Re: [PATCH 1/8] KVM: arm/arm64: vgic: Add LPI translation cache definition

[Marc Zyngier <marc.zyngier@arm.com>]

Hi Eric,

On 07/06/2019 13:09, Auger Eric wrote:
> Hi Marc,
> 
> On 6/6/19 6:54 PM, Marc Zyngier wrote:
>> Add the basic data structure that expresses an MSI to LPI
>> translation as well as the allocation/release hooks.
>>
>> THe size of the cache is arbitrarily defined as 4*nr_vcpus.
>>
>> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
>> ---
>>  include/kvm/arm_vgic.h        | 10 ++++++++++
>>  virt/kvm/arm/vgic/vgic-init.c | 34 ++++++++++++++++++++++++++++++++++
>>  virt/kvm/arm/vgic/vgic-its.c  |  2 ++
>>  virt/kvm/arm/vgic/vgic.h      |  3 +++
>>  4 files changed, 49 insertions(+)
>>
>> diff --git a/include/kvm/arm_vgic.h b/include/kvm/arm_vgic.h
>> index c36c86f1ec9a..5a0d6b07c5ef 100644
>> --- a/include/kvm/arm_vgic.h
>> +++ b/include/kvm/arm_vgic.h
>> @@ -173,6 +173,14 @@ struct vgic_io_device {
>>  	struct kvm_io_device dev;
>>  };
>>  
>> +struct vgic_translation_cache_entry {
>> +	struct list_head	entry;
>> +	phys_addr_t		db;
> it is not obvious to me why you do need the db field? Isn't the LPI
> uniquely identfiied by the devid and eventid. If I recall correctly
> theorically the architecture allows to handle LPIs even without ITS.

Only having DID+EID is unfortunately not enough, and the translation has
to be per ITS. Think of a system with two ITSs, and a PCI device in
front of each of the ITSs. There is no reason why the two devices would
have different IDs, as they belong to different PCI hierarchies.

So the cache must take the source ITS into account. The alternative
would be to keep a separate cache per ITS, but that would lead to more
overhead on the fast path, having to lookup the ITS first.

As for LPIs without ITS, that wouldn't need a cache at all.

