[PATCH v4 2/2] xen: delay xen_hvm_init_time_ops() if kdump is boot on vcpu>=32

[Dongli Zhang]

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 calls xen_hvm_init_time_ops() again later in
xen_hvm_smp_prepare_boot_cpu() after the 'vcpu_info' for boot vcpu is
registered when the boot vcpu is >= 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"

The bugfix for PVM is not implemented due to the lack of testing
environment.

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)
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)
Changed since v3:
  - Re-use v2 but move the login into xen_hvm_init_time_ops() (Suggested
    by Boris Ostrovsky)

 arch/x86/xen/smp_hvm.c |  6 ++++++
 arch/x86/xen/time.c    | 25 ++++++++++++++++++++++++-
 2 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/arch/x86/xen/smp_hvm.c b/arch/x86/xen/smp_hvm.c
index 6ff3c887e0b9..b70afdff419c 100644
--- a/arch/x86/xen/smp_hvm.c
+++ b/arch/x86/xen/smp_hvm.c
@@ -19,6 +19,12 @@ static void __init xen_hvm_smp_prepare_boot_cpu(void)
 	 */
 	xen_vcpu_setup(0);
 
+	/*
+	 * Called again in case the kernel boots on vcpu >= MAX_VIRT_CPUS.
+	 * Refer to comments in xen_hvm_init_time_ops().
+	 */
+	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
diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c
index 55b3407358a9..dcf292cc859e 100644
--- a/arch/x86/xen/time.c
+++ b/arch/x86/xen/time.c
@@ -558,16 +558,36 @@ static void xen_hvm_setup_cpu_clockevents(void)
 
 void __init xen_hvm_init_time_ops(void)
 {
+	static bool hvm_time_initialized;
+
+	if (hvm_time_initialized)
+		return;
+
 	/*
 	 * vector callback is needed otherwise we cannot receive interrupts
 	 * on cpu > 0 and at this point we don't know how many cpus are
 	 * available.
 	 */
 	if (!xen_have_vector_callback)
-		return;
+		goto exit;
 
 	if (!xen_feature(XENFEAT_hvm_safe_pvclock)) {
 		pr_info("Xen doesn't support pvclock on HVM, disable pv timer");
+		goto exit;
+	}
+
+	/*
+	 * Only MAX_VIRT_CPUS 'vcpu_info' are embedded inside 'shared_info'.
+	 * The __this_cpu_read(xen_vcpu) is still NULL when Xen HVM guest
+	 * boots on vcpu >= MAX_VIRT_CPUS (e.g., kexec), To access
+	 * __this_cpu_read(xen_vcpu) via xen_clocksource_read() will panic.
+	 *
+	 * The xen_hvm_init_time_ops() should be called again later after
+	 * __this_cpu_read(xen_vcpu) is available.
+	 */
+	if (!__this_cpu_read(xen_vcpu)) {
+		pr_info("Delay xen_init_time_common() as kernel is running on vcpu=%d\n",
+			xen_vcpu_nr(0));
 		return;
 	}
 
@@ -577,6 +597,9 @@ void __init xen_hvm_init_time_ops(void)
 	x86_cpuinit.setup_percpu_clockev = xen_hvm_setup_cpu_clockevents;
 
 	x86_platform.set_wallclock = xen_set_wallclock;
+
+exit:
+	hvm_time_initialized = true;
 }
 #endif
 
-- 
2.17.1