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Date:   Wed, 5 Sep 2018 10:02:35 +0200 (CEST)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     "H. Peter Anvin" <hpa@zytor.com>
cc:     "Kirill A. Shutemov" <kirill@shutemov.name>,
        Baoquan He <bhe@redhat.com>, x86@kernel.org,
        kexec@lists.infradead.org, linux-kernel@vger.kernel.org,
        mingo@kernel.org, kirill.shutemov@linux.intel.com
Subject: Re: [PATCH 1/3] x86/boot: Add bit fields into xloadflags for 5-level
 kernel checking
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References: <20180829141624.13985-1-bhe@redhat.com> <20180829141624.13985-2-bhe@redhat.com> <6ea94875-ae07-6220-eb3e-d3f830cdac03@zytor.com> <20180904034414.GI1740@192.168.1.3> <4546fc39-4982-4c91-c812-0df1e9bc9e20@zytor.com> <20180904052036.GJ1740@192.168.1.3>
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On Tue, 4 Sep 2018, H. Peter Anvin wrote:

> On 09/04/18 01:42, Kirill A. Shutemov wrote:
> > 
> > Switching between 4- and 5-level paging modes (in either direction)
> > requires paing disabling. It means the code that does the switching has to
> > be under 4G otherwise we would lose control.
> > 
> > We handle the switching correctly in kernel decompression code, but not on
> > kexec caller side.
> > 
> > XLF_5LEVEL indicates that kernel decompression code can deal with
> > switching between paging modes and it's safe to jump there in 5-level
> > paging mode.
> > 
> > As an alternative we can change kexec to switch to 4-level paging mode
> > before starting the new kernel. Not sure how hard it will be.
> > 
> 
> Have a flag saying entering in 5-level mode is fine.  However, you really
> should support returning to 4-level mode in kexec.  It is *much* easier to do
> on the caller side as you have total control of memory allocation there.

Works for a regular kexec, but not for starting a crash kernel....

Thanks,

	tglx