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Date:   Tue, 4 Sep 2018 13:20:36 +0800
From:   Baoquan He <bhe@redhat.com>
To:     "H. Peter Anvin" <hpa@zytor.com>
Cc:     x86@kernel.org, kexec@lists.infradead.org,
        linux-kernel@vger.kernel.org, tglx@linutronix.de, 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
Message-ID: <20180904052036.GJ1740@192.168.1.3>
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>
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On 09/03/18 at 09:13pm, H. Peter Anvin wrote:
> On 09/03/18 20:44, Baoquan He wrote:
> > 
> > 1) in arch/x86/kernel/relocate_kernel_64.S, we set X86_CR4_LA57 into cr4
> > if the 1st kernel is in 5-level mode. Then in
> > arch/x86/boot/compressed/head_64.S, paging_prepare() is called to decide
> > if 5-level mode will be enabled, and prepare the trampoline. If
> > kexec/kdump kernel is expected to be in 4-level, e.g with 'nolv5'
> > specified, it still can handle well. But for the old kernel w/o these
> >  5-level codes, it will ignore the fact that X86_CR4_LA57 has been set
> > in CR4 and proceed anyway, then #GP is triggered. That's why XLF_5LEVEL
> > is used to mark. 
> > 
> 
> That's what I'm saying, don't do that.  Always jump into the second kernel in
> 4-level mode, i.e. X86_CR4_LA57 unset.  That's the only sane thing.

Well, this might not be suggested. Kexec has been a formal feature in
our distro, our customers usually use it to reboot high end servers
because those machines may take one hour to boot up from firmware. And
5-level may be also supported very soon, if people want to do a fast
reboot from the current kernel in 5-level, and expect to see it's in
5-level too in the 2nd kernel, this always kexec jumping to the 2nd
kernel in 4-level mode might be unaccepted.

Thanks
Baoquan