Re: [PATCH v9 04/11] x86/entry/64: Adapt assembly for PIE support

Peter Zijlstra
2019-08-06T08:30:42Z

On Tue, Aug 06, 2019 at 07:08:51AM +0200, Borislav Petkov wrote:
> On Mon, Aug 05, 2019 at 10:50:30AM -0700, Thomas Garnier wrote:
> > I saw that %rdx was used for temporary usage and restored before the
> > end so I assumed that it was not an option.
>
> PUSH_AND_CLEAR_REGS saves all regs earlier so I think you should be
> able to use others. Like SAVE_AND_SWITCH_TO_KERNEL_CR3/RESTORE_CR3, for
> example, uses %r15 and %r14.

AFAICT the CONFIG_DEBUG_ENTRY thing he's changing is before we setup
pt_regs.

Also consider the UNWIND hint that's in there, it states we only have
the IRET frame on stack, not a full regs set.