Re: [PATCH 13/14] x86: BHI stubs

[Josh Poimboeuf]

On Mon, Sep 30, 2024 at 11:23:38PM +0100, Andrew Cooper wrote:
> On 30/09/2024 10:30 pm, Josh Poimboeuf wrote:
> > On Fri, Sep 27, 2024 at 09:49:09PM +0200, Peter Zijlstra wrote:
> >> +SYM_INNER_LABEL(__bhi_args_0, SYM_L_LOCAL)
> >> +	UNWIND_HINT_FUNC
> >> +	cmovne %r10, %rdi
> > IIUC, this works because if the "jz" in the CFI preamble mispredicts to
> > the __bhi_args_* code, "cmovne" will zero out the speculative value of
> > rdi.
> >
> > Why use %r10 instead of a literal $0?  Also how do you know %r10 is 0?
> 
> There's no encoding for CMOVcc which takes an $imm.

Ah.

> %r10 is guaranteed zero after the FineIBT prologue

If the "jz" in the FineIBT prologue mispredicts, isn't %r10 non-zero by
definition?

> , but I don't see
> anything in patch 11 which makes this true in the !FineIBT case.

I thought this code is only used by FineIBT?

-- 
Josh