linux-kernel - Re: [PATCH v3] KVM: nVMX: Fix attempting to emulate "Acknowledge interrupt on exit" when there is no interrupt which L1 requires to inject to L2

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Date:   Thu, 3 Aug 2017 07:01:28 +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 v3] KVM: nVMX: Fix attempting to emulate "Acknowledge
 interrupt on exit" when there is no interrupt which L1 requires to inject to L2

2017-08-03 4:26 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
> 2017-08-02 03:48-0700, Wanpeng Li:
>> From: Wanpeng Li <wanpeng.li@...mail.com>
>>
>> ------------[ cut here ]------------
>>  WARNING: CPU: 5 PID: 2288 at arch/x86/kvm/vmx.c:11124 nested_vmx_vmexit+0xd64/0xd70 [kvm_intel]
>>  CPU: 5 PID: 2288 Comm: qemu-system-x86 Not tainted 4.13.0-rc2+ #7
>>  RIP: 0010:nested_vmx_vmexit+0xd64/0xd70 [kvm_intel]
>> Call Trace:
>>   vmx_check_nested_events+0x131/0x1f0 [kvm_intel]
>>   ? vmx_check_nested_events+0x131/0x1f0 [kvm_intel]
>>   kvm_arch_vcpu_ioctl_run+0x5dd/0x1be0 [kvm]
>>   ? vmx_vcpu_load+0x1be/0x220 [kvm_intel]
>>   ? kvm_arch_vcpu_load+0x62/0x230 [kvm]
>>   kvm_vcpu_ioctl+0x340/0x700 [kvm]
>>   ? kvm_vcpu_ioctl+0x340/0x700 [kvm]
>>   ? __fget+0xfc/0x210
>>   do_vfs_ioctl+0xa4/0x6a0
>>   ? __fget+0x11d/0x210
>>   SyS_ioctl+0x79/0x90
>>   do_syscall_64+0x8f/0x750
>>   ? trace_hardirqs_on_thunk+0x1a/0x1c
>>   entry_SYSCALL64_slow_path+0x25/0x25
>>
>> This can be reproduced by booting L1 guest w/ 'noapic' grub parameter, which
>> means that tells the kernel to not make use of any IOAPICs that may be present
>> in the system.
>>
>> Actually external_intr variable in nested_vmx_vmexit() is the req_int_win
>> variable passed from vcpu_enter_guest() which means that the L0's userspace
>> requests an irq window. I observed the scenario (!kvm_cpu_has_interrupt(vcpu) &&
>> L0's userspace reqeusts an irq window) is true, so there is no interrupt which
>> L1 requires to inject to L2, we should not attempt to emualte "Acknowledge
>> interrupt on exit" for the irq window requirement in this scenario.
>>
>> SDM says that with acknowledge interrupt on exit, bit 31 of the VM-exit
>> interrupt information (valid interrupt) is always set to 1 on
>> EXIT_REASON_EXTERNAL_INTERRUPT.  We don't want to break hypervisors
>> expecting an interrupt in that case, so we should do a userspace VM exit
>> when the window is open and then inject the userspace interrupt with a
>> VM exit.
>>
>> Cc: Paolo Bonzini <pbonzini@...hat.com>
>> Cc: Radim Krčmář <rkrcmar@...hat.com>
>> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
>> ---
>> v2 -> v3:
>>  * request an irq window and don't nested vmexit
>> v1 -> v2:
>>  * update patch description
>>  * check nested_exit_intr_ack_set() first
>>
>>  arch/x86/kvm/vmx.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index 80b20e8..9ef2ec3 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -10761,7 +10761,8 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu, bool external_intr)
>>               return 0;
>>       }
>>
>> -     if ((kvm_cpu_has_interrupt(vcpu) || external_intr) &&
>> +     if ((kvm_cpu_has_interrupt(vcpu) ||
>> +         (external_intr && !nested_exit_intr_ack_set(vcpu))) &&
>
> I think it would be safer to also add something like the second hunk I
> posted (that also takes nested_exit_on_intr() into account).
>
> The issue is that we're allowing L2's GUEST_RFLAGS and
> GUEST_INTERRUPTIBILITY_INFO to disable userspace interrupt injection
> even though neither affect delivery of interrupts into L1.
> This means that L2 can block/postpone the delivery to L1 by doing "cli;
> busy_loop/normal_critical_section".

Ouch! My fault, the v3 patch w/o the second hunk and w/ the second
hunk both can result in L1 guest softlockup. I just tested the patch
with L2 windows guest yesterday, however, the softlockup can happen
when the L2 is the linux guest. So should we still take the v2 for the
moment?

Regards,
Wanpeng Li

>
> Thanks.
>
>>           nested_exit_on_intr(vcpu)) {
>>               if (vmx->nested.nested_run_pending)
>>                       return -EBUSY;
>> --
>> 2.7.4
>>