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Date:   Sat, 6 Apr 2019 09:39:57 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, kirill.shutemov@...ux.intel.com,
        mingo@...nel.org, bp@...en8.de, hpa@...or.com, dyoung@...hat.com,
        x86@...nel.org
Subject: Re: [PATCH v3 2/3] x86/kexec/64: Error out if try to jump to old
 4-level kernel from 5-level kernel

On 04/05/19 at 10:38pm, Thomas Gleixner wrote:
> On Tue, 12 Mar 2019, Baoquan He wrote:
> 
> > In relocate_kernel() CR4.LA57 flag is set before kexec jumping if
> > the kernel has 5-level paging enabled. Then in boot/compressed/head_64.S,
> > it will check if the booting kernel is in 4-level or 5-level paging
> > mode, and handle accordingly. However, the old kernel which doesn't
> > contain the 5-level codes doesn't know how to cope with it, then #GP
> > triggered.
> 
> The above is more than confusing. I assume you want to say:
> 
>   If the running kernel has 5-level paging activated, the 5-level paging
>   mode is preserved across kexec. If the kexec'ed kernel does not contain
>   support for handling active 5-level paging mode in the decompressor, the
>   decompressor will crash with #GP.
> 
> > Instead of triggering #GP during kexec kernel boot, error out during
> > kexec loading if find out we are trying to jump to old 4-level kernel
> > from 5-level kernel.
> 
> Prevent this situation at load time. If 5-level paging is active, check the
> xloadflags whether the kexec kernel can handle 5-level paging at least in
> the decompressor. If not, reject the load attempt.

Yes, exactly. I will rewrite patch log with simpler sentences as you
have demonstrated. Thanks.
>  
> > Signed-off-by: Baoquan He <bhe@...hat.com>
> > Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > ---
> >  arch/x86/kernel/kexec-bzimage64.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/arch/x86/kernel/kexec-bzimage64.c b/arch/x86/kernel/kexec-bzimage64.c
> > index 1f3b77367948..4c9c079b5673 100644
> > --- a/arch/x86/kernel/kexec-bzimage64.c
> > +++ b/arch/x86/kernel/kexec-bzimage64.c
> > @@ -321,6 +321,11 @@ static int bzImage64_probe(const char *buf, unsigned long len)
> >  		return ret;
> >  	}
> >  
> > +	if (!(header->xloadflags & XLF_5LEVEL) && pgtable_l5_enabled()) {
> > +		pr_err("Can not jump to old 4-level kernel from 5-level kernel.\n");
> 
> This is confusing at best.
> 
>      	"bzImage cannot handle 5-level paging mode\n"
> 
> or something like this.
> 
> > +		return ret;

Will change too as you suggested.

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