linux-kernel - Re: [KVM PATCH v3 2/3] KVM: export lockless GSI attribute

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Date:	Wed, 28 Oct 2009 09:24:30 -0400
From:	Gregory Haskins <gregory.haskins@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Gregory Haskins <ghaskins@...ell.com>, kvm@...r.kernel.org,
	alacrityvm-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: [KVM PATCH v3 2/3] KVM: export lockless GSI attribute

Michael S. Tsirkin wrote:
> On Mon, Oct 26, 2009 at 12:22:03PM -0400, Gregory Haskins wrote:
>> Certain GSI's support lockless injecton, but we have no way to detect
>> which ones at the GSI level.  Knowledge of this attribute will be
>> useful later in the series so that we can optimize irqfd injection
>> paths for cases where we know the code will not sleep.  Therefore,
>> we provide an API to query a specific GSI.
>>
>> Signed-off-by: Gregory Haskins <ghaskins@...ell.com>
>> ---
>>
>>  include/linux/kvm_host.h |    2 ++
>>  virt/kvm/irq_comm.c      |   35 ++++++++++++++++++++++++++++++++++-
>>  2 files changed, 36 insertions(+), 1 deletions(-)
>>
>> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
>> index 1fe135d..01151a6 100644
>> --- a/include/linux/kvm_host.h
>> +++ b/include/linux/kvm_host.h
>> @@ -119,6 +119,7 @@ struct kvm_memory_slot {
>>  struct kvm_kernel_irq_routing_entry {
>>  	u32 gsi;
>>  	u32 type;
>> +	bool lockless;
> 
> So lockless is the same as type == MSI from below?

Yep, today anyway.

> If the idea is to make it extensible for the future,
> let's just add an inline function, we don't need a field for this.
> 

This makes sense.

>>  	int (*set)(struct kvm_kernel_irq_routing_entry *e,
>>  		   struct kvm *kvm, int irq_source_id, int level);
>>  	union {
>> @@ -420,6 +421,7 @@ void kvm_get_intr_delivery_bitmask(struct kvm_ioapic *ioapic,
>>  				   unsigned long *deliver_bitmask);
>>  #endif
>>  int kvm_set_irq(struct kvm *kvm, int irq_source_id, u32 irq, int level);
>> +int kvm_irq_check_lockless(struct kvm *kvm, u32 irq);
>>  void kvm_notify_acked_irq(struct kvm *kvm, unsigned irqchip, unsigned pin);
>>  void kvm_register_irq_ack_notifier(struct kvm *kvm,
>>  				   struct kvm_irq_ack_notifier *kian);
>> diff --git a/virt/kvm/irq_comm.c b/virt/kvm/irq_comm.c
>> index db2553f..a7fd487 100644
>> --- a/virt/kvm/irq_comm.c
>> +++ b/virt/kvm/irq_comm.c
>> @@ -173,6 +173,35 @@ int kvm_set_irq(struct kvm *kvm, int irq_source_id, u32 irq, int level)
>>  	return ret;
>>  }
>>  
>> +int kvm_irq_check_lockless(struct kvm *kvm, u32 irq)
>> +{
>> +	struct kvm_kernel_irq_routing_entry *e;
>> +	struct kvm_irq_routing_table *irq_rt;
>> +	struct hlist_node *n;
>> +	int ret = -ENOENT;
>> +	int idx;
>> +
>> +	idx = srcu_read_lock(&kvm->irq_routing.srcu);
>> +	irq_rt = rcu_dereference(kvm->irq_routing.table);
>> +	if (irq < irq_rt->nr_rt_entries)
>> +		hlist_for_each_entry(e, n, &irq_rt->map[irq], link) {
>> +			if (!e->lockless) {
>> +				/*
>> +				 * all destinations need to be lockless to
>> +				 * declare that the GSI as a whole is also
>> +				 * lockless
>> +				 */
>> +				ret = 0;
>> +				break;
>> +			}
>> +
>> +			ret = 1;
>> +		}
>> +	srcu_read_unlock(&kvm->irq_routing.srcu, idx);
>> +
>> +	return ret;
>> +}
>> +
>>  void kvm_notify_acked_irq(struct kvm *kvm, unsigned irqchip, unsigned pin)
>>  {
>>  	struct kvm_irq_ack_notifier *kian;
>> @@ -310,18 +339,22 @@ static int setup_routing_entry(struct kvm_irq_routing_table *rt,
>>  	int delta;
>>  	struct kvm_kernel_irq_routing_entry *ei;
>>  	struct hlist_node *n;
>> +	bool lockless = ue->type == KVM_IRQ_ROUTING_MSI;
>>  
>>  	/*
>>  	 * Do not allow GSI to be mapped to the same irqchip more than once.
>>  	 * Allow only one to one mapping between GSI and MSI.
>> +	 * Do not allow mixed lockless vs locked variants to coexist.
> 
> Userspace has no idea which entries are lockless and which are not:
> this is an implementation detail - so it might not be able to avoid
> illegal combinations.
> Since this is called on an ioctl, can the rule be formulated in a way
> that makes sense for userspace?
> 

I'm not sure.

>>  	 */
>>  	hlist_for_each_entry(ei, n, &rt->map[ue->gsi], link)
>>  		if (ei->type == KVM_IRQ_ROUTING_MSI ||
>> -		    ue->u.irqchip.irqchip == ei->irqchip.irqchip)
>> +		    ue->u.irqchip.irqchip == ei->irqchip.irqchip ||
>> +		    ei->lockless != lockless)
> 
> So this check seems like it does nothing, because lockless is same as
> MSI, and MSI is always 1:1? Intentional?
> 

Yeah, it was more to guard-against/document the dependency, in case the
1:1 with MSI ever changes in the future.

Kind Regards,
-Greg


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