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

[Thomas Gleixner]

On Tue, 4 Sep 2018, H. Peter Anvin wrote:

> On 09/04/18 01:42, Kirill A. Shutemov wrote:
> > 
> > Switching between 4- and 5-level paging modes (in either direction)
> > requires paing disabling. It means the code that does the switching has to
> > be under 4G otherwise we would lose control.
> > 
> > We handle the switching correctly in kernel decompression code, but not on
> > kexec caller side.
> > 
> > XLF_5LEVEL indicates that kernel decompression code can deal with
> > switching between paging modes and it's safe to jump there in 5-level
> > paging mode.
> > 
> > As an alternative we can change kexec to switch to 4-level paging mode
> > before starting the new kernel. Not sure how hard it will be.
> > 
> 
> Have a flag saying entering in 5-level mode is fine.  However, you really
> should support returning to 4-level mode in kexec.  It is *much* easier to do
> on the caller side as you have total control of memory allocation there.

Works for a regular kexec, but not for starting a crash kernel....

Thanks,

	tglx