netdev - Re: [PATCH net-next v2] ibmveth: Add multi buffers rx replenishment hcall support

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Message-ID: <20250720104540.GU2459@horms.kernel.org>
Date: Sun, 20 Jul 2025 11:45:40 +0100
From: Simon Horman <horms@...nel.org>
To: Mingming Cao <mmc@...ux.ibm.com>
Cc: netdev@...r.kernel.org, bjking1@...ux.ibm.com, haren@...ux.ibm.com,
	ricklind@...ux.ibm.com, davemarq@...ux.ibm.com, maddy@...ux.ibm.com,
	mpe@...erman.id.au, npiggin@...il.com, christophe.leroy@...roup.eu,
	andrew+netdev@...n.ch, davem@...emloft.net, kuba@...nel.org,
	edumazet@...gle.com, pabeni@...hat.com,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH net-next v2] ibmveth: Add multi buffers rx replenishment
 hcall support

On Sat, Jul 19, 2025 at 05:13:56AM -0400, Mingming Cao wrote:
> This patch enables batched RX buffer replenishment in ibmveth by
> using the new firmware-supported h_add_logical_lan_buffers() hcall
>  to submit up to 8 RX buffers in a single call, instead of repeatedly
> calling the single-buffer h_add_logical_lan_buffer() hcall.
> 
> During the probe, with the patch, the driver queries ILLAN attributes
> to detect IBMVETH_ILLAN_RX_MULTI_BUFF_SUPPORT bit. If the attribute is
> present, rx_buffers_per_hcall is set to 8, enabling batched replenishment.
> Otherwise, it defaults to 1, preserving the original upstream behavior
>  with no change in code flow for unsupported systems.
> 
> The core rx replenish logic remains the same. But when batching
> is enabled, the driver aggregates up to 8 fully prepared descriptors
> into a single h_add_logical_lan_buffers() hypercall. If any allocation
> or DMA mapping fails while preparing a batch, only the successfully
> prepared buffers are submitted, and the remaining are deferred for
> the next replenish cycle.
> 
> If at runtime the firmware stops accepting the batched hcall—e,g,
> after a Live Partition Migration (LPM) to a host that does not
> support h_add_logical_lan_buffers(), the hypercall returns H_FUNCTION.
> In that case, the driver transparently disables batching, resets
> rx_buffers_per_hcall to 1, and falls back to the single-buffer hcall
> in next future replenishments to take care of these and future buffers.
> 
> Test were done on systems with firmware that both supports and
> does not support the new h_add_logical_lan_buffers hcall.
> 
> On supported firmware, this reduces hypercall overhead significantly
> over multiple buffers. SAR measurements showed about a 15% improvement
> in packet processing rate under moderate RX load, with heavier traffic
> seeing gains more than 30%
> 
> Signed-off-by: Mingming Cao <mmc@...ux.ibm.com>
> Reviewed-by: Brian King <bjking1@...ux.ibm.com>
> Reviewed-by: Haren Myneni <haren@...ux.ibm.com>
> Reviewed-by: Dave Marquardt <davemarq@...ux.ibm.com>

Thanks for the update.

Reviewed-by: Simon Horman <horms@...nel.org>

...