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Date:   Fri, 18 Mar 2022 16:03:01 -0700
From:   "H. Peter Anvin" <hpa@zytor.com>
To:     David Laight <David.Laight@ACULAB.COM>,
        "'Linus Torvalds'" <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>
CC:     Andrew Cooper <Andrew.Cooper3@citrix.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Bill Wendling <morbo@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Juergen Gross <jgross@suse.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "llvm@lists.linux.dev" <llvm@lists.linux.dev>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-toolchains <linux-toolchains@vger.kernel.org>
Subject: RE: [PATCH v5] x86: use builtins to read eflags
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On March 18, 2022 3:43:13 PM PDT, David Laight <David=2ELaight@ACULAB=2ECOM=
> wrote:
>From: Linus Torvalds
>> Sent: 18 March 2022 18:19
>>=20
>> On Fri, Mar 18, 2022 at 10:59 AM Andy Lutomirski <luto@kernel=2Eorg> wr=
ote:
>> >
>> > I generally agree=2E  In this particular case, though, I will keep us=
ing
>> > the builtin in tools/testing/selftests/x86/helpers=2Eh unless we actu=
ally
>> > hit breakage=2E  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=2E
>>=20
>> Yeah, redzoning is a problem for "pushf"=2E
>>=20
>> 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=2E
>>=20
>> I think it would be lovely to solve that in inline asm itself some way
>> - by marking the stack pointer clobbered or something=2E
>
>Something that generates:
>	mov	%rax,-8(%rsp)
>	pushf
>	xchg	%rax,0(%rsp)
>	add	%rsp,8
>should work with or without a redzone=2E
>Will be a bit slower :-(
>
>	David
>
>-
>Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1=
 1PT, UK
>Registration No: 1397386 (Wales)
>

*Much* slower (XCHG is implicitly locked, for hysterical raisins) and you =
are using the redzone between the mov and the push, so it seems like you ju=
st made things worse for no reason=2E