Re: [PATCH v2] KVM: x86: Rework INIT and SIPI handling

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, Mar 13, 2013 at 12:42:34PM +0100, Jan Kiszka wrote:
> 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.
> 
> This introduces APIC events for those two signals, keeping them in
> kvm_apic until kvm_apic_accept_events is run over the target vcpu
> context. kvm_apic_has_events reports to kvm_arch_vcpu_runnable if there
> are pending events, thus if vcpu blocking should end.
> 
> The patch comes with the side effect of effectively obsoleting
> KVM_MP_STATE_SIPI_RECEIVED. We still accept it from user space, but
> immediately translate it to KVM_MP_STATE_INIT_RECEIVED + KVM_APIC_SIPI.
> The vcpu itself will no longer enter the KVM_MP_STATE_SIPI_RECEIVED
> state. That also means we no longer exit to user space after receiving a
> SIPI event.
> 
> Furthermore, we already reset the VCPU on INIT, only fixing up the code
> segment later on when SIPI arrives. Moreover, we fix INIT handling for
> the BSP: it never enter wait-for-SIPI but directly starts over on INIT.
> 
> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> ---
> 
> Changes in v2:
>  - drop cmpxchg from INIT signaling
>  - rmb on SIPI processing
>  - reset sets all CPUs to 0xf000:0xfff0, RIP is fixed up on SIPI reception
> 
>  arch/x86/include/asm/kvm_host.h |    3 +-
>  arch/x86/kvm/lapic.c            |   48 +++++++++++++++++++++++++++-----
>  arch/x86/kvm/lapic.h            |   11 +++++++
>  arch/x86/kvm/svm.c              |    6 ----
>  arch/x86/kvm/vmx.c              |   12 +-------
>  arch/x86/kvm/x86.c              |   58 +++++++++++++++++++++++++-------------
>  6 files changed, 93 insertions(+), 45 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 348d859..ef7f4a5 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -345,7 +345,6 @@ struct kvm_vcpu_arch {
>  	unsigned long apic_attention;
>  	int32_t apic_arb_prio;
>  	int mp_state;
> -	int sipi_vector;
>  	u64 ia32_misc_enable_msr;
>  	bool tpr_access_reporting;
>  
> @@ -819,6 +818,7 @@ int kvm_emulate_wbinvd(struct kvm_vcpu *vcpu);
>  
>  void kvm_get_segment(struct kvm_vcpu *vcpu, struct kvm_segment *var, int seg);
>  int kvm_load_segment_descriptor(struct kvm_vcpu *vcpu, u16 selector, int seg);
> +void kvm_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, unsigned int vector);
>  
>  int kvm_task_switch(struct kvm_vcpu *vcpu, u16 tss_selector, int idt_index,
>  		    int reason, bool has_error_code, u32 error_code);
> @@ -1002,6 +1002,7 @@ int kvm_cpu_has_injectable_intr(struct kvm_vcpu *v);
>  int kvm_cpu_has_interrupt(struct kvm_vcpu *vcpu);
>  int kvm_arch_interrupt_allowed(struct kvm_vcpu *vcpu);
>  int kvm_cpu_get_interrupt(struct kvm_vcpu *v);
> +void kvm_vcpu_reset(struct kvm_vcpu *vcpu);
>  
>  void kvm_define_shared_msr(unsigned index, u32 msr);
>  void kvm_set_shared_msr(unsigned index, u64 val, u64 mask);
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 02b51dd..a8e9369 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -731,7 +731,11 @@ 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;
> +			/* assumes that there are only KVM_APIC_INIT/SIPI */
> +			apic->pending_events = (1UL << KVM_APIC_INIT);
> +			/* make sure pending_events is visible before sending
> +			 * the request */
> +			smp_wmb();
>  			kvm_make_request(KVM_REQ_EVENT, vcpu);
>  			kvm_vcpu_kick(vcpu);
>  		} else {
> @@ -743,13 +747,13 @@ 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) {
> -			result = 1;
> -			vcpu->arch.sipi_vector = vector;
> -			vcpu->arch.mp_state = KVM_MP_STATE_SIPI_RECEIVED;
> -			kvm_make_request(KVM_REQ_EVENT, vcpu);
> -			kvm_vcpu_kick(vcpu);
> -		}
> +		result = 1;
> +		apic->sipi_vector = vector;
> +		/* make sure sipi_vector is visible for the receiver */
> +		smp_wmb();
> +		set_bit(KVM_APIC_SIPI, &apic->pending_events);
> +		kvm_make_request(KVM_REQ_EVENT, vcpu);
> +		kvm_vcpu_kick(vcpu);

Why are APIC_DM_STARTUP / APIC_DM_INIT setting KVM_REQ_EVENT again?
Can't see any direct connection. See below.

> +static inline bool kvm_apic_has_events(struct kvm_vcpu *vcpu)
> +{
> +	return vcpu->arch.apic->pending_events;
> +}

vcpu->arch.apic = NULL?

>  	if (kvm_check_request(KVM_REQ_EVENT, vcpu) || req_int_win) {
> +		kvm_apic_accept_events(vcpu);
> +		if (vcpu->arch.mp_state == KVM_MP_STATE_INIT_RECEIVED) {
> +			r = 1;
> +			goto out;
> +		}
> +

A separate request bit makes sense, because nothing in this
(KVM_REQ_EVENT conditional) code sequence handles the work from
APIC_DM_STARTUP / APIC_DM_INIT sites (well, before your patch). See
below.

>  
> -	if (unlikely(vcpu->arch.mp_state == KVM_MP_STATE_SIPI_RECEIVED)) {
> -		pr_debug("vcpu %d received sipi with vector # %x\n",
> -			 vcpu->vcpu_id, vcpu->arch.sipi_vector);
> -		kvm_lapic_reset(vcpu);
> -		kvm_vcpu_reset(vcpu);
> -		vcpu->arch.mp_state = KVM_MP_STATE_RUNNABLE;
> -	}
> -
>  	vcpu->srcu_idx = srcu_read_lock(&kvm->srcu);
>  	r = vapic_enter(vcpu);

vmx_vcpu_reset overwrites vcpu->srcu_idx if ->vcpu_reset is called from
within srcu section.

Also, this is not part of the hotpath and therefore it could be farther
from vcpu_enter_guest. What about processing a new request bit here?
(KVM_REQ_EVENT <-> apic->pending_events relationship is cloudy).

Also the fact kvm_apic_accept_events() is called from sites is annoying,
why is that necessary again?