Re: [PATCH v2] KVM: x86: Convert INIT and SIPI signals into synchronously handled requests

[Jan Kiszka <jan.kiszka@web.de>]

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On 2013-03-05 09:46, Gleb Natapov wrote:
> On Tue, Mar 05, 2013 at 09:24:40AM +0100, Jan Kiszka wrote:
>> On 2013-03-05 08:57, Gleb Natapov wrote:
>>> On Tue, Mar 05, 2013 at 12:00:10AM +0100, Jan Kiszka wrote:
>>>> On 2013-03-04 22:41, Jan Kiszka wrote:
>>>>> From: Jan Kiszka <jan.kiszka@siemens.com>
>>>>>
>>>>> A VCPU sending INIT or SIPI to some other VCPU races for setting the
>>>>> remote VCPU's mp_state. When we were unlucky, KVM_MP_STATE_INIT_RECEIVED
>>>>> was overwritten by kvm_emulate_halt and, thus, got lost.
>>>>>
>>>>> Fix this by raising requests on the sender side that will then be
>>>>> handled synchronously over the target VCPU context.
>>>>>
>>>>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
>>>>> ---
>>>>>
>>>>> Changes in v2:
>>>>>  - check transition to INIT_RECEIVED in vcpu_enter_guest
>>>>>  - removed return value of kvm_check_init_and_sipi - caller has to
>>>>>    check for relevant transition afterward
>>>>>  - add write barrier after setting sipi_vector
>>>>>
>>>>>  arch/x86/kvm/lapic.c     |   11 ++++++-----
>>>>>  arch/x86/kvm/x86.c       |   15 +++++++++++++++
>>>>>  include/linux/kvm_host.h |    2 ++
>>>>>  3 files changed, 23 insertions(+), 5 deletions(-)
>>>>>
>>>>> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
>>>>> index 02b51dd..7986c9f 100644
>>>>> --- a/arch/x86/kvm/lapic.c
>>>>> +++ b/arch/x86/kvm/lapic.c
>>>>> @@ -731,8 +731,7 @@ static int __apic_accept_irq(struct kvm_lapic *apic, int delivery_mode,
>>>>>  	case APIC_DM_INIT:
>>>>>  		if (!trig_mode || level) {
>>>>>  			result = 1;
>>>>> -			vcpu->arch.mp_state = KVM_MP_STATE_INIT_RECEIVED;
>>>>> -			kvm_make_request(KVM_REQ_EVENT, vcpu);
>>>>> +			kvm_make_request(KVM_REQ_INIT, vcpu);
>>>>>  			kvm_vcpu_kick(vcpu);
>>>>>  		} else {
>>>>>  			apic_debug("Ignoring de-assert INIT to vcpu %d\n",
>>>>> @@ -743,11 +742,13 @@ static int __apic_accept_irq(struct kvm_lapic *apic, int delivery_mode,
>>>>>  	case APIC_DM_STARTUP:
>>>>>  		apic_debug("SIPI to vcpu %d vector 0x%02x\n",
>>>>>  			   vcpu->vcpu_id, vector);
>>>>> -		if (vcpu->arch.mp_state == KVM_MP_STATE_INIT_RECEIVED) {
>>>>> +		if (vcpu->arch.mp_state == KVM_MP_STATE_INIT_RECEIVED ||
>>>>> +		    test_bit(KVM_REQ_INIT, &vcpu->requests)) {
>>>>>  			result = 1;
>>>>>  			vcpu->arch.sipi_vector = vector;
>>>>> -			vcpu->arch.mp_state = KVM_MP_STATE_SIPI_RECEIVED;
>>>>> -			kvm_make_request(KVM_REQ_EVENT, vcpu);
>>>>> +			/* make sure sipi_vector is visible for the receiver */
>>>>> +			smp_wmb();
>>>>> +			kvm_make_request(KVM_REQ_SIPI, vcpu);
>>>>>  			kvm_vcpu_kick(vcpu);
>>>>>  		}
>>>>>  		break;
>>>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>>>> index d0cf737..0be04b9 100644
>>>>> --- a/arch/x86/kvm/x86.c
>>>>> +++ b/arch/x86/kvm/x86.c
>>>>> @@ -5641,6 +5641,15 @@ static void update_eoi_exitmap(struct kvm_vcpu *vcpu)
>>>>>  	kvm_x86_ops->load_eoi_exitmap(vcpu, eoi_exit_bitmap);
>>>>>  }
>>>>>  
>>>>> +static void kvm_check_init_and_sipi(struct kvm_vcpu *vcpu)
>>>>> +{
>>>>> +	if (kvm_check_request(KVM_REQ_INIT, vcpu))
>>>>> +		vcpu->arch.mp_state = KVM_MP_STATE_INIT_RECEIVED;
>>>>
>>>> And here is a small race between clearing REQ_INIT and setting
>>>> INIT_RECEIVED. It can make the LAPIC drop the SIPI incorrectly. Need to
>>>> break up test and clear, doing the clear after mp_state update. Yeah...
>>>>
>>> You also need to call kvm_check_init_and_sipi() in
>>> kvm_arch_vcpu_ioctl_get_mpstate(), 
>>
>> Indeed.
>>
>>> which means you now have three places
>>> where you transfer INIT/SIPI state from requests to mp_state. All the
>>> problems arise from the fact that now you have two places where you
>>> are storing current state.
>>
>> Not at all. I'm keeping the state in a single place, mp_state. I just
>> have to make sure that I do not loose asynchronous events - what INIT
>> and SIPI are.
>>
> As evident from this code:
>  +           if (vcpu->arch.mp_state == KVM_MP_STATE_INIT_RECEIVED ||
>  +               test_bit(KVM_REQ_INIT, &vcpu->requests)) {
> the state is in two places.

That's just to protect the content of sipi_vector during delivery. We
could drop the complete if clause if we protected that variable differently.

> 
>>> To overcome this we can either deprecated
>>> KVM_MP_STATE_INIT_RECEIVED/KVM_MP_STATE_SIPI_RECEIVED values for mp_state
>>> (use it only for migration purposes) and use separate state in APIC
>>> to hold those event, like with nmi, or why not go with Paolo's simple
>>> cmpxchg one?
>>
>> We need to replace most, if not all, manipulations of mp_state with
>> cmpxchg, verifying the state transitions there. And the request-based
>> approach still looks cleaner to me when it comes to implementing INIT
>> handling for nested modes. That will just trivially hook into
>> kvm_check_init_and_sipi.
>>
> The mp_state changes are rare, do not see the problem replacing all
> state changes with cmpxchg. I do not like request-based approach as
> implemented since we keep state in two places and constantly sync it
> back.

And I like it more as it avoids spurious state changes toward INIT. That
will happen if we misuse mp_state for event signaling, like we do so
far, having to fix it up later again because the INIT event turned out
to become an INIT VM-exit.

Jan



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