linux-kernel - Re: [PATCH 02/10] x86/cet: Introduce WRUSS instruction

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Message-ID: <1528729376.4526.0.camel@2b52.sc.intel.com>
Date:   Mon, 11 Jun 2018 08:02:56 -0700
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-doc@...r.kernel.org,
        Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. J. Lu" <hjl.tools@...il.com>,
        "Shanbhogue, Vedvyas" <vedvyas.shanbhogue@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jonathan Corbet <corbet@....net>,
        Oleg Nesterov <oleg@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        mike.kravetz@...cle.com
Subject: Re: [PATCH 02/10] x86/cet: Introduce WRUSS instruction

On Mon, 2018-06-11 at 10:17 +0200, Peter Zijlstra wrote:
> 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@...el.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.

Thanks!  I will fix it.

Yu-cheng