Re: [Qemu-devel] [virtio-dev] Re: [PATCH v3 1/2] virtio-net: enable configurable tx queue size

[Jason Wang <jasowang@redhat.com>]

On 2017年06月27日 05:21, Michael S. Tsirkin wrote:
> On Mon, Jun 26, 2017 at 06:34:25PM +0800, Wei Wang wrote:
>> On 06/26/2017 04:05 PM, Jason Wang wrote:
>>>
>>> On 2017年06月26日 12:55, Wei Wang wrote:
>>>> On 06/26/2017 11:18 AM, Jason Wang wrote:
>>>>> On 2017年06月23日 10:32, Wei Wang wrote:
>>>>>> This patch enables the virtio-net tx queue size to be configurable
>>>>>> between 256 (the default queue size) and 1024 by the user when the
>>>>>> vhost-user backend is used.
>>>>>>
>>>>>> Currently, the maximum tx queue size for other backends is 512 due
>>>>>> to the following limitations:
>>>>>> - QEMU backend: the QEMU backend implementation in some cases may
>>>>>> send 1024+1 iovs to writev.
>>>>>> - Vhost_net backend: there are possibilities that the guest sends
>>>>>> a vring_desc of memory which corsses a MemoryRegion thereby
>>>>>> generating more than 1024 iovs after translattion from guest-physical
>>>>>> address in the backend.
>>>>>>
>>>>>> Signed-off-by: Wei Wang <wei.w.wang@intel.com>
>>>>>> ---
>>>>>>    hw/net/virtio-net.c            | 45
>>>>>> +++++++++++++++++++++++++++++++++---------
>>>>>>    include/hw/virtio/virtio-net.h |  1 +
>>>>>>    2 files changed, 37 insertions(+), 9 deletions(-)
>>>>>>
>>>>>> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
>>>>>> index 91eddaf..d13ca60 100644
>>>>>> --- a/hw/net/virtio-net.c
>>>>>> +++ b/hw/net/virtio-net.c
>>>>>> @@ -34,8 +34,11 @@
>>>>>>      /* previously fixed value */
>>>>>>    #define VIRTIO_NET_RX_QUEUE_DEFAULT_SIZE 256
>>>>>> +
>>>>>> +    /*
>>>>>> +     * Currently, backends other than vhost-user don't
>>>>>> support 1024 queue
>>>>>> +     * size.
>>>>>> +     */
>>>>>> +    if (n->net_conf.tx_queue_size == VIRTQUEUE_MAX_SIZE &&
>>>>>> +        nc->peer->info->type != NET_CLIENT_DRIVER_VHOST_USER) {
>>>>>> +        n->net_conf.tx_queue_size = VIRTIO_NET_TX_QUEUE_DEFAULT_SIZE;
>>>>> Do we really want assume all vhost-user backend support 1024
>>>>> queue size?
>>>>>
>>>>   Do you know any vhost-user backend implementation that doesn't support
>>>> 1024 tx queue size?
>>>>
>>>>
>>> I don't but the issue is vhost-user uditis an open protocol, there could
>>> be even closed source implementation on this. So we can't audit all
>>> kinds of backends.
>>>
>> This is the discussion left before. Here is my thought:
>>
>> Until now, nobody is officially using 1024 tx queue size, including the
>> unknown
>> vhost-user backend implementation. So, applying this patch won't affect any
>> already deployed products.
>>
>> If someday, people whose products are based on the unknown vhost-user
>> implementation change to try with 1024 tx queue size and apply this patch,
>> and find 1024 doesn't work for them, they can simply set the value to 512
>> (which
>> is better than the 256 size they were using) or apply the 2nd patch entitled
>> "VIRTIO_F_MAX_CHAIN_SIZE" to use 1024 queue size.
>>
>> I didn't see any big issue here. Would you see any issues?
>>
>> @Michael, what is your opinion?
> Since the default does not change, I think it's fine.
> You need to both have a non-default value and be using
> a backend that does not support the 1024 size.
> Seems unlikely to me.

But this patch in fact allows 1024 to be used even for vhost-kernel 
after migration from vhost-user?

Thanks

>
>
>>>>>> +    }
>>>>>> +
>>>>>> +    for (i = 0; i < n->max_queues; i++) {
>>>>>> +        virtio_net_add_queue(n, i);
>>>>>> +    }
>>>>> Any reason to move virtio_net_add_queue() here?
>>>>>
>>>> Please check the whole init steps.  It was moved here (after
>>>> qemu_new_nic())
>>>> to make sure nc->peer is not NULL.
>>> I don't get here, can't you get peer just from nic_conf.peers?
>>>
>> Correct, nic_conf.peers also works. Thanks.
>>
>> Best,
>> Wei
>>
>>
>>
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