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

Peter Zijlstra
2019-06-14T08:28:26Z

On Wed, Jun 12, 2019 at 12:17:24PM +0200, Vitaly Kuznetsov wrote:
> Dmitry Safonov <dima@arista.com> 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};