[PATCHv2] x86/hyperv: Hold cpus_read_lock() on assigning reenlightenment vector

Dmitry Safonov dima at arista.com
Mon Jun 17 16:39:55 UTC 2019

KVM support may be compiled as dynamic module, which triggers the
following splat on modprobe (under CONFIG_DEBUG_PREEMPT):

 KVM: vmx: using Hyper-V Enlightened VMCS
 BUG: using smp_processor_id() in preemptible [00000000] code: modprobe/466
 caller is debug_smp_processor_id+0x17/0x19
 CPU: 0 PID: 466 Comm: modprobe Kdump: loaded Not tainted 4.19.43 #1
 Hardware name: Microsoft Corporation Virtual Machine/Virtual Machine, BIOS 090007  06/02/2017
 Call Trace:
  kvm_arch_init+0x14a/0x163 [kvm]
  kvm_init+0x30/0x259 [kvm]
  vmx_init+0xed/0x3db [kvm_intel]

Hold cpus_read_lock() so that MSR will be written for an online CPU,
even if set_hv_tscchange_cb() gets being preempted.
While at it, cleanup smp_processor_id()'s in hv_cpu_init() and add a
lockdep assert into hv_cpu_die().

Fixes: 93286261de1b4 ("x86/hyperv: Reenlightenment notifications

Cc: Andy Lutomirski <luto at kernel.org>
Cc: Borislav Petkov <bp at alien8.de>
Cc: Cathy Avery <cavery at redhat.com>
Cc: Haiyang Zhang <haiyangz at microsoft.com>
Cc: "H. Peter Anvin" <hpa at zytor.com>
Cc: Ingo Molnar <mingo at redhat.com>
Cc: "K. Y. Srinivasan" <kys at microsoft.com>
Cc: "Michael Kelley (EOSG)" <Michael.H.Kelley at microsoft.com>
Cc: Mohammed Gamal <mmorsy at redhat.com>
Cc: Paolo Bonzini <pbonzini at redhat.com>
Cc: Peter Zijlstra <peterz at infradead.org>
Cc: Radim Krčmář <rkrcmar at redhat.com>
Cc: Roman Kagan <rkagan at virtuozzo.com>
Cc: Sasha Levin <sashal at kernel.org>
Cc: Stephen Hemminger <sthemmin at microsoft.com>
Cc: Thomas Gleixner <tglx at linutronix.de>
Cc: Vitaly Kuznetsov <vkuznets at redhat.com>

Cc: devel at linuxdriverproject.org
Cc: kvm at vger.kernel.org
Cc: linux-hyperv at vger.kernel.org
Cc: x86 at kernel.org
Reported-by: Prasanna Panchamukhi <panchamukhi at arista.com>
Signed-off-by: Dmitry Safonov <dima at arista.com>
v1 link: lkml.kernel.org/r/20190611212003.26382-1-dima at arista.com

NOTE that I hadn't a chance to test v2 on hyperv machine so far,
ONLY BUILD TESTED. (In hope that the patch still makes sense and Kbuild
bot will report any issue).

 arch/x86/hyperv/hv_init.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
index 1608050e9df9..ec7fd7d6c125 100644
--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c
@@ -20,6 +20,7 @@
 #include <linux/clockchips.h>
 #include <linux/hyperv.h>
 #include <linux/slab.h>
+#include <linux/cpu.h>
 #include <linux/cpuhotplug.h>
@@ -91,7 +92,7 @@ EXPORT_SYMBOL_GPL(hv_max_vp_index);
 static int hv_cpu_init(unsigned int cpu)
 	u64 msr_vp_index;
-	struct hv_vp_assist_page **hvp = &hv_vp_assist_page[smp_processor_id()];
+	struct hv_vp_assist_page **hvp = &hv_vp_assist_page[cpu];
 	void **input_arg;
 	struct page *pg;
@@ -103,7 +104,7 @@ static int hv_cpu_init(unsigned int cpu)
-	hv_vp_index[smp_processor_id()] = msr_vp_index;
+	hv_vp_index[cpu] = msr_vp_index;
 	if (msr_vp_index > hv_max_vp_index)
 		hv_max_vp_index = msr_vp_index;
@@ -182,7 +183,6 @@ void set_hv_tscchange_cb(void (*cb)(void))
 	struct hv_reenlightenment_control re_ctrl = {
 		.enabled = 1,
-		.target_vp = hv_vp_index[smp_processor_id()]
 	struct hv_tsc_emulation_control emu_ctrl = {.enabled = 1};
@@ -196,7 +196,16 @@ void set_hv_tscchange_cb(void (*cb)(void))
 	/* Make sure callback is registered before we write to MSRs */
+	/*
+	 * As reenlightenment vector is global, there is no difference which
+	 * CPU will register MSR, though it should be an online CPU.
+	 * hv_cpu_die() callback guarantees that on CPU teardown
+	 * another CPU will re-register MSR back.
+	 */
+	cpus_read_lock();
+	re_ctrl.target_vp = hv_vp_index[raw_smp_processor_id()];
 	wrmsrl(HV_X64_MSR_REENLIGHTENMENT_CONTROL, *((u64 *)&re_ctrl));
+	cpus_read_unlock();
 	wrmsrl(HV_X64_MSR_TSC_EMULATION_CONTROL, *((u64 *)&emu_ctrl));
@@ -239,6 +248,7 @@ static int hv_cpu_die(unsigned int cpu)
 	rdmsrl(HV_X64_MSR_REENLIGHTENMENT_CONTROL, *((u64 *)&re_ctrl));
 	if (re_ctrl.target_vp == hv_vp_index[cpu]) {
+		lockdep_assert_cpus_held();
 		/* Reassign to some other online CPU */
 		new_cpu = cpumask_any_but(cpu_online_mask, cpu);

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