Re: [PATCHv2 08/10] x86/mm: Replace compile-time checks for 5-level with runtime-time

[Juergen Gross]

On 25/07/17 11:05, Kirill A. Shutemov wrote:
> On Tue, Jul 18, 2017 at 04:24:06PM +0200, Juergen Gross wrote:
>> Xen PV guests will never run with 5-level-paging enabled. So I guess you
>> can drop the complete if (IS_ENABLED(CONFIG_X86_5LEVEL)) {} block.
> 
> There is more code to drop from mmu_pv.c.
> 
> But while there, I thought if with boot-time 5-level paging switching we
> can allow kernel to compile with XEN_PV and XEN_PVH, so the kernel image
> can be used in these XEN modes with 4-level paging.
> 
> Could you check if with the patch below we can boot in XEN_PV and XEN_PVH
> modes?

We can't. I have used your branch:

git://git.kernel.org/pub/scm/linux/kernel/git/kas/linux.git
la57/boot-switching/v2

with this patch applied on top.

Doesn't boot PV guest with X86_5LEVEL configured (very early crash).
Doesn't build with X86_5LEVEL not configured:

  AS      arch/x86/kernel/head_64.o
/home/gross/linux/arch/x86/kernel/head_64.S: Assembler messages:
/home/gross/linux/arch/x86/kernel/head_64.S:350: Error: attempt to move
.org backwards
/home/gross/linux/arch/x86/kernel/head_64.S:352: Error: attempt to move
.org backwards
/home/gross/linux/arch/x86/kernel/head_64.S:43: Error: invalid operands
(*ABS* and *UND* sections) for `>>'
/home/gross/linux/arch/x86/kernel/head_64.S:44: Error: invalid operands
(*ABS* and *UND* sections) for `>>'
/home/gross/linux/scripts/Makefile.build:403: recipe for target
'arch/x86/kernel/head_64.o' failed
make[7]: *** [arch/x86/kernel/head_64.o] Error 1


Juergen