Re: [PATCHv4 06/14] x86/kexec: Keep CR4.MCE set during kexec for TDX guest

[kirill.shutemov@xxxxxxxxxxxxxxx]

On Tue, Dec 05, 2023 at 11:58:45PM +0000, Huang, Kai wrote:
> On Tue, 2023-12-05 at 03:45 +0300, Kirill A. Shutemov wrote:
> > TDX guests are not 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>
> > Reviewed-by: Kai Huang <kai.huang@xxxxxxxxx>
> > ---
> >  arch/x86/kernel/relocate_kernel_64.S | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
> > index 56cab1bb25f5..cd6a53667c6b 100644
> > --- a/arch/x86/kernel/relocate_kernel_64.S
> > +++ b/arch/x86/kernel/relocate_kernel_64.S
> > @@ -145,12 +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. Clearing MCE is not allowed
> > +	 *    in TDX guests.
> >  	 */
> >  	movl	$X86_CR4_PAE, %eax
> >  	testq	$X86_CR4_LA57, %r13
> >  	jz	1f
> >  	orl	$X86_CR4_LA57, %eax
> >  1:
> > +	ALTERNATIVE "", __stringify(orl $X86_CR4_MCE, %eax), X86_FEATURE_TDX_GUEST
> >  	movq	%rax, %cr4
> >  
> >  	jmp 1f
> 
> Nit:
> 
> It seems <asm/alternative.h> isn't included in relocate_kernel_64.S.  Maybe it's
> better to do it explicitly.
> 
> Maybe even better to explicitly include <linux/stringify.h> too, but I see
> <asm/alternative.h> already does that.

Okay, I will add both.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov