Re: [PATCHv11 05/19] x86/relocate_kernel: Use named labels for less confusion

[H. Peter Anvin]

On June 12, 2024 4:06:07 PM PDT, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>On 12/06/2024 10:22 am, Kirill A. Shutemov wrote:
>> On Tue, Jun 11, 2024 at 11:26:17AM -0700, H. Peter Anvin wrote:
>>> On 6/4/24 08:21, Kirill A. Shutemov wrote:
>>>>  From b45fe48092abad2612c2bafbb199e4de80c99545 Mon Sep 17 00:00:00 2001
>>>> From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
>>>> Date: Fri, 10 Feb 2023 12:53:11 +0300
>>>> Subject: [PATCHv11.1 06/19] x86/kexec: Keep CR4.MCE set during kexec for TDX guest
>>>>
>>>> TDX guests run with MCA enabled (CR4.MCE=1b) from the very start. If
>>>> that bit is cleared during CR4 register reprogramming during boot or
>>>> kexec flows, a #VE exception will be raised which the guest kernel
>>>> cannot handle it.
>>>>
>>>> Therefore, make sure the CR4.MCE setting is preserved over kexec too and
>>>> avoid raising any #VEs.
>>>>
>>>> The change doesn't affect non-TDX-guest environments.
>>>>
>>>> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
>>>> ---
>>>>   arch/x86/kernel/relocate_kernel_64.S | 17 ++++++++++-------
>>>>   1 file changed, 10 insertions(+), 7 deletions(-)
>>>>
>>>> diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
>>>> index 085eef5c3904..9c2cf70c5f54 100644
>>>> --- a/arch/x86/kernel/relocate_kernel_64.S
>>>> +++ b/arch/x86/kernel/relocate_kernel_64.S
>>>> @@ -5,6 +5,8 @@
>>>>    */
>>>>   #include <linux/linkage.h>
>>>> +#include <linux/stringify.h>
>>>> +#include <asm/alternative.h>
>>>>   #include <asm/page_types.h>
>>>>   #include <asm/kexec.h>
>>>>   #include <asm/processor-flags.h>
>>>> @@ -145,14 +147,15 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
>>>>   	 * Set cr4 to a known state:
>>>>   	 *  - physical address extension enabled
>>>>   	 *  - 5-level paging, if it was enabled before
>>>> +	 *  - Machine check exception on TDX guest, if it was enabled before.
>>>> +	 *    Clearing MCE might not be allowed in TDX guests, depending on setup.
>>>> +	 *
>>>> +	 * Use R13 that contains the original CR4 value, read in relocate_kernel().
>>>> +	 * PAE is always set in the original CR4.
>>>>   	 */
>>>> -	movl	$X86_CR4_PAE, %eax
>>>> -	testq	$X86_CR4_LA57, %r13
>>>> -	jz	.Lno_la57
>>>> -	orl	$X86_CR4_LA57, %eax
>>>> -.Lno_la57:
>>>> -
>>>> -	movq	%rax, %cr4
>>>> +	andl	$(X86_CR4_PAE | X86_CR4_LA57), %r13d
>>>> +	ALTERNATIVE "", __stringify(orl $X86_CR4_MCE, %r13d), X86_FEATURE_TDX_GUEST
>>>> +	movq	%r13, %cr4
>>> If this is the case, I don't really see a reason to clear MCE per se as I'm
>>> guessing a machine check here will be fatal anyway? It just changes the
>>> method of death.
>> Andrew had a strong opinion on method of death here.
>>
>> https://lore.kernel.org/all/1144340e-dd95-ee3b-dabb-579f9a65b3c7@xxxxxxxxxx
>
>Not sure if I intended it to come across that strongly, but given a
>choice, the !CR4.MCE death is cleaner because at least you're not
>interpreting garbage and trying to use it as a valid IDT.
>
>~Andrew

Zorch the IDT if it isn't valid?