Re: [Qemu-devel] [PATCH] virtio-serial: Allow one MSI-X vector per virtqueue

[Zang Hongyong <zanghongyong@huawei.com>]

于 2011/12/19,星期一 15:26, Amit Shah 写道:
> On (Mon) 19 Dec 2011 [14:09:43], Zang Hongyong wrote:
>> 于 2011/12/16,星期五 17:39, Amit Shah 写道:
>>> On (Fri) 16 Dec 2011 [09:14:26], zanghongyong@huawei.com wrote:
>>>> From: Hongyong Zang<zanghongyong@huawei.com>
>>>>
>>>> In pci_enable_msix(), the guest's virtio-serial driver tries to set msi-x
>>>> with one vector per queue. But it fails and eventually all virtio-serial
>>>> ports share one MSI-X vector. Because every virtio-serial port has *two*
>>>> virtqueues, virtio-serial needs (port+1)*2 vectors other than (port+1).
>>> Ouch, good catch.
>>>
>>> One comment below:
>>>
>>>> This patch allows every virtqueue to have its own MSI-X vector.
>>>> (When the MSI-X vectors needed are more than MSIX_MAX_ENTRIES defined in
>>>> qemu: msix.c, all the queues still share one MSI-X vector as before.)
>>>>
>>>> Signed-off-by: Hongyong Zang<zanghongyong@huawei.com>
>>>> ---
>>>>   hw/virtio-pci.c |    5 ++++-
>>>>   1 files changed, 4 insertions(+), 1 deletions(-)
>>>>
>>>> diff --git a/hw/virtio-pci.c b/hw/virtio-pci.c
>>>> index 77b75bc..2c9c6fb 100644
>>>> --- a/hw/virtio-pci.c
>>>> +++ b/hw/virtio-pci.c
>>>> @@ -718,8 +718,11 @@ static int virtio_serial_init_pci(PCIDevice *pci_dev)
>>>>           return -1;
>>>>       }
>>>>       vdev->nvectors = proxy->nvectors == DEV_NVECTORS_UNSPECIFIED
>>>> -                                        ? proxy->serial.max_virtserial_ports + 1
>>>> +                                        ? (proxy->serial.max_virtserial_ports + 1) * 2
>>>>                                           : proxy->nvectors;
>>>> +    /*msix.c: #define MSIX_MAX_ENTRIES 32*/
>>>> +    if (vdev->nvectors>   32)
>>>> +        vdev->nvectors = 32;
>>> This change isn't needed: if the proxy->nvectors value exceeds the max
>>> allowed, virtio_init_pci() will end up using a shared vector instead
>>> of separate ones.
>>>
>> Hi Amit,
>> If the nvectors exceeds the max, msix_init() will return -EINVAL in QEMU,
>> and the front-end driver in Guest will use regular interrupt instead
>> of MSI-X.
> In that case, I believe msix_init() should be changed to attempt to
> share interrupts instead of drivers doing this by themselves.
>
> 		Amit
>
> .
>
Yup, if the vectors exceeds the max, msix_init() should try to share one 
vector or use the max vectors directly.

Or we may change the MSIX_MAX_ENTRIES defined in misx.c:
It allocs one page for the msix table(reserve second half of the page 
for pending bits), and one table entry only needs four DWORDs in PCI 
SPEC, so I think the max entries could be 128 instead of 32.