86/helpers.h unless we actually
> > hit breakage.  This is because this is *user* code, it is compiled with
> > the redzone enabled, and the asm code to do the right thing when the
> > redzone is enabled is too hairy for me to want to deal with it.
> 
> Yeah, redzoning is a problem for "pushf".
> 
> Or rather, it's not the redzoning itself, but the fact that the
> compiler might use the word under the stack for random other things,
> and the pushf will then corrupt some local variable storage.
> 
> I think it would be lovely to solve that in inline asm itself some way
> - by marking the stack pointer clobbered or something.

Something that generates:
	mov	%rax,-8(%rsp)
	pushf
	xchg	%rax,0(%rsp)
	add	%rsp,8
should work with or without a redzone.
Will be a bit slower :-(

	David

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