Subject: Re: [net-next 05/16] ixgbevf: enable multiple queue support

From: Vlad Zolotarov <[email-masked]>
Date: Wed, 25 Mar 2015 21:49:39 +0200

On 03/25/15 20:30, Alexander Duyck wrote:
> On 03/25/2015 01:44 AM, Vlad Zolotarov wrote:
>>
>>
>> On 02/05/15 11:35, Jeff Kirsher wrote:
>>> From: Emil Tantilov <[email-masked]>
>>>
>>> This patch enables multiple queues and RSS support for the VF.
>>> Maximum of 2 queues are supported due to available vectors.
>>>
>>> Signed-off-by: Emil Tantilov <[email-masked]>
>>> Tested-by: Krishneil Singh <[email-masked]>
>>> Signed-off-by: Jeff Kirsher <[email-masked]>
>>> ---
>>>   drivers/net/ethernet/intel/ixgbevf/ixgbevf.h      |  1 +
>>>   drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c | 22 
>>> +++++++++++++++++++---
>>>   2 files changed, 20 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/net/ethernet/intel/ixgbevf/ixgbevf.h 
>>> b/drivers/net/ethernet/intel/ixgbevf/ixgbevf.h
>>> index 8c44ab2..65cda34 100644
>>> --- a/drivers/net/ethernet/intel/ixgbevf/ixgbevf.h
>>> +++ b/drivers/net/ethernet/intel/ixgbevf/ixgbevf.h
>>> @@ -124,6 +124,7 @@ struct ixgbevf_ring {
>>>     #define MAX_RX_QUEUES IXGBE_VF_MAX_RX_QUEUES
>>>   #define MAX_TX_QUEUES IXGBE_VF_MAX_TX_QUEUES
>>> +#define IXGBEVF_MAX_RSS_QUEUES 2
>>
>> According to the MRQE register description in 82599 and x540 specs 2 
>> (the value of IXGBEVF_MAX_RSS_QUEUES) is the minimal allowed number 
>> of queues in a single pool thus it's a minimal number of VF queues - 
>> not the maximal (see more comments on this below).
>
> This is the maximum since the VF only supports 3 interrupts, one for 
> mailbox and 2 for queues, and RSS requires an interrupt per queue 
> pair.  The other possibility is that DCB has been enabled in which 
> case there RSS is reduced to one queue pair per pool with one RSS pool 
> for each traffic class.
>
>>
>>>     #define IXGBEVF_DEFAULT_TXD   1024
>>>   #define IXGBEVF_DEFAULT_RXD   512
>>> diff --git a/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c 
>>> b/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c
>>> index c9b49bf..815808f 100644
>>> --- a/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c
>>> +++ b/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c
>>> @@ -1794,7 +1794,8 @@ static int ixgbevf_configure_dcb(struct 
>>> ixgbevf_adapter *adapter)
>>>       struct ixgbe_hw *hw = &adapter->hw;
>>>       unsigned int def_q = 0;
>>>       unsigned int num_tcs = 0;
>>> -    unsigned int num_rx_queues = 1;
>>> +    unsigned int num_rx_queues = adapter->num_rx_queues;
>>> +    unsigned int num_tx_queues = adapter->num_tx_queues;
>>>       int err;
>>>         spin_lock_bh(&adapter->mbx_lock);
>>> @@ -1808,6 +1809,9 @@ static int ixgbevf_configure_dcb(struct 
>>> ixgbevf_adapter *adapter)
>>>           return err;
>>>         if (num_tcs > 1) {
>>> +        /* we need only one Tx queue */
>>> +        num_tx_queues = 1;
>>> +
>>>           /* update default Tx ring register index */
>>>           adapter->tx_ring[0]->reg_idx = def_q;
>>>   @@ -1816,7 +1820,8 @@ static int ixgbevf_configure_dcb(struct 
>>> ixgbevf_adapter *adapter)
>>>       }
>>>         /* if we have a bad config abort request queue reset */
>>> -    if (adapter->num_rx_queues != num_rx_queues) {
>>> +    if ((adapter->num_rx_queues != num_rx_queues) ||
>>> +        (adapter->num_tx_queues != num_tx_queues)) {
>>>           /* force mailbox timeout to prevent further messages */
>>>           hw->mbx.timeout = 0;
>>>   @@ -2181,8 +2186,19 @@ static void ixgbevf_set_num_queues(struct 
>>> ixgbevf_adapter *adapter)
>>>           return;
>>>         /* we need as many queues as traffic classes */
>>> -    if (num_tcs > 1)
>>> +    if (num_tcs > 1) {
>>>           adapter->num_rx_queues = num_tcs;
>>> +    } else {
>>> +        u16 rss = min_t(u16, num_online_cpus(), 
>>> IXGBEVF_MAX_RSS_QUEUES);
>>
>> Emil,
>> Here u ignore the IXGBE_VF_RX_QUEUES value returned the PF (stored in 
>> the hw->mac.max_rx_queues). This will accidentally work if PF is 
>> driven by a current ixgbe Linux driver but what would happen if HV is 
>> not a Linux box? What if HV decides to configure the PSRTYPE[x].RQPL 
>> to 0 and decide that VF should be configured with a single queue? I 
>> maybe miss something here but the above line seems bogus. Please, 
>> comment.
>
> It isn't ignored.  The fact that num_tcs is being tested for checks 
> for the one case where the number of RSS queues available would be 1, 
> otherwise the hardware will provide at least 2 queues. 

> If the RETA is not configured for 2 queues one of them will not be 
> active, but that should not be any sort of actual issue and would be 
> more of an administrative choice anyway as it would also limit the PF.

This would be an administrative choice ignored by the ixgbevf driver.
Frankly, I don't understand why are we arguing about this for so long? 
It's obvious that the safest and the most correct way to write it would 
be to limit the num_rx_queues by the value returned by PF. It would also 
remove any further questions and it's a one line patch.
However, if u still insist to keep it the way it is then let's drop this 
discussion and move on.

> - Alex