linux-kernel - Re: [PATCH v2] KVM: nVMX: Fix trying to cancel vmlauch/vmresume

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Date:   Tue, 22 Aug 2017 06:55:17 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH v2] KVM: nVMX: Fix trying to cancel vmlauch/vmresume

2017-08-22 0:20 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
> 2017-08-18 07:11-0700, Wanpeng Li:
>> From: Wanpeng Li <wanpeng.li@...mail.com>
>>
>> ------------[ cut here ]------------
>> WARNING: CPU: 7 PID: 3861 at /home/kernel/ssd/kvm/arch/x86/kvm//vmx.c:11299 nested_vmx_vmexit+0x176e/0x1980 [kvm_intel]
>> CPU: 7 PID: 3861 Comm: qemu-system-x86 Tainted: G        W  OE   4.13.0-rc4+ #11
>> RIP: 0010:nested_vmx_vmexit+0x176e/0x1980 [kvm_intel]
>> Call Trace:
>>  ? kvm_multiple_exception+0x149/0x170 [kvm]
>>  ? handle_emulation_failure+0x79/0x230 [kvm]
>>  ? load_vmcs12_host_state+0xa80/0xa80 [kvm_intel]
>>  ? check_chain_key+0x137/0x1e0
>>  ? reexecute_instruction.part.168+0x130/0x130 [kvm]
>>  nested_vmx_inject_exception_vmexit+0xb7/0x100 [kvm_intel]
>>  ? nested_vmx_inject_exception_vmexit+0xb7/0x100 [kvm_intel]
>>  vmx_queue_exception+0x197/0x300 [kvm_intel]
>>  kvm_arch_vcpu_ioctl_run+0x1b0c/0x2c90 [kvm]
>>  ? kvm_arch_vcpu_runnable+0x220/0x220 [kvm]
>>  ? preempt_count_sub+0x18/0xc0
>>  ? restart_apic_timer+0x17d/0x300 [kvm]
>>  ? kvm_lapic_restart_hv_timer+0x37/0x50 [kvm]
>>  ? kvm_arch_vcpu_load+0x1d8/0x350 [kvm]
>>  kvm_vcpu_ioctl+0x4e4/0x910 [kvm]
>>  ? kvm_vcpu_ioctl+0x4e4/0x910 [kvm]
>>  ? kvm_dev_ioctl+0xbe0/0xbe0 [kvm]
>>
>> The flag "nested_run_pending", which can override the decision of which should run
>> next, L1 or L2. nested_run_pending=1 means that we *must* run L2 next, not L1. This
>> is necessary in particular when L1 did a VMLAUNCH of L2 and therefore expects L2 to
>> be run (and perhaps be injected with an event it specified, etc.). Nested_run_pending
>> is especially intended to avoid switching  to L1 in the injection decision-point.
>>
>> I catch this in the queue exception path, this patch fixes it by requesting
>> an immediate VM exit from L2 and keeping the exception for L1 pending for a
>> subsequent nested VM exit.
>>
>> Cc: Paolo Bonzini <pbonzini@...hat.com>
>> Cc: Radim Krčmář <rkrcmar@...hat.com>
>> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
>> ---
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> @@ -6356,8 +6356,8 @@ static int inject_pending_event(struct kvm_vcpu *vcpu, bool req_int_win)
>>                       kvm_update_dr7(vcpu);
>>               }
>
> Hm, we shouldn't execute the code above if exception won't be injected.
>
>>
>> -             kvm_x86_ops->queue_exception(vcpu);
>> -             return 0;
>
> vmx_complete_interrupts() assumes that the exception is always injected,
> so it would be dropped by kvm_clear_exception_queue().
>
> I'm starting to wonder whether getting rid of nested_run_pending
> wouldn't be nicer.

Yeah, I rethink of your concern for nested_run_pending w/ return value
is 0, actually the path in the calltrace is the else branch in
nested_vmx_check_exception(), an exception will be injected to L2 by
L1 if L1 owns this exception, otherwise injected by L0 directly. For
the nested_run_pending w/ return value is 0 stuff, we can treat it as
L0 injects the exception to L2 directly. So there is no exception is
injected to wrong guest.

Regards,
Wanpeng Li