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

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2013-03-14 01:08, Marcelo Tosatti wrote:
> 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.

Changing the mp_state may unblock pending events (IRQs, NMIs), I think
that is the original reason.

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

Not possible for the caller of this function.

> 
>>  	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.

OK. Was told to avoid new request bits.

> 
>>  
>> -	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.

Indeed.

Do you know what the look over vmx_set_cr0 actually protects?

> 
> 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).

I can pull the check from vcpu_enter_guest out at this level again, but
then we will generate the user space exits again.

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

- to avoid having pending events in the APIC when delivering mp_state
  to user space
- to keep mp_state correct after waking up from kvm_vcpu_block (we
  could otherwise walk through code paths with the wrong state)
- to process pending events when the VCPU was running

Jan

-- 
Siemens AG, Corporate Technology, CT RTC ITP SDP-DE
Corporate Competence Center Embedded Linux