Re: [Qemu-devel] [PATCH 1/2] return available ioeventfds count in kvm_has_many_ioeventfds()

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

On 2012-03-13 13:00, Amos Kong wrote:
> On 13/03/12 19:50, Jan Kiszka wrote:
>> Please tag uq/master patches with "PATCH uq/master".
>>
>> On 2012-03-13 11:42, Amos Kong wrote:
>>> Older kernels have a 6 device limit on the KVM io bus.
>>> This patch makes kvm_has_many_ioeventfds() return available
>>> ioeventfd count. ioeventfd will be disabled if there is
>>> no 7 available ioeventfds.
>>>
>>> Signed-off-by: Amos Kong<akong@redhat.com>
>>> ---
>>>   hw/virtio-pci.c |    2 +-
>>>   kvm-all.c       |    9 +++------
>>>   2 files changed, 4 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/hw/virtio-pci.c b/hw/virtio-pci.c
>>> index a0fb7c1..d63f303 100644
>>> --- a/hw/virtio-pci.c
>>> +++ b/hw/virtio-pci.c
>>> @@ -678,7 +678,7 @@ void virtio_init_pci(VirtIOPCIProxy *proxy, VirtIODevice *vdev)
>>>       pci_register_bar(&proxy->pci_dev, 0, PCI_BASE_ADDRESS_SPACE_IO,
>>>                        &proxy->bar);
>>>
>>> -    if (!kvm_has_many_ioeventfds()) {
>>> +    if (kvm_has_many_ioeventfds() != 7) {
>>>           proxy->flags&= ~VIRTIO_PCI_FLAG_USE_IOEVENTFD;
>>>       }
>>>
>>> diff --git a/kvm-all.c b/kvm-all.c
>>> index 3c6b4f0..d12694b 100644
>>> --- a/kvm-all.c
>>> +++ b/kvm-all.c
>>> @@ -78,7 +78,6 @@ struct KVMState
>>>       int pit_in_kernel;
>>>       int pit_state2;
>>>       int xsave, xcrs;
>>> -    int many_ioeventfds;
>>>       int irqchip_inject_ioctl;
>>>   #ifdef KVM_CAP_IRQ_ROUTING
>>>       struct kvm_irq_routing *irq_routes;
>>> @@ -510,8 +509,8 @@ static int kvm_check_many_ioeventfds(void)
>>>           }
>>>       }
>>>
>>> -    /* Decide whether many devices are supported or not */
>>> -    ret = i == ARRAY_SIZE(ioeventfds);
>>> +    /* If i equals to 7, many devices are supported */
>>> +    ret = i;
>>>
>>>       while (i-->  0) {
>>>           kvm_set_ioeventfd_pio_word(ioeventfds[i], 0, i, false);
>>> @@ -1078,8 +1077,6 @@ int kvm_init(void)
>>>       kvm_state = s;
>>>       memory_listener_register(&kvm_memory_listener, NULL);
>>>
>>> -    s->many_ioeventfds = kvm_check_many_ioeventfds();
>>> -
>>>       cpu_interrupt_handler = kvm_handle_interrupt;
>>>
>>>       return 0;
>>> @@ -1407,7 +1404,7 @@ int kvm_has_many_ioeventfds(void)
>>>       if (!kvm_enabled()) {
>>>           return 0;
>>>       }
>>> -    return kvm_state->many_ioeventfds;
>>> +    return kvm_check_many_ioeventfds();
>>
>> And why are you dropping the caching of the kvm_check_many_ioeventfds()
>> return value? Is kvm_has_many_ioeventfds not used outside init scopes?
> 
> Hi Jan,
> 
> In the past, kvm_state->many_ioeventfds is only updated once at the 
> beginning,

And the additional use case of patch 2 is not hot path either, right?

> I want to use kvm_check_many_ioeventfds() to check if available 
> ioeventfd exists
> before starting ioeventfd each time.

OK, but than kvm_has_many_ioeventfds is not the right name for this
function anymore. Call it "kvm_get_available_ioventfds" or so, but not
in a away that implies a boolean return value.

Jan

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux