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From:   Brian Gerst <brgerst@gmail.com>
Date:   Wed, 1 Apr 2020 10:38:51 -0400
Message-ID: <CAMzpN2h5u3gbRFfew-BSUH_=E509QirQaopiTBrVQc6Oq2AcvA@mail.gmail.com>
Subject: Re: [PATCH v2] x86/smpboot: Remove 486-isms from the modern AP boot path
To:     Andrew Cooper <andrew.cooper3@citrix.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Jan Kiszka <jan.kiszka@siemens.com>,
        James Morris <jmorris@namei.org>,
        David Howells <dhowells@redhat.com>,
        Matthew Garrett <mjg59@google.com>,
        Josh Boyer <jwboyer@redhat.com>,
        Steve Wahl <steve.wahl@hpe.com>,
        Mike Travis <mike.travis@hpe.com>,
        Dimitri Sivanich <dimitri.sivanich@hpe.com>,
        Arnd Bergmann <arnd@arndb.de>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Giovanni Gherdovich <ggherdovich@suse.cz>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Len Brown <len.brown@intel.com>,
        Kees Cook <keescook@chromium.org>,
        Martin Molnar <martin.molnar.programming@gmail.com>,
        Pingfan Liu <kernelfans@gmail.com>,
        jailhouse-dev@googlegroups.com
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On Wed, Apr 1, 2020 at 8:14 AM Andrew Cooper <andrew.cooper3@citrix.com> wrote:
>
> On 01/04/2020 12:39, Brian Gerst wrote:
> > On Wed, Apr 1, 2020 at 5:22 AM Andrew Cooper <andrew.cooper3@citrix.com> wrote:
> >> On 31/03/2020 23:53, Brian Gerst wrote:
> >>> On Tue, Mar 31, 2020 at 6:44 PM Andrew Cooper <andrew.cooper3@citrix.com> wrote:
> >>>> On 31/03/2020 23:23, Brian Gerst wrote:
> >>>>> On Tue, Mar 31, 2020 at 1:59 PM Andrew Cooper <andrew.cooper3@citrix.com> wrote:
> >>>>>> Linux has an implementation of the Universal Start-up Algorithm (MP spec,
> >>>>>> Appendix B.4, Application Processor Startup), which includes unconditionally
> >>>>>> writing to the Bios Data Area and CMOS registers.
> >>>>>>
> >>>>>> The warm reset vector is only necessary in the non-integrated Local APIC case.
> >>>>>> UV and Jailhouse already have an opt-out for this behaviour, but blindly using
> >>>>>> the BDA and CMOS on a UEFI or other reduced hardware system isn't clever.
> >>>>>>
> >>>>>> We could make this conditional on the integrated-ness of the Local APIC, but
> >>>>>> 486-era SMP isn't supported.  Drop the logic completely, tidying up the includ
> >>>>>> list and header files as appropriate.
> >>>>>>

> >>>>> You removed x86_platform.legacy.warm_reset in the original patch, but
> >>>>> that is missing in V2.
> >>>> Second hunk?  Or are you referring to something different?
> >>> Removing the warm_reset field from struct x86_legacy_features.
> >> Ok, but that is still present as the 2nd hunk of the patch.
> > My apologies, Gmail was hiding that section of the patch because it
> > was a reply to the original patch.  For future reference, add the
> > version number to the title when resubmitting a patch (ie. [PATCH
> > v2]).
>
> Erm... is Gmail hiding that too?
>
> Lore thinks it is there:
> https://lore.kernel.org/lkml/CAMzpN2g0LS5anGc7CXco4pgBHhGzc8hw+shMOg8WEWGsx+BHpg@mail.gmail.com/

Ugh, yes.  I thought it was the title that Gmail threaded on, but it
must be the In-Reply-To: header.  Sorry for the confusion.

That said, I think the v1 patch is probably the better way to go (but
adjusting the comments to include early Pentium-era systems without
integrated APICs).  Then the decision to drop support for external
APICs could be a separate patch.

--
Brian Gerst