Re: [PATCHv9 05/17] x86/kexec: Keep CR4.MCE set during kexec for TDX guest

[Kirill A. Shutemov]

On Thu, Apr 04, 2024 at 10:40:34AM +1300, Huang, Kai wrote:
> 
> 
> On 3/04/2024 4:42 am, Kirill A. Shutemov wrote:
> > On Fri, Mar 29, 2024 at 06:48:21PM +0200, Kirill A. Shutemov wrote:
> > > On Fri, Mar 29, 2024 at 11:21:32PM +0800, Xiaoyao Li wrote:
> > > > On 3/25/2024 6:38 PM, Kirill A. Shutemov wrote:
> > > > > TDX guests are not allowed to clear CR4.MCE. Attempt to clear it leads
> > > > > to #VE.
> > > > 
> > > > Will we consider making it more safe and compatible for future to guard
> > > > against X86_FEATURE_MCE as well?
> > > > 
> > > > If in the future, MCE becomes configurable for TD guest, then CR4.MCE might
> > > > not be fixed1.
> > > 
> > > Good point.
> > > 
> > > I guess we can leave it clear if it was clear. This should be easy
> > > enough. But we might want to clear even if was set if clearing is allowed.
> > > 
> > > It would require some kind of indication that clearing MCE is fine. We
> > > don't have such indication yet. Not sure we can reasonably future-proof
> > > the code at this point.
> > > 
> > > But let me think more.
> > 
> > I think I will go with the variant below.
> > 
> > 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.
> 
> Nit:  Perhaps we can just call out:
> 
> 	Clearing MCE is not allowed if it _was_ enabled before.
> 
> Which is always true I suppose.

It is true now. Future TDX will allow to clear CR4.MCE and we don't want
to flip it back on in this case.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov