linux-kernel - Re: [PATCH 22/43] x86/mm/kaiser: Prepare assembly for entry/exit CR3 switching

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Date:   Fri, 24 Nov 2017 14:04:45 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...capital.net>,
        "H . Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 22/43] x86/mm/kaiser: Prepare assembly for entry/exit
 CR3 switching

On Fri, 24 Nov 2017, Peter Zijlstra wrote:

> On Fri, Nov 24, 2017 at 01:17:06PM +0100, Ingo Molnar wrote:
> > 
> > * Peter Zijlstra <peterz@...radead.org> wrote:
> > 
> > > On Fri, Nov 24, 2017 at 10:14:27AM +0100, Ingo Molnar wrote:
> > > > @@ -1343,9 +1373,10 @@ ENTRY(error_entry)
> > > >  
> > > >  .Lerror_bad_iret:
> > > >  	/*
> > > > +	 * We came from an IRET to user mode, so we have user
> > > > +	 * gsbase and CR3.  Switch to kernel gsbase and CR3:
> > > >  	 */
> > > > +	SWITCH_TO_KERNEL_CR3 scratch_reg=%rax
> > > >  	SWAPGS
> > > >  
> > > >  	/*
> > > 
> > > The Changelog states SWAPGS must be done before, yet the code does
> > > after.
> > 
> > Yes, so this is the SWAPGS that is done before we go back to user-space.
> 
> The comment there clearly states we have user gs and we need to switch
> to kernel gs. The Changelog states we should switch gs before cr3.
> 
> So either the comment or the code needs fixing.

The code :)