Re: [PATCH] x86: Disable kexec for TDX guests

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Sat, Mar 25, 2023 at 09:25:36AM -0700, Dave Hansen wrote:
> On 3/25/23 09:01, Kirill A. Shutemov wrote:
> > The last item is tricky. TDX guests use ACPI MADT MPWK to bring up
> > secondary CPUs. The mechanism doesn't allow to put a CPU back offline if
> > it has woken up.
> ...
> > +int arch_kexec_load(void)
> > +{
> > +	if (cpu_feature_enabled(X86_FEATURE_TDX_GUEST)) {
> > +		pr_warn_once("Disable kexec: not yet supported in TDX guest\n");
> > +		return -EOPNOTSUPP;
> > +	}
> > +
> > +	return 0;
> > +}
> 
> So, let's put all this together:
> 
> 1. TDX implementations use MADT for wakeup exclusively right now (but
>    are not necessarily _required_ to do so forever)
> 2. MADT doesn't support CPU offlining
> 3. kexec() requires offlining
> 
> Thus, current TDX implementations can't support TDX guests.  This
> *doesn't* say that TDX will always use the MADT for wakeups.
> 
> Yet, the check you have here is for TDX and *not* for the MADT.

As I described in the commit message there are more than MADT that is
required to get kexec in TDX guest.

> That seems wrong.
> 
> Let's say SEV or arm64 comes along and uses the MADT for their guests.
> They'll add another arch_kexec_load(), with a check for *their* feature.
> 
> This all seems like you should be disabling kexec() the moment the MADT
> CPU wakeup is used instead of making it based on TDX.

I guess we can go this path if you are fine with taking CR4.MCE and shared
memory reverting patches (they require some rework, but I can get them
into shape quickly). After that we can forbid kexec on machines with MADT
if nr_cpus > 1.

Sounds good?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov