Date:   Fri, 24 Feb 2023 19:12:19 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     David Woodhouse <dwmw2@infradead.org>,
        Borislav Petkov <bp@alien8.de>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Isaku Yamahata <isaku.yamahata@intel.com>, x86@kernel.org,
        linux-coco@lists.linux.dev, kexec@lists.infradead.org,
        linux-kernel@vger.kernel.org, rafael@kernel.org
Subject: Re: [PATCH 0/2] Kexec enabling in TDX guest
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On Fri, Feb 24, 2023 at 07:22:18AM -0800, Dave Hansen wrote:
> On 2/24/23 06:30, Kirill A. Shutemov wrote:
> > Ideally, it has to be addressed on BIOS level: it has to provide a way to
> > offline CPUs, putting it back to pre-wakeup state.
> 
> Is there anything stopping us from just parking the CPUs in a loop
> looking at 'acpi_mp_wake_mailbox_paddr'?  Basically park them in a way
> which is indistinguishable from what the BIOS did.

+Rafael.

 - Forward compatibility can be an issue. Version 0 of mailbox supports
   only single Wakeup command. Future specs may define a new command that
   kernel implementation doesn't support.

 - BIOS owns the mailbox page and can re-use for something else after the
   last CPU has woken up. (I know it is very theoretical, but still.)

 - We can patch ACPI table to point to mailbox page in kernel allocated
   memory, but it brings other problem. If the first kernel didn't wake up
   all CPUs for some reason (CONFIG_SMP=n or nr_cpus= or something) the
   second kernel would not be able to wake up them too since they looping
   around the old address.

But ultimately, I think it is clearly missing BIOS functionality and has
to be addressed there. Hacking around it in kernel will lead to more
problems down the road.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov