Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   Bandan Das <bsd@redhat.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Chris Wilson <chris@chris-wilson.co.uk>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>
Subject: Re: Linux 5.3-rc7
References: <CAHk-=wi_nBULUyO=OKtNBCZ+VSqdOcEiUeFqXTQY_D5ga5k4gQ@mail.gmail.com>
        <156785100521.13300.14461504732265570003@skylake-alporthouse-com>
        <alpine.DEB.2.21.1909071628420.1902@nanos.tec.linutronix.de>
        <156786727951.13300.15226856788926071603@skylake-alporthouse-com>
        <alpine.DEB.2.21.1909071657430.1902@nanos.tec.linutronix.de>
        <CAHk-=wikdDMYqhygJYkoWw7YxpGNx7O2kFRxbG91NNeFO7yu3w@mail.gmail.com>
        <CAHk-=wimjeOCi4k7+nhzx3XWzvQUbMtNpcKNo8ZteodQ5c5APg@mail.gmail.com>
Date:   Mon, 09 Sep 2019 09:54:13 -0400
Message-ID: <jpga7bdpoca.fsf@linux.bootlegged.copy>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux)
MIME-Version: 1.0
Content-Type: text/plain
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Linus Torvalds <torvalds@linux-foundation.org> writes:

> On Sat, Sep 7, 2019 at 12:17 PM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
>>
>> I'm really not clear on why it's a good idea to clear the LDR bits on
>> shutdown, and commit 558682b52919 ("x86/apic: Include the LDR when
>> clearing out APIC registers") just looks pointless. And now it has
>> proven to break some machines.
>>
>> So why wouldn't we just revert it?
>
> Side note: looking around for the discussion about this patch, at
> least one version of the patch from Bandan had
>
> +       if (!x2apic_enabled) {
>
> rather than
>
> +       if (!x2apic_enabled()) {
>

I believe this crept up by accident when I was preparing the series, my testing
was with x2apic_enabled() but I didn't test CPU hotplug - only the kdump path
with 32 bit guest. In hindsight, I should have been more careful with testing,
sorry about that.

Bandan

> which meant that whatever Bandan tested at that point was actually a
> complete no-op, since "!x2apic_enabled" is never true (it tests a
> function pointer against NULL, which it won't be).
>
> Then that was fixed by the time it hit -tip (and eventually my tree),
> but it kind of shows how the patch history of this is all
> questionable. Further strengthened by a quote from that discussion:
>
>  "this is really a KVM bug but it doesn't hurt to clear out the LDR in
> the guest and then, it wouldn't need a hypervisor fix"
>
> and clearly it *does* hurt to clear the LDR in the guest, making the
> whole thinking behind the patch wrong and broken. The kernel clearly
> _does_ depend on LDR having the right contents.
>
> Now, I still suspect the boot problem then comes from our
> cpu0_logical_apicid use mentioned in that previous email, but at this
> point I think the proper fix is "revert for now, and we can look at
> this as a cleanup with the cpu0_logical_apicid thing for 5.4 instead".
>
> Hmm?
>
>                    Linus