Re: [PATCH v2 4/9] KVM: arm/arm64: vgic-its: Invalidate MSI-LPI translation cache on specific commands

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Hi Eric,

On 01/07/2019 13:38, Auger Eric wrote:
> Hi Marc,
> 
> On 6/11/19 7:03 PM, Marc Zyngier wrote:
>> The LPI translation cache needs to be discarded when an ITS command
>> may affect the translation of an LPI (DISCARD and MAPD with V=0) or
>> the routing of an LPI to a redistributor with disabled LPIs (MOVI,
>> MOVALL).
>>
>> We decide to perform a full invalidation of the cache, irrespective
>> of the LPI that is affected. Commands are supposed to be rare enough
>> that it doesn't matter.
>>
>> Signed-off-by: Marc Zyngier <marc.zyngier@xxxxxxx>
>> ---
>>  virt/kvm/arm/vgic/vgic-its.c | 8 ++++++++
>>  1 file changed, 8 insertions(+)
>>
>> diff --git a/virt/kvm/arm/vgic/vgic-its.c b/virt/kvm/arm/vgic/vgic-its.c
>> index 9b6b66204b97..5254bb762e1b 100644
>> --- a/virt/kvm/arm/vgic/vgic-its.c
>> +++ b/virt/kvm/arm/vgic/vgic-its.c
>> @@ -733,6 +733,8 @@ static int vgic_its_cmd_handle_discard(struct kvm *kvm, struct vgic_its *its,
>>  		 * don't bother here since we clear the ITTE anyway and the
>>  		 * pending state is a property of the ITTE struct.
>>  		 */
>> +		vgic_its_invalidate_cache(kvm);
>> +
>>  		its_free_ite(kvm, ite);
>>  		return 0;
>>  	}
>> @@ -768,6 +770,8 @@ static int vgic_its_cmd_handle_movi(struct kvm *kvm, struct vgic_its *its,
>>  	ite->collection = collection;
>>  	vcpu = kvm_get_vcpu(kvm, collection->target_addr);
>>  
>> +	vgic_its_invalidate_cache(kvm);
>> +
>>  	return update_affinity(ite->irq, vcpu);
>>  }
>>  
>> @@ -996,6 +1000,8 @@ static void vgic_its_free_device(struct kvm *kvm, struct its_device *device)
>>  	list_for_each_entry_safe(ite, temp, &device->itt_head, ite_list)
>>  		its_free_ite(kvm, ite);
>>  
>> +	vgic_its_invalidate_cache(kvm);
>> +
>>  	list_del(&device->dev_list);
>>  	kfree(device);
>>  }
>> @@ -1249,6 +1255,8 @@ static int vgic_its_cmd_handle_movall(struct kvm *kvm, struct vgic_its *its,
>>  		vgic_put_irq(kvm, irq);
>>  	}
>>  
>> +	vgic_its_invalidate_cache(kvm);
> All the commands are executed with the its_lock held. Now we don't take
> it anymore on the fast cache injection path. Don't we have a window
> where the move has been applied at table level and the cache is not yet
> invalidated? Same question for vgic_its_free_device().

There is definitely a race, but that race is invisible from the guest's
perspective. The guest can only assume that the command has taken effect
once a SYNC command has been executed, and it cannot observe that the
SYNC command has been executed before we have invalidated the cache.

Does this answer your question?

Thanks,

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



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