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?
