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

[Gleb Natapov <gleb@redhat.com>]

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.

> > 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.

--
			Gleb.