Re: [PATCH] x86/kexec: Make kexec work in 5-level paging mode

[Baoquan He]

On 01/29/18 at 02:08pm, Kirill A. Shutemov wrote:
> I've missed that we need to change relocate_kernel() to set CR4.LA57
> flag if the kernel has 5-level paging enabled.
> 
> I avoided to use ifdef CONFIG_X86_5LEVEL here and inferred if we need to
> enabled 5-level paging from previous CR4 value. This way the code is
> ready for boot-time switching between paging modes.
> 
> Fixes: 77ef56e4f0fb ("x86: Enable 5-level paging support via CONFIG_X86_5LEVEL=y")
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Reported-by: Baoquan He <bhe@xxxxxxxxxx>

Thanks, Kirill.

Tested on qemu with la57 support, kexec works well. Kdump kernel can
boot into kernel, while there's a memory allocation failure during
boot which I am trying to fix. The reason is kdump kernel need reserve
as small memory as possible. Will post soon.

For this patch, feel free to add my Tested-by.

Tested-by: Baoquan He <bhe@xxxxxxxxxx>

Thanks
Baoquan
> ---
>  arch/x86/kernel/relocate_kernel_64.S | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
> index 307d3bac5f04..11eda21eb697 100644
> --- a/arch/x86/kernel/relocate_kernel_64.S
> +++ b/arch/x86/kernel/relocate_kernel_64.S
> @@ -68,6 +68,9 @@ relocate_kernel:
>  	movq	%cr4, %rax
>  	movq	%rax, CR4(%r11)
>  
> +	/* Save CR4. Required to enable the right paging mode later. */
> +	movq	%rax, %r13
> +
>  	/* zero out flags, and disable interrupts */
>  	pushq $0
>  	popfq
> @@ -126,8 +129,13 @@ identity_mapped:
>  	/*
>  	 * Set cr4 to a known state:
>  	 *  - physical address extension enabled
> +	 *  - 5-level paging, if it was enabled before
>  	 */
>  	movl	$X86_CR4_PAE, %eax
> +	testq	$X86_CR4_LA57, %r13
> +	jz	1f
> +	orl	$X86_CR4_LA57, %eax
> +1:
>  	movq	%rax, %cr4
>  
>  	jmp 1f
> -- 
> 2.15.1
>