Re: [PATCH] x86/hyperv: Disable preemption while setting reenlightenment vector

[Peter Zijlstra]

On Wed, Jun 12, 2019 at 12:17:24PM +0200, Vitaly Kuznetsov wrote:
> Dmitry Safonov <dima@xxxxxxxxxx> writes:
> 
> > 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

> > @@ -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.

> > @@ -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};