[PATCH net-next 04/12] ibmvnic: Introduce xmit_more support using batched subCRQ hcalls

[Jakub Kicinski]

On Thu, 12 Nov 2020 13:09:59 -0600 Thomas Falcon wrote:
> Include support for the xmit_more feature utilizing the
> H_SEND_SUB_CRQ_INDIRECT hypervisor call which allows the sending
> of multiple subordinate Command Response Queue descriptors in one
> hypervisor call via a DMA-mapped buffer. This update reduces hypervisor
> calls and thus hypervisor call overhead per TX descriptor.
> 
> Signed-off-by: Thomas Falcon <tlfalcon at linux.ibm.com>

The common bug with xmit_more is not flushing the already queued
notifications when there is a drop. Any time you drop a skb you need 
to check it's not an skb that was the end of an xmit_more train and 
if so flush notifications (or just always flush on error).

Looking at the driver e.g. this starting goto:

        if (ibmvnic_xmit_workarounds(skb, netdev)) {                            
                tx_dropped++;                                                   
                tx_send_failed++;                                               
                ret = NETDEV_TX_OK;                                             
                goto out;                                                       
        }  

Does not seem to hit any flush on its way out AFAICS.