Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH 1/3] x86/boot: Add bit fields into xloadflags for 5-level
 kernel checking
To:     Baoquan He <bhe@redhat.com>
Cc:     tglx@linutronix.de, mingo@kernel.org,
        kirill.shutemov@linux.intel.com, x86@kernel.org,
        linux-kernel@vger.kernel.org, kexec@lists.infradead.org
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>
From:   "H. Peter Anvin" <hpa@zytor.com>
Message-ID: <4546fc39-4982-4c91-c812-0df1e9bc9e20@zytor.com>
Date:   Mon, 3 Sep 2018 21:13:54 -0700
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.9.1
MIME-Version: 1.0
In-Reply-To: <20180904034414.GI1740@192.168.1.3>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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.

	-hpa