linux-kernel - Re: [PATCH v2 3/3] x86/acpi, x86/boot: Add multiprocessor wake-up support

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Date:   Fri, 23 Apr 2021 10:58:33 -0700
From:   "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Rafael J Wysocki <rjw@...ysocki.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Len Brown <lenb@...nel.org>,
        Robert Moore <robert.moore@...el.com>,
        Erik Kaneda <erik.kaneda@...el.com>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [PATCH v2 3/3] x86/acpi, x86/boot: Add multiprocessor wake-up
 support

On 4/23/21 6:05 AM, Rafael J. Wysocki wrote:
> Although I've looked at this patch already, I now realize that
> according to the spec the mailbox is only suitable for the first AP
> wakeup during system initialization.
> 
> Shouldn't the original handler be restored then to handle subsequent wakeups?

For TDX use case, since it does not support CPU hotplug/offline features, once
the AP is turned on, it will never be off-lined and hence need not support
subsequent wakeups.

AFAIK, this MADT wake table is mainly defined for TDX use case.

Please also check the TDX GHCI spec, sec 4.1.

For each TD-guest, application processor, the mailbox can be used once for the
wakeup command. After the guest-application processor takes the action according
to the command, the intention is the mailbox will no longer be checked by the
guest-application processor. Other guest processors can continue using the mailbox
for the next command.

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer