On Tue, Apr 09, 2024 at 07:22:24AM -0700, Sean Christopherson wrote: > On Tue, Apr 09, 2024, Kirill A. Shutemov wrote: > > Depending on setup, TDX guests might be allowed to clear CR4.MCE. > > Attempt to clear it leads to #VE. > > > > Use alternatives to keep the flag during kexec for TDX guests. > > > > 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 | 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 56cab1bb25f5..8e2037d78a1f 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> > > @@ -145,11 +147,17 @@ 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 allowed in TDX guests, depending on setup. > > */ > > movl $X86_CR4_PAE, %eax > > testq $X86_CR4_LA57, %r13 > > jz 1f > > orl $X86_CR4_LA57, %eax > > +1: > > + testq $X86_CR4_MCE, %r13 > > + jz 1f > > + ALTERNATIVE "", __stringify(orl $X86_CR4_MCE, %eax), X86_FEATURE_TDX_GUEST > > The TEST+Jcc+OR sequences are rather odd, and require way more instructions and > thus way more copy+paste than is necessary. > > movl $X86_CR4_LA57, %eax > ALTERNATIVE "", __stringify(orl $X86_CR4_MCE, %eax), X86_FEATURE_TDX_GUEST > andl %r13d, %eax > orl $X86_CR4_PAE, %eax > movq %rax, %cr4 > > Then preserving new bits unconditionally only requires adding the flag to the > initial move, and feature-dependent bits only need a single ALTERNATIVE line. Thanks! It is much better. > And there's no branches, blazing fast kexec! ;-) kexec/sec STONKS! :D Updated patch is below. >From 6be428e3b1c6fb494b2c48ba6a7c133514a0b2b4 Mon Sep 17 00:00:00 2001 From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> Date: Fri, 10 Feb 2023 12:53:11 +0300 Subject: [PATCHv10.1 05/18] x86/kexec: Keep CR4.MCE set during kexec for TDX guest Depending on setup, TDX guests might be allowed to clear CR4.MCE. Attempt to clear it leads to #VE. Use alternatives to keep the flag during kexec for TDX guests. 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 | 15 +++++++++------ 1 file changed, 9 insertions(+), 6 deletions(-) diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S index 56cab1bb25f5..90246d544eb1 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> @@ -143,14 +145,15 @@ 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 allowed in TDX guests, depending on setup. + * - physical address extension enabled */ - movl $X86_CR4_PAE, %eax - testq $X86_CR4_LA57, %r13 - jz 1f - orl $X86_CR4_LA57, %eax -1: + movl $X86_CR4_LA57, %eax + ALTERNATIVE "", __stringify(orl $X86_CR4_MCE, %eax), X86_FEATURE_TDX_GUEST + andl %r13d, %eax + orl $X86_CR4_PAE, %eax movq %rax, %cr4 jmp 1f -- Kiryl Shutsemau / Kirill A. Shutemov _______________________________________________ kexec mailing list kexec@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/kexec
