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

[Peter Zijlstra]

On Tue, Jun 11, 2019 at 10:20:03PM +0100, Dmitry Safonov wrote:
> 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
> 
> 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;
> @@ -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();
> +
>  	wrmsrl(HV_X64_MSR_TSC_EMULATION_CONTROL, *((u64 *)&emu_ctrl));
>  }
>  EXPORT_SYMBOL_GPL(set_hv_tscchange_cb);

This looks bogus, MSRs are a per-cpu resource, you had better know what
CPUs you're on and be stuck to it when you do wrmsr. This just fudges
the code to make the warning go away and doesn't fix the actual problem
afaict.