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Date: Mon, 11 Jun 2018 10:17:04 +0200
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 02/10] x86/cet: Introduce WRUSS instruction
Message-ID: <20180611081704.GI12180@hirez.programming.kicks-ass.net>
References: <20180607143807.3611-1-yu-cheng.yu@intel.com>
 <20180607143807.3611-3-yu-cheng.yu@intel.com>
 <CALCETrU45Cuzvfz3c1+-+7=9KS2N33Bpp1JqBtaGxhPo8U+Fqg@mail.gmail.com>
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To: Andy Lutomirski <luto@kernel.org>
Cc: Yu-cheng Yu <yu-cheng.yu@intel.com>, LKML <linux-kernel@vger.kernel.org>, linux-doc@vger.kernel.org, Linux-MM <linux-mm@kvack.org>, linux-arch <linux-arch@vger.kernel.org>, X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, "H. J. Lu" <hjl.tools@gmail.com>, "Shanbhogue, Vedvyas" <vedvyas.shanbhogue@intel.com>, "Ravi V. Shankar" <ravi.v.shankar@intel.com>, Dave Hansen <dave.hansen@linux.intel.com>, Jonathan Corbet <corbet@lwn.net>, Oleg Nesterov <oleg@redhat.com>, Arnd Bergmann <arnd@arndb.de>, mike.kravetz@oracle.com

On Thu, Jun 07, 2018 at 09:40:02AM -0700, Andy Lutomirski wrote:
> On Thu, Jun 7, 2018 at 7:41 AM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:

> Peterz, isn't there some fancy better way we're supposed to handle the
> error return these days?

> > +       asm volatile("1:.byte 0x66, 0x0f, 0x38, 0xf5, 0x37\n"
> > +                    "xor %[err],%[err]\n"
> > +                    "2:\n"
> > +                    ".section .fixup,\"ax\"\n"
> > +                    "3: mov $-1,%[err]; jmp 2b\n"
> > +                    ".previous\n"
> > +                    _ASM_EXTABLE(1b, 3b)
> > +               : [err] "=a" (err)
> > +               : [val] "S" (val), [addr] "D" (addr)
> > +               : "memory");

So the alternative is something like:

__visible bool ex_handler_wuss(const struct exception_table_entry *fixup,
			       struct pt_regs *regs, int trapnr)
{
	regs->ip = ex_fixup_addr(fixup);
	regs->ax = -1L;

	return true;
}


	int err = 0;

	asm volatile("1: INSN_WUSS\n"
		     "2:\n"
		     _ASM_EXTABLE_HANDLE(1b, 2b, ex_handler_wuss)
		     : "=a" (err)
		     : "S" (val), "D" (addr));

But I'm not at all sure that's actually better.