RE: [PATCH] x86/hyperv: Initialize clockevents after LAPIC is initialized

From: Dexuan Cui <decui@microsoft.com> Sent: Saturday, January 16, 2021 2:32 PM

With commit 4df4cb9e99f8, the Hyper-V direct-mode STIMER is actually
initialized before LAPIC is initialized: see

  apic_intr_mode_init()

    x86_platform.apic_post_init()
      hyperv_init()
        hv_stimer_alloc()

    apic_bsp_setup()
      setup_local_APIC()

setup_local_APIC() temporarily disables LAPIC, initializes it and
re-eanble it.  The direct-mode STIMER depends on LAPIC, and when it's
registered, it can be programmed immediately and the timer can fire
very soon:

  hv_stimer_init
    clockevents_config_and_register
      clockevents_register_device
        tick_check_new_device
          tick_setup_device
            tick_setup_periodic(), tick_setup_oneshot()
              clockevents_program_event

When the timer fires in the hypervisor, if the LAPIC is in the
disabled state, new versions of Hyper-V ignore the event and don't inject
the timer interrupt into the VM, and hence the VM hangs when it boots.

Note: when the VM starts/reboots, the LAPIC is pre-enabled by the
firmware, so the window of LAPIC being temporarily disabled is pretty
small, and the issue can only happen once out of 100~200 reboots for
a 40-vCPU VM on one dev host, and on another host the issue doesn't
reproduce after 2000 reboots.

The issue is more noticeable for kdump/kexec, because the LAPIC is
disabled by the first kernel, and stays disabled until the kdump/kexec
kernel enables it. This is especially an issue to a Generation-2 VM
(for which Hyper-V doesn't emulate the PIT timer) when CONFIG_HZ=1000
(rather than CONFIG_HZ=250) is used.

Fix the issue by moving hv_stimer_alloc() to a later place where the
LAPIC timer is initialized.

Fixes: 4df4cb9e99f8 ("x86/hyperv: Initialize clockevents earlier in CPU onlining")
Signed-off-by: Dexuan Cui <decui@microsoft.com>
---
 arch/x86/hyperv/hv_init.c | 29 ++++++++++++++++++++++++++---
 1 file changed, 26 insertions(+), 3 deletions(-)

Reviewed-by:  Michael Kelley <redacted>