Re: [PATCH v4 4/9] KVM: Switch assigned device IRQ forwarding to threaded handler

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

Am 09.11.2010 13:26, Avi Kivity wrote:
> On 11/08/2010 01:21 PM, Jan Kiszka wrote:
>> This improves the IRQ forwarding for assigned devices: By using the
>> kernel's threaded IRQ scheme, we can get rid of the latency-prone work
>> queue and simplify the code in the same run.
>>
>> Moreover, we no longer have to hold assigned_dev_lock while raising the
>> guest IRQ, which can be a lenghty operation as we may have to iterate
>> over all VCPUs. The lock is now only used for synchronizing masking vs.
>> unmasking of INTx-type IRQs, thus is renames to intx_lock.
> 
> Nice stuff.
> 
>>
>> -#define KVM_ASSIGNED_MSIX_PENDING		0x1
>> -struct kvm_guest_msix_entry {
>> -	u32 vector;
>> -	u16 entry;
>> -	u16 flags;
>> -};
>> -
> 
> Ok, so .flags was used to handle the delay between the interrupt handler 
> and the work queue, which is now done automatically by threaded 
> interrupts.  Good.

Yes, a nice consolidating side effect.

> 
>> @@ -313,10 +276,10 @@ static int assigned_device_enable_host_msix(struct kvm *kvm,
>>   		return r;
>>
>>   	for (i = 0; i<  dev->entries_nr; i++) {
>> -		r = request_irq(dev->host_msix_entries[i].vector,
>> -				kvm_assigned_dev_intr, 0,
>> -				"kvm_assigned_msix_device",
>> -				(void *)dev);
>> +		r = request_threaded_irq(dev->host_msix_entries[i].vector,
>> +					 NULL, kvm_assigned_dev_thread,
>> +					 0, "kvm_assigned_msix_device",
>> +					 (void *)dev);
>>   		if (r)
>>   			goto err;
>>   	}
> 
> 
> Should eventually be done from interrupt context.  msix delivery only 
> needs a routing table lookup and smp reschedule IPI, which can be done 
> from interrupt context.
> 

Yes, definitely. But we would have to catch the case when the guest
configures the MSI-X message to be a broadcast to all its trillion VCPUs
(once we support trillions of them).

Jan

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