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

On 01/29/18 at 07:19pm, Baoquan He wrote:

On 01/29/18 at 02:08pm, Kirill A. Shutemov wrote:

quoted

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 <redacted>
Reported-by: Baoquan He <redacted>

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.

By the way, the kdump failure can be worked around by increasing
crashkernel memory, then kdump kernel can still work well. So this patch
is necessary fix for kexec/kdump.

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

Tested-by: Baoquan He <redacted>

Thanks
Baoquan

quoted

---
 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

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