linux-kernel - Re: [PATCH v2] x86/mm/kaiser: Prepare the x86/entry assembly code for entry/exit CR3 switching

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Message-ID: <alpine.DEB.2.20.1711271435570.1799@nanos>
Date:   Mon, 27 Nov 2017 14:36:47 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
cc:     Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...capital.net>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2] x86/mm/kaiser: Prepare the x86/entry assembly code
 for entry/exit CR3 switching

On Mon, 27 Nov 2017, Josh Poimboeuf wrote:
> On Sun, Nov 26, 2017 at 03:55:53PM +0100, Ingo Molnar wrote:
> > @@ -198,6 +201,14 @@ ENTRY(entry_SYSCALL_64)
> >  
> >  	swapgs
> >  	movq	%rsp, PER_CPU_VAR(rsp_scratch)
> > +
> > +	/*
> > +	 * The kernel CR3 is needed to map the process stack, but we
> > +	 * need a scratch register to be able to load CR3.  %rsp is
> > +	 * clobberable right now, so use it as a scratch register.
> > +	 * %rsp will look crazy here for a couple instructions.
> > +	 */
> > +	SWITCH_TO_KERNEL_CR3 scratch_reg=%rsp
> >  	movq	PER_CPU_VAR(cpu_current_top_of_stack), %rsp
> 
> I think it doesn't make sense to have a SWITCH_TO_KERNEL_CR3 here.  This
> code is the (future) kaiser-disabled path, where SWITCH_TO_KERNEL_CR3 is
> a no-op anyway.

Yes, Borislav noticed that as well. That's a left over from the early
versions where the trampoline did not exist.

Thanks,

	tglx