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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sat, 7 Sep 2019 12:27:42 -0700
Message-ID: <CAHk-=wimjeOCi4k7+nhzx3XWzvQUbMtNpcKNo8ZteodQ5c5APg@mail.gmail.com>
Subject: Re: Linux 5.3-rc7
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Chris Wilson <chris@chris-wilson.co.uk>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Bandan Das <bsd@redhat.com>
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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()) {

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