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 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.

> 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.

Jan



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