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 _______________________________________________ kexec mailing list kexec@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/kexec
