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

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