>> +	u32			devid;
>> +	u32			eventid;
>> +	struct vgic_irq		*irq;
>> +};
>> +
>>  struct vgic_its {
>>  	/* The base address of the ITS control register frame */
>>  	gpa_t			vgic_its_base;
>> @@ -260,6 +268,8 @@ struct vgic_dist {
>>  	struct list_head	lpi_list_head;
>>  	int			lpi_list_count;
>>  
>> +	struct list_head	lpi_translation_cache;
>> +
>>  	/* used by vgic-debug */
>>  	struct vgic_state_iter *iter;
>>  
>> diff --git a/virt/kvm/arm/vgic/vgic-init.c b/virt/kvm/arm/vgic/vgic-init.c
>> index 3bdb31eaed64..25ae25694a28 100644
>> --- a/virt/kvm/arm/vgic/vgic-init.c
>> +++ b/virt/kvm/arm/vgic/vgic-init.c
>> @@ -64,6 +64,7 @@ void kvm_vgic_early_init(struct kvm *kvm)
>>  	struct vgic_dist *dist = &kvm->arch.vgic;
>>  
>>  	INIT_LIST_HEAD(&dist->lpi_list_head);
>> +	INIT_LIST_HEAD(&dist->lpi_translation_cache);
>>  	raw_spin_lock_init(&dist->lpi_list_lock);
>>  }
>>  
>> @@ -260,6 +261,27 @@ static void kvm_vgic_vcpu_enable(struct kvm_vcpu *vcpu)
>>  		vgic_v3_enable(vcpu);
>>  }
>>  
>> +void vgic_lpi_translation_cache_init(struct kvm *kvm)
>> +{
>> +	struct vgic_dist *dist = &kvm->arch.vgic;
>> +	int i;
>> +
>> +	if (!list_empty(&dist->lpi_translation_cache))
>> +		return;
>> +
>> +	for (i = 0; i < LPI_CACHE_SIZE(kvm); i++) {
>> +		struct vgic_translation_cache_entry *cte;
>> +
>> +		/* An allocation failure is not fatal */
>> +		cte = kzalloc(sizeof(*cte), GFP_KERNEL);
>> +		if (WARN_ON(!cte))
>> +			break;
>> +
>> +		INIT_LIST_HEAD(&cte->entry);
>> +		list_add(&cte->entry, &dist->lpi_translation_cache);
>> +	}
>> +}
>> +
>>  /*
>>   * vgic_init: allocates and initializes dist and vcpu data structures
>>   * depending on two dimensioning parameters:
>> @@ -305,6 +327,7 @@ int vgic_init(struct kvm *kvm)
>>  	}
>>  
>>  	if (vgic_has_its(kvm)) {
>> +		vgic_lpi_translation_cache_init(kvm);
>>  		ret = vgic_v4_init(kvm);
>>  		if (ret)
>>  			goto out;
>> @@ -346,6 +369,17 @@ static void kvm_vgic_dist_destroy(struct kvm *kvm)
>>  		INIT_LIST_HEAD(&dist->rd_regions);
>>  	}
>>  
>> +	if (vgic_has_its(kvm)) {
>> +		struct vgic_translation_cache_entry *cte, *tmp;
>> +
>> +		list_for_each_entry_safe(cte, tmp,
>> +					 &dist->lpi_translation_cache, entry) {
>> +			list_del(&cte->entry);
>> +			kfree(cte);
>> +		}
>> +		INIT_LIST_HEAD(&dist->lpi_translation_cache);
>> +	}
>> +
>>  	if (vgic_supports_direct_msis(kvm))
>>  		vgic_v4_teardown(kvm);
>>  }
>> diff --git a/virt/kvm/arm/vgic/vgic-its.c b/virt/kvm/arm/vgic/vgic-its.c
>> index 44ceaccb18cf..5758504fd934 100644
>> --- a/virt/kvm/arm/vgic/vgic-its.c
>> +++ b/virt/kvm/arm/vgic/vgic-its.c
>> @@ -1696,6 +1696,8 @@ static int vgic_its_create(struct kvm_device *dev, u32 type)
>>  			kfree(its);
>>  			return ret;
>>  		}
>> +
>> +		vgic_lpi_translation_cache_init(dev->kvm);
>>  	}
>>  
>>  	mutex_init(&its->its_lock);
>> diff --git a/virt/kvm/arm/vgic/vgic.h b/virt/kvm/arm/vgic/vgic.h
>> index abeeffabc456..a58e1b263dca 100644
>> --- a/virt/kvm/arm/vgic/vgic.h
>> +++ b/virt/kvm/arm/vgic/vgic.h
>> @@ -316,6 +316,9 @@ int vgic_copy_lpi_list(struct kvm *kvm, struct kvm_vcpu *vcpu, u32 **intid_ptr);
>>  int vgic_its_resolve_lpi(struct kvm *kvm, struct vgic_its *its,
>>  			 u32 devid, u32 eventid, struct vgic_irq **irq);
>>  struct vgic_its *vgic_msi_to_its(struct kvm *kvm, struct kvm_msi *msi);
>> +void vgic_lpi_translation_cache_init(struct kvm *kvm);
>> +
>> +#define LPI_CACHE_SIZE(kvm)	(atomic_read(&(kvm)->online_vcpus) * 4)
> Couldn't the cache be a function of the number of allocated lpis. We
> could realloc the list accordingly. I miss why it is rather dependent on
> the number of vcpus and not on the number of assigned devices/MSIs?

How do you find out about the number of LPIs? That's really for the
guest to decide what it wants to do. Also, KVM itself doesn't have much
of a clue about the number of assigned devices or their MSI capability.
That's why I've suggested that userspace could be involved here.

So far, I've used the number of vcpus as MSIs are usually used to deal
with per-CPU queues. This allows the cache to scale with the number of
queues that the guest is expected to deal with. Ali's reply earlier seem
to indicate that this is a common pattern, but it is the multiplying
factor that is hard to express.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...
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