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 always delays xen_hvm_init_time_ops() to later in
xen_hvm_smp_prepare_boot_cpu() after the 'vcpu_info' for boot vcpu is
registered.
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"
Unfortunately, the 'soft_reset' (kexec) does not work with mainline xen
version so that I can test this patch only with HVM guest on old xen
hypervisor where 'soft_reset' is working. The bugfix for PVM is not
implemented due to the lack of testing environment.
Cc: Joe Jin <joe....@oracle.com>
Signed-off-by: Dongli Zhang <dongli.zh...@oracle.com>
---
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)
Changed since v2:
- Delay for all VCPUs. (Suggested by Boris Ostrovsky)
- Add commit message that why PVM is not supported by this patch
- Test if kexec/kdump works with mainline xen (HVM and PVM)
arch/x86/xen/enlighten_hvm.c | 1 -
arch/x86/xen/smp_hvm.c | 17 +++++++++++++++++
2 files changed, 17 insertions(+), 1 deletion(-)
diff --git a/arch/x86/xen/enlighten_hvm.c b/arch/x86/xen/enlighten_hvm.c
index 517a9d8d8f94..53f306ec1d3b 100644
--- a/arch/x86/xen/enlighten_hvm.c
+++ b/arch/x86/xen/enlighten_hvm.c
@@ -216,7 +216,6 @@ 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();
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..9a5efc1a1633 100644
--- a/arch/x86/xen/smp_hvm.c
+++ b/arch/x86/xen/smp_hvm.c
@@ -19,6 +19,23 @@ static void __init xen_hvm_smp_prepare_boot_cpu(void)
*/
xen_vcpu_setup(0);
+ /*
+ * xen_hvm_init_time_ops() used to be called at very early stage
+ * by xen_hvm_guest_init(). While only MAX_VIRT_CPUS 'vcpu_info'
+ * are embedded inside 'shared_info', the VM would use them until
+ * xen_vcpu_setup() is used to allocate/relocate them at arbitrary
+ * address.
+ *
+ * However, when Xen HVM guest boots on vcpu >= MAX_VIRT_CPUS
+ * (e.g., kexec kernel), per_cpu(xen_vcpu, cpu) is NULL at early
+ * stage. To access per_cpu(xen_vcpu, cpu) via
+ * xen_clocksource_read() would panic the kernel.
+ *
+ * Therefore we always delay xen_hvm_init_time_ops() to
+ * xen_hvm_smp_prepare_boot_cpu() to avoid the panic.
+ */
+ 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
