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. And there's no branches, blazing fast kexec! ;-) _______________________________________________ kexec mailing list kexec@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/kexec
