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

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Date:   Sat, 14 Nov 2020 15:46:32 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Thomas Falcon <tlfalcon@...ux.ibm.com>
Cc:     netdev@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        dnbanerg@...ibm.com, brking@...ux.vnet.ibm.com, pradeep@...ibm.com,
        drt@...ux.vnet.ibm.com, sukadev@...ux.vnet.ibm.com,
        ljp@...ux.vnet.ibm.com, cforno12@...ux.ibm.com,
        ricklind@...ux.ibm.com
Subject: Re: [PATCH net-next 04/12] ibmvnic: Introduce xmit_more support
 using batched subCRQ hcalls

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@...ux.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.