[PATCH v2 1/1] xen: delay xen_hvm_init_time_ops() if kdump is boot on vcpu>=32

[Dongli Zhang <dongli.zhang@xxxxxxxxxx>]

The sched_clock() can be used very early since
commit 857baa87b642 ("sched/clock: Enable sched clock early"). In addition,
with commit 38669ba205d1 ("x86/xen/time: Output xen sched_clock time from
0"), kdump kernel in Xen HVM guest may panic at very early stage when
accessing &__this_cpu_read(xen_vcpu)->time as in below:

setup_arch()
 -> init_hypervisor_platform()
     -> x86_init.hyper.init_platform = xen_hvm_guest_init()
         -> xen_hvm_init_time_ops()
             -> xen_clocksource_read()
                 -> src = &__this_cpu_read(xen_vcpu)->time;

This is because Xen HVM supports at most MAX_VIRT_CPUS=32 'vcpu_info'
embedded inside 'shared_info' during early stage until xen_vcpu_setup() is
used to allocate/relocate 'vcpu_info' for boot cpu at arbitrary address.

However, when Xen HVM guest panic on vcpu >= 32, since
xen_vcpu_info_reset(0) would set per_cpu(xen_vcpu, cpu) = NULL when
vcpu >= 32, xen_clocksource_read() on vcpu >= 32 would panic.

This patch delays xen_hvm_init_time_ops() to later in
xen_hvm_smp_prepare_boot_cpu() after the 'vcpu_info' for boot vcpu is
registered when the boot vcpu is >= 32.

Another option is to always delay xen_hvm_init_time_ops() for any vcpus
(including vcpu=0). Since to delay xen_hvm_init_time_ops() may lead to
clock backward issue, it is preferred to avoid that for regular boot (The
pv_sched_clock=native_sched_clock() is used at the very beginning until
xen_sched_clock() is registered). That requires to adjust
xen_sched_clock_offset. That's why we only delay xen_hvm_init_time_ops()
for vcpu>=32.

This issue can be reproduced on purpose via below command at the guest
side when kdump/kexec is enabled:

"taskset -c 33 echo c > /proc/sysrq-trigger"

Reference:
https://lists.xenproject.org/archives/html/xen-devel/2021-10/msg00571.html
Cc: Joe Jin <joe.jin@xxxxxxxxxx>
Signed-off-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
---
Changed since v1:
  - Add commit message to explain why xen_hvm_init_time_ops() is delayed
    for any vcpus. (Suggested by Boris Ostrovsky)
  - Add a comment in xen_hvm_smp_prepare_boot_cpu() referencing the related
    code in xen_hvm_guest_init(). (suggested by Juergen Gross)

 arch/x86/xen/enlighten_hvm.c | 20 +++++++++++++++++++-
 arch/x86/xen/smp_hvm.c       |  8 ++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/arch/x86/xen/enlighten_hvm.c b/arch/x86/xen/enlighten_hvm.c
index e68ea5f4ad1c..7734dec52794 100644
--- a/arch/x86/xen/enlighten_hvm.c
+++ b/arch/x86/xen/enlighten_hvm.c
@@ -216,7 +216,25 @@ static void __init xen_hvm_guest_init(void)
        WARN_ON(xen_cpuhp_setup(xen_cpu_up_prepare_hvm, xen_cpu_dead_hvm));
        xen_unplug_emulated_devices();
        x86_init.irqs.intr_init = xen_init_IRQ;
-       xen_hvm_init_time_ops();
+
+       /*
+        * Only MAX_VIRT_CPUS 'vcpu_info' are embedded inside 'shared_info'
+        * and the VM would use them until xen_vcpu_setup() is used to
+        * allocate/relocate them at arbitrary address.
+        *
+        * However, when Xen HVM guest panic on vcpu >= MAX_VIRT_CPUS,
+        * per_cpu(xen_vcpu, cpu) is still NULL at this stage. To access
+        * per_cpu(xen_vcpu, cpu) via xen_clocksource_read() would panic.
+        *
+        * Therefore we delay xen_hvm_init_time_ops() to
+        * xen_hvm_smp_prepare_boot_cpu() when boot vcpu is >= MAX_VIRT_CPUS.
+        */
+       if (xen_vcpu_nr(0) >= MAX_VIRT_CPUS)
+               pr_info("Delay xen_hvm_init_time_ops() as kernel is running on 
vcpu=%d\n",
+                       xen_vcpu_nr(0));
+       else
+               xen_hvm_init_time_ops();
+
        xen_hvm_init_mmu_ops();
 
 #ifdef CONFIG_KEXEC_CORE
diff --git a/arch/x86/xen/smp_hvm.c b/arch/x86/xen/smp_hvm.c
index 6ff3c887e0b9..f99043df8bb5 100644
--- a/arch/x86/xen/smp_hvm.c
+++ b/arch/x86/xen/smp_hvm.c
@@ -19,6 +19,14 @@ static void __init xen_hvm_smp_prepare_boot_cpu(void)
         */
        xen_vcpu_setup(0);
 
+       /*
+        * The xen_hvm_init_time_ops() is delayed from
+        * xen_hvm_guest_init() to here to avoid panic when the kernel
+        * boots from vcpu>=MAX_VIRT_CPUS (32).
+        */
+       if (xen_vcpu_nr(0) >= MAX_VIRT_CPUS)
+               xen_hvm_init_time_ops();
+
        /*
         * The alternative logic (which patches the unlock/lock) runs before
         * the smp bootup up code is activated. Hence we need to set this up
-- 
2.17.1