Re: [PATCH net-next RESEND v4] ibmvnic: Increase max subcrq indirect entries with fallback

[Paolo Abeni <pabeni@redhat.com>]

On 8/21/25 3:02 PM, Mingming Cao wrote:
> POWER8 support a maximum of 16 subcrq indirect descriptor entries per
>  H_SEND_SUB_CRQ_INDIRECT call, while POWER9 and newer hypervisors
>  support up to 128 entries. Increasing the max number of indirect
> descriptor entries improves batching efficiency and reduces
> hcall overhead, which enhances throughput under large workload on POWER9+.
> 
> Currently, ibmvnic driver always uses a fixed number of max indirect
> descriptor entries (16). send_subcrq_indirect() treats all hypervisor
> errors the same:
>  - Cleanup and Drop the entire batch of descriptors.
>  - Return an error to the caller.
>  - Rely on TCP/IP retransmissions to recover.
>  - If the hypervisor returns H_PARAMETER (e.g., because 128
>    entries are not supported on POWER8), the driver will continue
>    to drop batches, resulting in unnecessary packet loss.
> 
> In this patch:
> Raise the default maximum indirect entries to 128 to improve ibmvnic
> batching on morden platform. But also gracefully fall back to
> 16 entries for Power 8 systems.
> 
> Since there is no VIO interface to query the hypervisor’s supported
> limit, vnic handles send_subcrq_indirect() H_PARAMETER errors:
>  - On first H_PARAMETER failure, log the failure context
>  - Reduce max_indirect_entries to 16 and allow the single batch to drop.
>  - Subsequent calls automatically use the correct lower limit,
>     avoiding repeated drops.
> 
> The goal is to  optimizes performance on modern systems while handles
> falling back for older POWER8 hypervisors.
> 
> Performance shows 40% improvements with MTU (1500) on largework load.
> 
> Signed-off-by: Mingming Cao <mmc@linux.ibm.com>
> Reviewed-by: Brian King <bjking1@linux.ibm.com>
> Reviewed-by: Haren Myneni <haren@linux.ibm.com>
> Reviewed-by: Simon Horman <horms@kernel.org>
> --------------------------------------
> Changes since v3:
> Link to v3: https://www.spinics.net/lists/netdev/msg1112828.html

For future memory: please use lore links instead.

Thanks,

Paolo