linux-kernel - Re: [PATCH v3 3/3] x86/acpi, x86/boot: Add multiprocessor wake-up support

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Date:   Mon, 10 May 2021 19:24:38 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        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>,
        "open list:ACPI COMPONENT ARCHITECTURE (ACPICA)" <devel@...ica.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [PATCH v3 3/3] x86/acpi, x86/boot: Add multiprocessor wake-up support

On Mon, May 10, 2021 at 7:22 PM Andi Kleen <ak@...ux.intel.com> wrote:
>
> On Mon, May 10, 2021 at 10:10:24AM -0700, Kuppuswamy, Sathyanarayanan wrote:
> >
> >
> > On 5/10/21 9:55 AM, Rafael J. Wysocki wrote:
> > > I'm not sure how my comment regarding the fact that for a given CPU
> > > this function is only usable once has been addressed.
> > >
> > > While it may not be a practical concern in the use case that you are
> > > after (TDX), this is a generic mechanism and it needs to cover other
> > > possible usage scenarios.
> >
> > For the same CPU, if we try to use mailbox again, firmware will not
> > respond to it. So the command will timeout and return error.
>
> Right because the firmware code doesn't run anymore.
>
> The only possibility would be for Linux to put back some code that spins
> and waits again, but that would be quite pointless and wasteful.

The wakeup function can return an error when it is called for the
second time on the same CPU.