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From: Linus Torvalds <>
To: Thomas Gleixner <>
Cc: Chris Wilson <>,
	Linux List Kernel Mailing <>,
	Bandan Das <>
Subject: Re: Linux 5.3-rc7
Date: Sat, 7 Sep 2019 12:27:42 -0700
Message-ID: <> (raw)
In-Reply-To: <>

On Sat, Sep 7, 2019 at 12:17 PM Linus Torvalds
<> 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".



  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 [this message]
2019-09-09 13:54             ` Bandan Das
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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