[PATCHv10 05/18] x86/kexec: Keep CR4.MCE set during kexec for TDX guest

[Kirill A. Shutemov]

On Sun, Apr 28, 2024 at 07:11:11PM +0200, Borislav Petkov wrote:
> On Tue, Apr 09, 2024 at 06:26:05PM +0300, Kirill A. Shutemov wrote:
> > From 6be428e3b1c6fb494b2c48ba6a7c133514a0b2b4 Mon Sep 17 00:00:00 2001
> > From: "Kirill A. Shutemov" <kirill.shutemov at linux.intel.com>
> > 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.
> 
> This is all fine and dandy but nothing explains *why* TDX needs this
> special dance.
> 
> Why can't TDX do the usual CR4.MCE diddling like the normal kernel
> during init and needs to do that here immediately?

As I mentioned above, clearing CR4.MCE triggers #VE. It is quirk of the
platform.

There's plan to allow it in newer TDX modules, but kernel still has to
assume we cannot touch it in TDX guest case.

> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov at linux.intel.com>
> > ---
> >  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.
> 
> 			 ... might not be allowed ...
> 

Oopsie. Thanks.

> > +	 *  - 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
> 
> %r13 needs a comment here that it contains %cr4 read above in
> relocate_kernel()

Okay.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov