Revert "x86/apic: Include the LDR when clearing out APIC registers" [Linux 4.4.192]

This Linux kernel change "Revert “x86/apic: Include the LDR when clearing out APIC registers”" is included in the Linux 4.4.192 release. This change is authored by Linus Torvalds <torvalds [at] linux-foundation.org> on Sat Sep 7 14:25:54 2019 -0700. The commit for this change in Linux stable tree is 94eb535 (patch) which is from upstream commit 950b07c. The same Linux upstream change may have been applied to various maintained Linux releases and you can find all Linux releases containing changes from upstream 950b07c.

Revert "x86/apic: Include the LDR when clearing out APIC registers"

[ Upstream commit 950b07c14e8c59444e2359f15fd70ed5112e11a0 ]

This reverts commit 558682b5291937a70748d36fd9ba757fb25b99ae.

Chris Wilson reports that it breaks his CPU hotplug test scripts.  In
particular, it breaks offlining and then re-onlining the boot CPU, which
we treat specially (and the BIOS does too).

The symptoms are that we can offline the CPU, but it then does not come
back online again:

    smpboot: CPU 0 is now offline
    smpboot: Booting Node 0 Processor 0 APIC 0x0
    smpboot: do_boot_cpu failed(-1) to wakeup CPU#0

Thomas says he knows why it's broken (my personal suspicion: our magic
handling of the "cpu0_logical_apicid" thing), but for 5.3 the right fix
is to just revert it, since we've never touched the LDR bits before, and
it's not worth the risk to do anything else at this stage.

[ Hotpluging of the boot CPU is special anyway, and should be off by
  default. See the "BOOTPARAM_HOTPLUG_CPU0" config option and the
  cpu0_hotplug kernel parameter.

  In general you should not do it, and it has various known limitations
  (hibernate and suspend require the boot CPU, for example).

  But it should work, even if the boot CPU is special and needs careful
  treatment       - Linus ]

Link: https://lore.kernel.org/lkml/156785100521.13300.14461504732265570003@skylake-alporthouse-com/
Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Bandan Das <bsd@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

There are 4 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 arch/x86/kernel/apic/apic.c | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index 80c94fc..834d1b5 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -1031,10 +1031,6 @@ void clear_local_APIC(void)
    apic_write(APIC_LVT0, v | APIC_LVT_MASKED);
    v = apic_read(APIC_LVT1);
    apic_write(APIC_LVT1, v | APIC_LVT_MASKED);
-   if (!x2apic_enabled()) {
-       v = apic_read(APIC_LDR) & ~APIC_LDR_MASK;
-       apic_write(APIC_LDR, v);
-   }
    if (maxlvt >= 4) {
        v = apic_read(APIC_LVTPC);
        apic_write(APIC_LVTPC, v | APIC_LVT_MASKED);

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