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Subject: Re: [PATCH 1/3] x86/boot: Add bit fields into xloadflags for 5-level
 kernel checking
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
Cc:     Baoquan He <bhe@redhat.com>, x86@kernel.org,
        kexec@lists.infradead.org, linux-kernel@vger.kernel.org,
        tglx@linutronix.de, mingo@kernel.org,
        kirill.shutemov@linux.intel.com
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 <c4f65956-ad99-84ee-d4bf-044a6f0c5010@zytor.com>
 <20180904084231.ubyjaqp4xhqcnper@kshutemo-mobl1>
From:   "H. Peter Anvin" <hpa@zytor.com>
Message-ID: <7a1147d7-05af-98fe-3a14-b18ea445c2b5@zytor.com>
Date:   Tue, 4 Sep 2018 21:06:07 -0700
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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.

	-hpa