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

[Vitaly Kuznetsov]

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.

>
> The easiest solution seems to be disabling preemption while setting up
> reenlightment MSRs. While at it, fix hv_cpu_*() callbacks.
>
> Fixes: 93286261de1b4 ("x86/hyperv: Reenlightenment notifications
> support")
>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: Cathy Avery <cavery@xxxxxxxxxx>
> Cc: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: "K. Y. Srinivasan" <kys@xxxxxxxxxxxxx>
> Cc: "Michael Kelley (EOSG)" <Michael.H.Kelley@xxxxxxxxxxxxx>
> Cc: Mohammed Gamal <mmorsy@xxxxxxxxxx>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Cc: Roman Kagan <rkagan@xxxxxxxxxxxxx>
> Cc: Sasha Levin <sashal@xxxxxxxxxx>
> Cc: Stephen Hemminger <sthemmin@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
>
> Cc: devel@xxxxxxxxxxxxxxxxxxxxxx
> Cc: kvm@xxxxxxxxxxxxxxx
> Cc: linux-hyperv@xxxxxxxxxxxxxxx
> Cc: x86@xxxxxxxxxx
> Reported-by: Prasanna Panchamukhi <panchamukhi@xxxxxxxxxx>
> Signed-off-by: Dmitry Safonov <dima@xxxxxxxxxx>
> ---
>  arch/x86/hyperv/hv_init.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> index 1608050e9df9..0bdd79ecbff8 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;

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.

> @@ -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();

instead, there are no long-running things in the whole function. But
what you've done should work too, so

Reviewed-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>

>  	wrmsrl(HV_X64_MSR_TSC_EMULATION_CONTROL, *((u64 *)&emu_ctrl));
>  }
>  EXPORT_SYMBOL_GPL(set_hv_tscchange_cb);

-- 
Vitaly