linux-kernel - Re: [PATCH] x86: Disable kexec for TDX guests

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Message-ID: <ebead33b-0594-73df-56ae-f40473ac0ffc@intel.com>
Date:   Sat, 25 Mar 2023 09:25:36 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        Eric Biederman <ebiederm@...ssion.com>,
        kexec@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: Disable kexec for TDX guests

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.