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

[Baoquan He]

On 09/03/18 at 10:46pm, H. Peter Anvin wrote:
> 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.

OK, I didn't get your point. I forget what difficulty was met so that
Kirill need to take this way. In that way, we will never have chance to
put kernel above 64TB even from 5-level kernel to jump to 5-level
kernel.

Hi Kirill,

Could you help to explain why the current implementation is decided?

Thanks
Baoquan