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

[Yu-cheng Yu]

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@xxxxxxxxx> 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