On Wed, Jun 12, 2019 at 12:17:24PM +0200, Vitaly Kuznetsov wrote:

Dmitry Safonov [off-list ref] writes:

quoted

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

 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:
  dump_stack+0x61/0x7e
  check_preemption_disabled+0xd4/0xe6
  debug_smp_processor_id+0x17/0x19
  set_hv_tscchange_cb+0x1b/0x89
  kvm_arch_init+0x14a/0x163 [kvm]
  kvm_init+0x30/0x259 [kvm]
  vmx_init+0xed/0x3db [kvm_intel]
  do_one_initcall+0x89/0x1bc
  do_init_module+0x5f/0x207
  load_module+0x1b34/0x209b
  __ia32_sys_init_module+0x17/0x19
  do_fast_syscall_32+0x121/0x1fa
  entry_SYSENTER_compat+0x7f/0x91

Hm, I never noticed this one, you probably need something like
CONFIG_PREEMPT enabled so see it.

CONFIG_DEBUG_PREEMPT

quoted

@@ -91,7 +91,7 @@ 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 +103,7 @@ static int hv_cpu_init(unsigned int cpu)
 
 	hv_get_vp_index(msr_vp_index);
 
-	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;

The above is unrelated cleanup (as cpu == smp_processor_id() for
CPUHP_AP_ONLINE_DYN callbacks), right? As I'm pretty sure these can'd be
preempted.

Yeah, makes sense though.

quoted

@@ -182,7 +182,6 @@ void set_hv_tscchange_cb(void (*cb)(void))
 	struct hv_reenlightenment_control re_ctrl = {
 		.vector = HYPERV_REENLIGHTENMENT_VECTOR,
 		.enabled = 1,
-		.target_vp = hv_vp_index[smp_processor_id()]
 	};
 	struct hv_tsc_emulation_control emu_ctrl = {.enabled = 1};
 

@@ -196,7 +195,11 @@ void set_hv_tscchange_cb(void (*cb)(void))
 	/* Make sure callback is registered before we write to MSRs */
 	wmb();
 
+	preempt_disable();
+	re_ctrl.target_vp = hv_vp_index[smp_processor_id()];
 	wrmsrl(HV_X64_MSR_REENLIGHTENMENT_CONTROL, *((u64 *)&re_ctrl));
+	preempt_enable();
+

My personal preference would be to do something like
   int cpu = get_cpu();

   ... set things up ...

   put_cpu();

If it doesn't matter, how about this then?

---

diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
index 1608050e9df9..e58c693a9fce 100644
--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c

@@ -91,7 +91,7 @@ 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 +103,7 @@ static int hv_cpu_init(unsigned int cpu)
 
 	hv_get_vp_index(msr_vp_index);
 
-	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 +182,7 @@ void set_hv_tscchange_cb(void (*cb)(void))
 	struct hv_reenlightenment_control re_ctrl = {
 		.vector = HYPERV_REENLIGHTENMENT_VECTOR,
 		.enabled = 1,
-		.target_vp = hv_vp_index[smp_processor_id()]
+		.target_vp = hv_vp_index[raw_smp_processor_id()]
 	};
 	struct hv_tsc_emulation_control emu_ctrl = {.enabled = 1};