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

Dave Hansen
2023-03-25T16:25:46Z

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.

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.