linux-kernel - Re: [PATCH 1/3] x86/boot: Add bit fields into xloadflags for 5-level kernel checking

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Date:   Tue, 4 Sep 2018 11:44:14 +0800
From:   Baoquan He <bhe@...hat.com>
To:     "H. Peter Anvin" <hpa@...or.com>
Cc:     tglx@...utronix.de, mingo@...nel.org,
        kirill.shutemov@...ux.intel.com, x86@...nel.org,
        linux-kernel@...r.kernel.org, kexec@...ts.infradead.org
Subject: Re: [PATCH 1/3] x86/boot: Add bit fields into xloadflags for 5-level
 kernel checking

Hi hpa,

Thanks for looking into this. 

On 09/03/18 at 07:41pm, H. Peter Anvin wrote:
> I don't understand why there is any reason not to always enter the target
> kernel in 4-level mode.  There certainly is no point whatsoever in having two
> xloadflags: the only thing that could possibly matter is whether or not the
> kernel in question *can* be entered in 5-level mode should that ever be necessary.

This patchset is only used to fix kexec/kdump issues. We never stop
kernel booting in 4-level mode from firmware as 1st kernel. However,
there are issues when jump from the 1st kernel which is in 5-level mode
to 2nd kernel. The reason is:

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. 

2) For kexec_load interface, we put kernel/initrd at top of system RAM
in kexec_tools utility. If the 1st kernel is in 5-level mode, the
kexec-ed kernel has "CONFIG_X86_5LEVEL=n", we have to detect this and
limit the kernel to be loaded under 64TB, since kexec-ed kernel will
definitely run in 4-level mode. Putting kernel above 64TB will fail
kexec-ed kernel booting. That's why *XLF_5LEVEL_ENABLED* is needed.

Thanks
Baoquan