Re: [PATCH v4 9/9] KVM: Allow host IRQ sharing for passed-through PCI 2.3 devices

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

Am 09.11.2010 14:27, Avi Kivity wrote:
> On 11/08/2010 01:21 PM, Jan Kiszka wrote:
>> PCI 2.3 allows to generically disable IRQ sources at device level. This
>> enables us to share IRQs of such devices between on the host side when
>> passing them to a guest. This feature is optional, user space has to
>> request it explicitly. Moreover, user space can inform us about its view
>> of PCI_COMMAND_INTX_DISABLE so that we can avoid unmasking the interrupt
>> and signaling it if the guest masked it via the PCI config space.
>>
> 
> It's a pity this cannot be done transparently.  We could detect multiple 
> devices sharing the line,

Even that is not possible. Assigned or host devices may be activated
after we registered exclusively, pushing the breakage from VM start-up
to a different operation.

> but what about PCI_COMMAND_INTX_DISABLE?
> 
> Perhaps we can hook the kernel's handler for this bit?

Some IRQ registration notifier that would allow us to reregister our
handler with IRQ sharing support? Maybe.

> 
>>
>>   /* Depends on KVM_CAP_IOMMU */
>>   #define KVM_DEV_ASSIGN_ENABLE_IOMMU	(1<<  0)
>> +/* The following two depend on KVM_CAP_PCI_2_3 */
>> +#define KVM_DEV_ASSIGN_PCI_2_3		(1<<  1)
>> +#define KVM_DEV_ASSIGN_MASK_INTX	(1<<  2)
>> +
>> +If KVM_DEV_ASSIGN_PCI_2_3 is set, the kernel will manage legacy INTx interrupts
>> +via the PCI-2.3-compliant device-level mask, thus enable IRQ sharing with other
>> +assigned devices or host devices. KVM_DEV_ASSIGN_MASK_INTX specifies the
>> +guest's view on the INTx mask, see KVM_ASSIGN_SET_INTX_MASK for details.
>>
>>   4.48 KVM_DEASSIGN_PCI_DEVICE
>>
>> @@ -1263,6 +1271,23 @@ struct kvm_assigned_msix_entry {
>>   	__u16 padding[3];
>>   };
>>
>> +5.54 KVM_ASSIGN_SET_INTX_MASK
> 
> 4.54?

Of course.

> 
> (54? wow.)

And I don't think all IOCTLs are already documented (though the majority
now).

> 
>> +
>> +Capability: KVM_CAP_PCI_2_3
>> +Architectures: x86
>> +Type: vm ioctl
>> +Parameters: struct kvm_assigned_pci_dev (in)
>> +Returns: 0 on success, -1 on error
>> +
>> +Informs the kernel about the guest's view on the INTx mask. As long as the
>> +guest masks the legacy INTx, the kernel will refrain from unmasking it at
>> +hardware level and will not assert the guest's IRQ line. User space is still
>> +responsible for applying this state to the assigned device's real config space.
> 
> What if userspace lies?

User space problem. We will at worst receive one IRQ, mask it, and then
user space need to react again.

> 
>> +
>> +See KVM_ASSIGN_DEV_IRQ for the data structure. The target device is specified
>> +by assigned_dev_id. In the flags field, only KVM_DEV_ASSIGN_MASK_INTX is
>> +evaluated.
>> +
>>
>> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
>> index fe83eb0..7f1627c 100644
>> --- a/include/linux/kvm_host.h
>> +++ b/include/linux/kvm_host.h
>> @@ -468,6 +468,7 @@ struct kvm_assigned_dev_kernel {
>>   	unsigned int entries_nr;
>>   	int host_irq;
>>   	bool host_irq_disabled;
>> +	bool pci_2_3;
>>   	struct msix_entry *host_msix_entries;
>>   	int guest_irq;
>>   	struct msix_entry *guest_msix_entries;
>> @@ -477,6 +478,7 @@ struct kvm_assigned_dev_kernel {
>>   	struct pci_dev *dev;
>>   	struct kvm *kvm;
>>   	spinlock_t intx_lock;
>> +	struct mutex intx_mask_lock;
>>   	char irq_name[32];
>>   };
> 
> I saw no reason this can't be a spinlock, but perhaps I missed 
> something.  This would allow us to avoid srcu, which is slightly more 
> expensive than rcu.  Since pci 2.3 assigned devices are not a major use 
> case, I'd like not to penalize the mainstream users for this.

The lock has to be held across kvm_set_irq, which is the potentially
expensive (O(n), n == number of VCPUs) operation.

> 
> This patch undoes some of the niceness of the previous patches, but I 
> have no alternative to suggest.
> 

Yes, it surely does not make things simpler. But much of the complexity
is avoided during runtime when MSIs are used.

Jan

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