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

Vitaly Kuznetsov
2019-06-12T10:17:34Z

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.

>
> 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@kernel.org>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Cathy Avery <cavery@redhat.com>
> Cc: Haiyang Zhang <haiyangz@microsoft.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: "K. Y. Srinivasan" <kys@microsoft.com>
> Cc: "Michael Kelley (EOSG)" <Michael.H.Kelley@microsoft.com>
> Cc: Mohammed Gamal <mmorsy@redhat.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Cc: Roman Kagan <rkagan@virtuozzo.com>
> Cc: Sasha Levin <sashal@kernel.org>
> Cc: Stephen Hemminger <sthemmin@microsoft.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
>
> Cc: devel@linuxdriverproject.org
> Cc: kvm@vger.kernel.org
> Cc: linux-hyperv@vger.kernel.org
> Cc: x86@kernel.org
> Reported-by: Prasanna Panchamukhi <panchamukhi@arista.com>
> Signed-off-by: Dmitry Safonov <dima@arista.com>
> ---
> 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@redhat.com>

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

--
Vitaly