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Date: Tue, 6 Aug 2019 14:35:53 +0200
From: Borislav Petkov <>
To: Peter Zijlstra <>,
	Steven Rostedt <>
Cc: Thomas Garnier <>,
	Kernel Hardening <>,
	Kristen Carlson Accardi <>,
	Kees Cook <>,
	Andy Lutomirski <>,
	Thomas Gleixner <>,
	Ingo Molnar <>, "H. Peter Anvin" <>,
	the arch/x86 maintainers <>,
	LKML <>
Subject: Re: [PATCH v9 04/11] x86/entry/64: Adapt assembly for PIE support

+ rostedt.

Steve, pls have a look at the patch at the beginning of this thread as
it touches the reentrant NMI magic. :)


On Tue, Aug 06, 2019 at 10:30:32AM +0200, Peter Zijlstra wrote:
> 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.

Ok, after discussing it on IRC, I guess let's leave it like that. I
guess little ugly is better than a lot more ugly if we're wanting to
attempt to free up some regs here to save us the rax preservation.
Probably not worth the effort by a long shot.



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