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

[Baoquan He <bhe@redhat.com>]

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