linux-kernel - Re: [PATCH 0/2] Kexec enabling in TDX guest

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Date:   Fri, 24 Feb 2023 17:30:04 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Isaku Yamahata <isaku.yamahata@...el.com>, x86@...nel.org,
        linux-coco@...ts.linux.dev, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] Kexec enabling in TDX guest

On Wed, Feb 22, 2023 at 10:26:22AM +0000, David Woodhouse wrote:
> On Tue, 2023-02-14 at 02:48 +0300, Kirill A. Shutemov wrote:
> > The patch brings basic enabling of kexec in TDX guests.
> > 
> > By "basic enabling" I mean, kexec in the guests with a single CPU.
> > 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.
> > 
> > We are looking into this, but it might take time.
> 
> Can't we park the secondary CPUs in a purgatory-like thing of their own
> and wake them from there when we want them?
> 
> Patches for that were floating around once, although the primary reason
> then was latency, and we decided to address that differently by doing
> the bringup in parallel instead.

That's plan B. It is suboptimal. kexec() can happen into something that is
not Linux which will not be able to wake up CPUs.

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.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov