[patch 0/4] VMX: configure posted interrupt descriptor when assigning device (v3)

[patch 0/4] VMX: configure posted interrupt descriptor when assigning device (v3)

[Marcelo Tosatti]

Configuration of the posted interrupt descriptor is incorrect when devices
are hotplugged to the guest (and vcpus are halted).

See patch 4 for details.

---

v3: improved comments (Sean)
    use kvm_vcpu_wake_up (Sean)
    drop device_count from start_assignment function (Peter Xu)

v2: rather than using a potentially racy IPI (vs vcpu->cpu switches),
    kick the vcpus when assigning a device and let the blocked per-CPU
    list manipulation happen locally at ->pre_block and ->post_block
    (Sean Christopherson).

[patch 1/4] KVM: x86: add start_assignment hook to kvm_x86_ops

[Marcelo Tosatti]

Add a start_assignment hook to kvm_x86_ops, which is called when 
kvm_arch_start_assignment is done.

The hook is required to update the wakeup vector of a sleeping vCPU
when a device is assigned to the guest.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

Index: kvm/arch/x86/include/asm/kvm_host.h
===================================================================
--- kvm.orig/arch/x86/include/asm/kvm_host.h
+++ kvm/arch/x86/include/asm/kvm_host.h
@@ -1322,6 +1322,7 @@ struct kvm_x86_ops {
 
 	int (*update_pi_irte)(struct kvm *kvm, unsigned int host_irq,
 			      uint32_t guest_irq, bool set);
+	void (*start_assignment)(struct kvm *kvm);
 	void (*apicv_post_state_restore)(struct kvm_vcpu *vcpu);
 	bool (*dy_apicv_has_pending_interrupt)(struct kvm_vcpu *vcpu);
 
Index: kvm/arch/x86/kvm/svm/svm.c
===================================================================
--- kvm.orig/arch/x86/kvm/svm/svm.c
+++ kvm/arch/x86/kvm/svm/svm.c
@@ -4601,6 +4601,7 @@ static struct kvm_x86_ops svm_x86_ops __
 	.deliver_posted_interrupt = svm_deliver_avic_intr,
 	.dy_apicv_has_pending_interrupt = svm_dy_apicv_has_pending_interrupt,
 	.update_pi_irte = svm_update_pi_irte,
+	.start_assignment = NULL,
 	.setup_mce = svm_setup_mce,
 
 	.smi_allowed = svm_smi_allowed,
Index: kvm/arch/x86/kvm/vmx/vmx.c
===================================================================
--- kvm.orig/arch/x86/kvm/vmx/vmx.c
+++ kvm/arch/x86/kvm/vmx/vmx.c
@@ -7732,6 +7732,7 @@ static struct kvm_x86_ops vmx_x86_ops __
 	.nested_ops = &vmx_nested_ops,
 
 	.update_pi_irte = pi_update_irte,
+	.start_assignment = NULL,
 
 #ifdef CONFIG_X86_64
 	.set_hv_timer = vmx_set_hv_timer,
Index: kvm/arch/x86/kvm/x86.c
===================================================================
--- kvm.orig/arch/x86/kvm/x86.c
+++ kvm/arch/x86/kvm/x86.c
@@ -11295,7 +11295,11 @@ bool kvm_arch_can_dequeue_async_page_pre
 
 void kvm_arch_start_assignment(struct kvm *kvm)
 {
-	atomic_inc(&kvm->arch.assigned_device_count);
+	int ret;
+
+	ret = atomic_inc_return(&kvm->arch.assigned_device_count);
+	if (ret == 1)
+		static_call_cond(kvm_x86_start_assignment)(kvm);
 }
 EXPORT_SYMBOL_GPL(kvm_arch_start_assignment);
 
Index: kvm/arch/x86/include/asm/kvm-x86-ops.h
===================================================================
--- kvm.orig/arch/x86/include/asm/kvm-x86-ops.h
+++ kvm/arch/x86/include/asm/kvm-x86-ops.h
@@ -99,6 +99,7 @@ KVM_X86_OP_NULL(post_block)
 KVM_X86_OP_NULL(vcpu_blocking)
 KVM_X86_OP_NULL(vcpu_unblocking)
 KVM_X86_OP_NULL(update_pi_irte)
+KVM_X86_OP_NULL(start_assignment)
 KVM_X86_OP_NULL(apicv_post_state_restore)
 KVM_X86_OP_NULL(dy_apicv_has_pending_interrupt)
 KVM_X86_OP_NULL(set_hv_timer)

Re: [patch 1/4] KVM: x86: add start_assignment hook to kvm_x86_ops

[Peter Xu]

On Mon, May 10, 2021 at 02:26:47PM -0300, Marcelo Tosatti wrote:
> Add a start_assignment hook to kvm_x86_ops, which is called when 
> kvm_arch_start_assignment is done.
> 
> The hook is required to update the wakeup vector of a sleeping vCPU
> when a device is assigned to the guest.
> 
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> 
> Index: kvm/arch/x86/include/asm/kvm_host.h
> ===================================================================
> --- kvm.orig/arch/x86/include/asm/kvm_host.h
> +++ kvm/arch/x86/include/asm/kvm_host.h
> @@ -1322,6 +1322,7 @@ struct kvm_x86_ops {
>  
>  	int (*update_pi_irte)(struct kvm *kvm, unsigned int host_irq,
>  			      uint32_t guest_irq, bool set);
> +	void (*start_assignment)(struct kvm *kvm);
>  	void (*apicv_post_state_restore)(struct kvm_vcpu *vcpu);
>  	bool (*dy_apicv_has_pending_interrupt)(struct kvm_vcpu *vcpu);
>  
> Index: kvm/arch/x86/kvm/svm/svm.c
> ===================================================================
> --- kvm.orig/arch/x86/kvm/svm/svm.c
> +++ kvm/arch/x86/kvm/svm/svm.c
> @@ -4601,6 +4601,7 @@ static struct kvm_x86_ops svm_x86_ops __
>  	.deliver_posted_interrupt = svm_deliver_avic_intr,
>  	.dy_apicv_has_pending_interrupt = svm_dy_apicv_has_pending_interrupt,
>  	.update_pi_irte = svm_update_pi_irte,
> +	.start_assignment = NULL,

Can this be dropped (as default NULL)?

>  	.setup_mce = svm_setup_mce,
>  
>  	.smi_allowed = svm_smi_allowed,
> Index: kvm/arch/x86/kvm/vmx/vmx.c
> ===================================================================
> --- kvm.orig/arch/x86/kvm/vmx/vmx.c
> +++ kvm/arch/x86/kvm/vmx/vmx.c
> @@ -7732,6 +7732,7 @@ static struct kvm_x86_ops vmx_x86_ops __
>  	.nested_ops = &vmx_nested_ops,
>  
>  	.update_pi_irte = pi_update_irte,
> +	.start_assignment = NULL,

Same here?

>  
>  #ifdef CONFIG_X86_64
>  	.set_hv_timer = vmx_set_hv_timer,
> Index: kvm/arch/x86/kvm/x86.c
> ===================================================================
> --- kvm.orig/arch/x86/kvm/x86.c
> +++ kvm/arch/x86/kvm/x86.c
> @@ -11295,7 +11295,11 @@ bool kvm_arch_can_dequeue_async_page_pre
>  
>  void kvm_arch_start_assignment(struct kvm *kvm)
>  {
> -	atomic_inc(&kvm->arch.assigned_device_count);
> +	int ret;
> +
> +	ret = atomic_inc_return(&kvm->arch.assigned_device_count);
> +	if (ret == 1)
> +		static_call_cond(kvm_x86_start_assignment)(kvm);

Maybe "ret" can be dropped too?

void kvm_arch_start_assignment(struct kvm *kvm)
{
	if (atomic_inc_return(&kvm->arch.assigned_device_count) == 1)
		static_call_cond(kvm_x86_start_assignment)(kvm);
}

Otherwise looks good to me.  Thanks,

-- 
Peter Xu

Re: [patch 1/4] KVM: x86: add start_assignment hook to kvm_x86_ops

[Marcelo Tosatti]

On Tue, May 11, 2021 at 12:26:08PM -0400, Peter Xu wrote:
> On Mon, May 10, 2021 at 02:26:47PM -0300, Marcelo Tosatti wrote:
> > Add a start_assignment hook to kvm_x86_ops, which is called when 
> > kvm_arch_start_assignment is done.
> > 
> > The hook is required to update the wakeup vector of a sleeping vCPU
> > when a device is assigned to the guest.
> > 
> > Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> > 
> > Index: kvm/arch/x86/include/asm/kvm_host.h
> > ===================================================================
> > --- kvm.orig/arch/x86/include/asm/kvm_host.h
> > +++ kvm/arch/x86/include/asm/kvm_host.h
> > @@ -1322,6 +1322,7 @@ struct kvm_x86_ops {
> >  
> >  	int (*update_pi_irte)(struct kvm *kvm, unsigned int host_irq,
> >  			      uint32_t guest_irq, bool set);
> > +	void (*start_assignment)(struct kvm *kvm);
> >  	void (*apicv_post_state_restore)(struct kvm_vcpu *vcpu);
> >  	bool (*dy_apicv_has_pending_interrupt)(struct kvm_vcpu *vcpu);
> >  
> > Index: kvm/arch/x86/kvm/svm/svm.c
> > ===================================================================
> > --- kvm.orig/arch/x86/kvm/svm/svm.c
> > +++ kvm/arch/x86/kvm/svm/svm.c
> > @@ -4601,6 +4601,7 @@ static struct kvm_x86_ops svm_x86_ops __
> >  	.deliver_posted_interrupt = svm_deliver_avic_intr,
> >  	.dy_apicv_has_pending_interrupt = svm_dy_apicv_has_pending_interrupt,
> >  	.update_pi_irte = svm_update_pi_irte,
> > +	.start_assignment = NULL,
> 
> Can this be dropped (as default NULL)?

Done.

> 
> >  	.setup_mce = svm_setup_mce,
> >  
> >  	.smi_allowed = svm_smi_allowed,
> > Index: kvm/arch/x86/kvm/vmx/vmx.c
> > ===================================================================
> > --- kvm.orig/arch/x86/kvm/vmx/vmx.c
> > +++ kvm/arch/x86/kvm/vmx/vmx.c
> > @@ -7732,6 +7732,7 @@ static struct kvm_x86_ops vmx_x86_ops __
> >  	.nested_ops = &vmx_nested_ops,
> >  
> >  	.update_pi_irte = pi_update_irte,
> > +	.start_assignment = NULL,
> 
> Same here?

Done.

> >  
> >  #ifdef CONFIG_X86_64
> >  	.set_hv_timer = vmx_set_hv_timer,
> > Index: kvm/arch/x86/kvm/x86.c
> > ===================================================================
> > --- kvm.orig/arch/x86/kvm/x86.c
> > +++ kvm/arch/x86/kvm/x86.c
> > @@ -11295,7 +11295,11 @@ bool kvm_arch_can_dequeue_async_page_pre
> >  
> >  void kvm_arch_start_assignment(struct kvm *kvm)
> >  {
> > -	atomic_inc(&kvm->arch.assigned_device_count);
> > +	int ret;
> > +
> > +	if (atomic_inc_return(&kvm->arch.assigned_device_count) == 1)
> > +	if (ret == 1)
> > +		static_call_cond(kvm_x86_start_assignment)(kvm);
> 
> Maybe "ret" can be dropped too?
> 
> void kvm_arch_start_assignment(struct kvm *kvm)
> {
> 	if (atomic_inc_return(&kvm->arch.assigned_device_count) == 1)
> 		static_call_cond(kvm_x86_start_assignment)(kvm);
> }
> 
> Otherwise looks good to me.  Thanks,

Done.

[patch 2/4] KVM: add arch specific vcpu_check_block callback

[Marcelo Tosatti]

Add callback in kvm_vcpu_check_block, so that architectures
can direct a vcpu to exit the vcpu block loop without requiring
events that would unhalt it.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

Index: kvm/include/linux/kvm_host.h
===================================================================
--- kvm.orig/include/linux/kvm_host.h
+++ kvm/include/linux/kvm_host.h
@@ -971,6 +971,13 @@ static inline int kvm_arch_flush_remote_
 }
 #endif
 
+#ifndef __KVM_HAVE_ARCH_VCPU_CHECK_BLOCK
+static inline int kvm_arch_vcpu_check_block(struct kvm_vcpu *vcpu)
+{
+	return 0;
+}
+#endif
+
 #ifdef __KVM_HAVE_ARCH_NONCOHERENT_DMA
 void kvm_arch_register_noncoherent_dma(struct kvm *kvm);
 void kvm_arch_unregister_noncoherent_dma(struct kvm *kvm);
Index: kvm/virt/kvm/kvm_main.c
===================================================================
--- kvm.orig/virt/kvm/kvm_main.c
+++ kvm/virt/kvm/kvm_main.c
@@ -2794,6 +2794,8 @@ static int kvm_vcpu_check_block(struct k
 		goto out;
 	if (signal_pending(current))
 		goto out;
+	if (kvm_arch_vcpu_check_block(vcpu))
+		goto out;
 
 	ret = 0;
 out:

[patch 3/4] KVM: x86: implement kvm_arch_vcpu_check_block callback

[Marcelo Tosatti]

Implement kvm_arch_vcpu_check_block for x86. Next patch will add
implementation of kvm_x86_ops.vcpu_check_block for VMX.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

Index: kvm/arch/x86/include/asm/kvm_host.h
===================================================================
--- kvm.orig/arch/x86/include/asm/kvm_host.h
+++ kvm/arch/x86/include/asm/kvm_host.h
@@ -1320,6 +1320,8 @@ struct kvm_x86_ops {
 	void (*vcpu_blocking)(struct kvm_vcpu *vcpu);
 	void (*vcpu_unblocking)(struct kvm_vcpu *vcpu);
 
+	int (*vcpu_check_block)(struct kvm_vcpu *vcpu);
+
 	int (*update_pi_irte)(struct kvm *kvm, unsigned int host_irq,
 			      uint32_t guest_irq, bool set);
 	void (*start_assignment)(struct kvm *kvm);
@@ -1801,6 +1803,15 @@ static inline bool kvm_irq_is_postable(s
 		irq->delivery_mode == APIC_DM_LOWEST);
 }
 
+#define __KVM_HAVE_ARCH_VCPU_CHECK_BLOCK
+static inline int kvm_arch_vcpu_check_block(struct kvm_vcpu *vcpu)
+{
+	if (kvm_x86_ops.vcpu_check_block)
+		return static_call(kvm_x86_vcpu_check_block)(vcpu);
+
+	return 0;
+}
+
 static inline void kvm_arch_vcpu_blocking(struct kvm_vcpu *vcpu)
 {
 	static_call_cond(kvm_x86_vcpu_blocking)(vcpu);
Index: kvm/arch/x86/kvm/vmx/vmx.c
===================================================================
--- kvm.orig/arch/x86/kvm/vmx/vmx.c
+++ kvm/arch/x86/kvm/vmx/vmx.c
@@ -7727,6 +7727,7 @@ static struct kvm_x86_ops vmx_x86_ops __
 
 	.pre_block = vmx_pre_block,
 	.post_block = vmx_post_block,
+	.vcpu_check_block = NULL,
 
 	.pmu_ops = &intel_pmu_ops,
 	.nested_ops = &vmx_nested_ops,
Index: kvm/arch/x86/include/asm/kvm-x86-ops.h
===================================================================
--- kvm.orig/arch/x86/include/asm/kvm-x86-ops.h
+++ kvm/arch/x86/include/asm/kvm-x86-ops.h
@@ -98,6 +98,7 @@ KVM_X86_OP_NULL(pre_block)
 KVM_X86_OP_NULL(post_block)
 KVM_X86_OP_NULL(vcpu_blocking)
 KVM_X86_OP_NULL(vcpu_unblocking)
+KVM_X86_OP_NULL(vcpu_check_block)
 KVM_X86_OP_NULL(update_pi_irte)
 KVM_X86_OP_NULL(start_assignment)
 KVM_X86_OP_NULL(apicv_post_state_restore)
Index: kvm/arch/x86/kvm/svm/svm.c
===================================================================
--- kvm.orig/arch/x86/kvm/svm/svm.c
+++ kvm/arch/x86/kvm/svm/svm.c
@@ -4517,6 +4517,7 @@ static struct kvm_x86_ops svm_x86_ops __
 	.vcpu_put = svm_vcpu_put,
 	.vcpu_blocking = svm_vcpu_blocking,
 	.vcpu_unblocking = svm_vcpu_unblocking,
+	.vcpu_check_block = NULL,
 
 	.update_exception_bitmap = svm_update_exception_bitmap,
 	.get_msr_feature = svm_get_msr_feature,

[patch 4/4] KVM: VMX: update vcpu posted-interrupt descriptor when assigning device

[Marcelo Tosatti]

For VMX, when a vcpu enters HLT emulation, pi_post_block will:

1) Add vcpu to per-cpu list of blocked vcpus.

2) Program the posted-interrupt descriptor "notification vector" 
to POSTED_INTR_WAKEUP_VECTOR

With interrupt remapping, an interrupt will set the PIR bit for the 
vector programmed for the device on the CPU, test-and-set the 
ON bit on the posted interrupt descriptor, and if the ON bit is clear
generate an interrupt for the notification vector.

This way, the target CPU wakes upon a device interrupt and wakes up
the target vcpu.

Problem is that pi_post_block only programs the notification vector
if kvm_arch_has_assigned_device() is true. Its possible for the
following to happen:

1) vcpu V HLTs on pcpu P, kvm_arch_has_assigned_device is false,
notification vector is not programmed
2) device is assigned to VM
3) device interrupts vcpu V, sets ON bit
(notification vector not programmed, so pcpu P remains in idle)
4) vcpu 0 IPIs vcpu V (in guest), but since pi descriptor ON bit is set,
kvm_vcpu_kick is skipped
5) vcpu 0 busy spins on vcpu V's response for several seconds, until
RCU watchdog NMIs all vCPUs.

To fix this, use the start_assignment kvm_x86_ops callback to kick
vcpus out of the halt loop, so the notification vector is 
properly reprogrammed to the wakeup vector.

Reported-by: Pei Zhang <pezhang@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>


Index: kvm/arch/x86/kvm/vmx/posted_intr.c
===================================================================
--- kvm.orig/arch/x86/kvm/vmx/posted_intr.c
+++ kvm/arch/x86/kvm/vmx/posted_intr.c
@@ -204,6 +204,32 @@ void pi_post_block(struct kvm_vcpu *vcpu
 }
 
 /*
+ * Bail out of the block loop if the VM has an assigned
+ * device, but the blocking vCPU didn't reconfigure the
+ * PI.NV to the wakeup vector, i.e. the assigned device
+ * came along after the initial check in vcpu_block().
+ */
+
+int vmx_vcpu_check_block(struct kvm_vcpu *vcpu)
+{
+	struct pi_desc *pi_desc = vcpu_to_pi_desc(vcpu);
+
+	if (!irq_remapping_cap(IRQ_POSTING_CAP))
+		return 0;
+
+	if (!kvm_vcpu_apicv_active(vcpu))
+		return 0;
+
+	if (!kvm_arch_has_assigned_device(vcpu->kvm))
+		return 0;
+
+	if (pi_desc->nv == POSTED_INTR_WAKEUP_VECTOR)
+		return 0;
+
+	return 1;
+}
+
+/*
  * Handler for POSTED_INTERRUPT_WAKEUP_VECTOR.
  */
 void pi_wakeup_handler(void)
@@ -236,6 +262,25 @@ bool pi_has_pending_interrupt(struct kvm
 		(pi_test_sn(pi_desc) && !pi_is_pir_empty(pi_desc));
 }
 
+void vmx_pi_start_assignment(struct kvm *kvm)
+{
+	struct kvm_vcpu *vcpu;
+	int i;
+
+	if (!irq_remapping_cap(IRQ_POSTING_CAP))
+		return;
+
+	/*
+	 * Wakeup will cause the vCPU to bail out of kvm_vcpu_block() and
+	 * go back through vcpu_block().
+	 */
+	kvm_for_each_vcpu(i, vcpu, kvm) {
+		if (!kvm_vcpu_apicv_active(vcpu))
+			continue;
+
+		kvm_vcpu_wake_up(vcpu);
+	}
+}
 
 /*
  * pi_update_irte - set IRTE for Posted-Interrupts
Index: kvm/arch/x86/kvm/vmx/posted_intr.h
===================================================================
--- kvm.orig/arch/x86/kvm/vmx/posted_intr.h
+++ kvm/arch/x86/kvm/vmx/posted_intr.h
@@ -95,5 +95,7 @@ void __init pi_init_cpu(int cpu);
 bool pi_has_pending_interrupt(struct kvm_vcpu *vcpu);
 int pi_update_irte(struct kvm *kvm, unsigned int host_irq, uint32_t guest_irq,
 		   bool set);
+void vmx_pi_start_assignment(struct kvm *kvm);
+int vmx_vcpu_check_block(struct kvm_vcpu *vcpu);
 
 #endif /* __KVM_X86_VMX_POSTED_INTR_H */
Index: kvm/arch/x86/kvm/vmx/vmx.c
===================================================================
--- kvm.orig/arch/x86/kvm/vmx/vmx.c
+++ kvm/arch/x86/kvm/vmx/vmx.c
@@ -7727,13 +7727,13 @@ static struct kvm_x86_ops vmx_x86_ops __
 
 	.pre_block = vmx_pre_block,
 	.post_block = vmx_post_block,
-	.vcpu_check_block = NULL,
+	.vcpu_check_block = vmx_vcpu_check_block,
 
 	.pmu_ops = &intel_pmu_ops,
 	.nested_ops = &vmx_nested_ops,
 
 	.update_pi_irte = pi_update_irte,
-	.start_assignment = NULL,
+	.start_assignment = vmx_pi_start_assignment,
 
 #ifdef CONFIG_X86_64
 	.set_hv_timer = vmx_set_hv_timer,

Re: [patch 4/4] KVM: VMX: update vcpu posted-interrupt descriptor when assigning device

[Paolo Bonzini]

On 10/05/21 19:26, Marcelo Tosatti wrote:
> +void vmx_pi_start_assignment(struct kvm *kvm)
> +{
> +	struct kvm_vcpu *vcpu;
> +	int i;
> +
> +	if (!irq_remapping_cap(IRQ_POSTING_CAP))
> +		return;
> +
> +	/*
> +	 * Wakeup will cause the vCPU to bail out of kvm_vcpu_block() and
> +	 * go back through vcpu_block().
> +	 */
> +	kvm_for_each_vcpu(i, vcpu, kvm) {
> +		if (!kvm_vcpu_apicv_active(vcpu))
> +			continue;
> +
> +		kvm_vcpu_wake_up(vcpu);

Would you still need the check_block callback, if you also added a 
kvm_make_request(KVM_REQ_EVENT)?

In fact, since this is entirely not a hot path, can you just do 
kvm_make_all_cpus_request(kvm, KVM_REQ_EVENT) instead of this loop?

Thanks,

Paolo

> +	}
> +}
>   
>   /*
>    * pi_update_irte - set IRTE for Posted-Interrupts
> Index: kvm/arch/x86/kvm/vmx/posted_intr.h
> ===================================================================
> --- kvm.orig/arch/x86/kvm/vmx/posted_intr.h
> +++ kvm/arch/x86/kvm/vmx/posted_intr.h
> @@ -95,5 +95,7 @@ void __init pi_init_cpu(int cpu);
>   bool pi_has_pending_interrupt(struct kvm_vcpu *vcpu);
>   int pi_update_irte(struct kvm *kvm, unsigned int host_irq, uint32_t guest_irq,
>   		   bool set);
> +void vmx_pi_start_assignment(struct kvm *kvm);
> +int vmx_vcpu_check_block(struct kvm_vcpu *vcpu);
>   
>   #endif /* __KVM_X86_VMX_POSTED_INTR_H */
> Index: kvm/arch/x86/kvm/vmx/vmx.c
> ===================================================================
> --- kvm.orig/arch/x86/kvm/vmx/vmx.c
> +++ kvm/arch/x86/kvm/vmx/vmx.c
> @@ -7727,13 +7727,13 @@ static struct kvm_x86_ops vmx_x86_ops __
>   
>   	.pre_block = vmx_pre_block,
>   	.post_block = vmx_post_block,
> -	.vcpu_check_block = NULL,
> +	.vcpu_check_block = vmx_vcpu_check_block,
>   
>   	.pmu_ops = &intel_pmu_ops,
>   	.nested_ops = &vmx_nested_ops,
>   
>   	.update_pi_irte = pi_update_irte,
> -	.start_assignment = NULL,
> +	.start_assignment = vmx_pi_start_assignment,
>   
>   #ifdef CONFIG_X86_64
>   	.set_hv_timer = vmx_set_hv_timer,
> 
> 

Re: [patch 4/4] KVM: VMX: update vcpu posted-interrupt descriptor when assigning device

[Marcelo Tosatti]

On Mon, May 24, 2021 at 05:55:18PM +0200, Paolo Bonzini wrote:
> On 10/05/21 19:26, Marcelo Tosatti wrote:
> > +void vmx_pi_start_assignment(struct kvm *kvm)
> > +{
> > +	struct kvm_vcpu *vcpu;
> > +	int i;
> > +
> > +	if (!irq_remapping_cap(IRQ_POSTING_CAP))
> > +		return;
> > +
> > +	/*
> > +	 * Wakeup will cause the vCPU to bail out of kvm_vcpu_block() and
> > +	 * go back through vcpu_block().
> > +	 */
> > +	kvm_for_each_vcpu(i, vcpu, kvm) {
> > +		if (!kvm_vcpu_apicv_active(vcpu))
> > +			continue;
> > +
> > +		kvm_vcpu_wake_up(vcpu);
> 
> Would you still need the check_block callback, if you also added a
> kvm_make_request(KVM_REQ_EVENT)?
> 
> In fact, since this is entirely not a hot path, can you just do
> kvm_make_all_cpus_request(kvm, KVM_REQ_EVENT) instead of this loop?
> 
> Thanks,
> 
> Paolo

Hi Paolo,

Don't think so:

int kvm_arch_vcpu_runnable(struct kvm_vcpu *vcpu)
{
        return kvm_vcpu_running(vcpu) || kvm_vcpu_has_events(vcpu);
}

static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu)
{
        int ret = -EINTR;
        int idx = srcu_read_lock(&vcpu->kvm->srcu);

        if (kvm_arch_vcpu_runnable(vcpu)) {
                kvm_make_request(KVM_REQ_UNHALT, vcpu);  <---- don't want KVM_REQ_UNHALT
                goto out;
        }
        if (kvm_cpu_has_pending_timer(vcpu))
                goto out;
        if (signal_pending(current))
                goto out;

        ret = 0;
out:
        srcu_read_unlock(&vcpu->kvm->srcu, idx);
        return ret;
}

