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

[H. Peter Anvin]

On 09/03/18 22:20, Baoquan He wrote:
> 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.
> 

That makes no sense.  I'm talking about *entering* the kernel; the second
kernel should switch to 5-level mode as necessary.

	-hpa