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SubjectRe: [PATCH] x86/hyperv: Disable preemption while setting reenlightenment vector
Date
Peter Zijlstra <peterz@infradead.org> writes:

> 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@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;
>> @@ -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.

Actually, we don't care which CPU will receive the reenlightenment
notification and TSC Emulation in Hyper-V is, of course, global. We have
code which re-assignes the notification to some other CPU in case the
one it's currently assigned to goes away (see hv_cpu_die()).

--
Vitaly

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