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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
Date: Mon, 09 Sep 2019 09:54:13 -0400
Message-ID: <jpga7bdpoca.fsf@linux.bootlegged.copy> (raw)
In-Reply-To: <CAHk-=wimjeOCi4k7+nhzx3XWzvQUbMtNpcKNo8ZteodQ5c5APg@mail.gmail.com>

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

  reply index

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-02 17:28 Linus Torvalds
2019-09-07 10:10 ` Chris Wilson
2019-09-07 14:29   ` Thomas Gleixner
2019-09-07 14:41     ` Chris Wilson
2019-09-07 15:00       ` Thomas Gleixner
2019-09-07 15:24         ` Chris Wilson
2019-09-07 20:12           ` Thomas Gleixner
2019-09-07 19:17         ` Linus Torvalds
2019-09-07 19:27           ` Linus Torvalds
2019-09-09 13:54             ` Bandan Das [this message]
2019-09-07 20:44           ` Thomas Gleixner
2019-09-07 21:13             ` Linus Torvalds
2019-09-08 11:02               ` Sasha Levin

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