See previous discussion:


Date: Wed, 12 May 2021 14:41:56 +0000                                                                                   
From: Sean Christopherson <seanjc@google.com>                                                                           
To: Marcelo Tosatti <mtosatti@redhat.com>                                                                               
Cc: Peter Xu <peterx@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, kvm@vger.kernel.org, Alex Williamson             
        <alex.williamson@redhat.com>, Pei Zhang <pezhang@redhat.com>                                                    
Subject: Re: [patch 4/4] KVM: VMX: update vcpu posted-interrupt descriptor when assigning device                        

On Tue, May 11, 2021, Marcelo Tosatti wrote:
> > The KVM_REQ_UNBLOCK patch will resume execution even any such event                                                 
>                                                                                                                       
>                                                 even without any such event                                           
>                                                                                                                       
> > occuring. So the behaviour would be different from baremetal.                                                       

I agree with Marcelo, we don't want to spuriously unhalt the vCPU.  It's legal,
albeit risky, to do something like

       	hlt
       	/* #UD to triple fault if this CPU is awakened. */
       	ud2

when offlining a CPU, in which case the spurious wake event will crash the guest.

Re: [patch 4/4] KVM: VMX: update vcpu posted-interrupt descriptor when assigning device

[Paolo Bonzini]

On 24/05/21 19:53, Marcelo Tosatti wrote:
> On Mon, May 24, 2021 at 05:55:18PM +0200, Paolo Bonzini wrote:
>> On 10/05/21 19:26, Marcelo Tosatti wrote:
>>> +void vmx_pi_start_assignment(struct kvm *kvm)
>>> +{
>>> +	struct kvm_vcpu *vcpu;
>>> +	int i;
>>> +
>>> +	if (!irq_remapping_cap(IRQ_POSTING_CAP))
>>> +		return;
>>> +
>>> +	/*
>>> +	 * Wakeup will cause the vCPU to bail out of kvm_vcpu_block() and
>>> +	 * go back through vcpu_block().
>>> +	 */
>>> +	kvm_for_each_vcpu(i, vcpu, kvm) {
>>> +		if (!kvm_vcpu_apicv_active(vcpu))
>>> +			continue;
>>> +
>>> +		kvm_vcpu_wake_up(vcpu);
>>
>> Would you still need the check_block callback, if you also added a
>> kvm_make_request(KVM_REQ_EVENT)?
>>
>> In fact, since this is entirely not a hot path, can you just do
>> kvm_make_all_cpus_request(kvm, KVM_REQ_EVENT) instead of this loop?
>>
>> Thanks,
>>
>> Paolo
> 
> Hi Paolo,
> 
> Don't think so:
> 
> static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu)
> {
>          int ret = -EINTR;
>          int idx = srcu_read_lock(&vcpu->kvm->srcu);
> 
>          if (kvm_arch_vcpu_runnable(vcpu)) {
>                  kvm_make_request(KVM_REQ_UNHALT, vcpu);  <---- don't want KVM_REQ_UNHALT

UNHALT is incorrect indeed, but requests don't have to unhalt the vCPU.

This case is somewhat similar to signal_pending(), where the next 
KVM_RUN ioctl resumes the halt.  It's also similar to 
KVM_REQ_PENDING_TIMER.  So you can:

- rename KVM_REQ_PENDING_TIMER to KVM_REQ_UNBLOCK except in 
arch/powerpc, where instead you add KVM_REQ_PENDING_TIMER to 
arch/powerpc/include/asm/kvm_host.h

- here, you add

	if (kvm_check_request(KVM_REQ_UNBLOCK, vcpu))
		goto out;

- then vmx_pi_start_assignment only needs to

	if (!irq_remapping_cap(IRQ_POSTING_CAP))
		return;
	kvm_make_all_cpus_request(kvm, KVM_REQ_UNBLOCK);

kvm_arch_vcpu_runnable() would still return false, so the mp_state would 
not change.

Paolo