> -----Original Message-----
> From: Michael Kelley <mhkli...@outlook.com>
> Sent: Wednesday, August 14, 2024 7:57 PM
> To: Erni Sri Satya Vennela <er...@linux.microsoft.com>; KY Srinivasan
> <k...@microsoft.com>; Haiyang Zhang <haiya...@microsoft.com>;
> wei....@kernel.org; Dexuan Cui <de...@microsoft.com>; da...@davemloft.net;
> eduma...@google.com; k...@kernel.org; pab...@redhat.com; linux-
> hyp...@vger.kernel.org; net...@vger.kernel.org; linux-
> ker...@vger.kernel.org
> Cc: Erni Sri Satya Vennela <er...@microsoft.com>
> Subject: RE: [PATCH v2] net: netvsc: Update default VMBus channels
>
> From: Erni Sri Satya Vennela <er...@linux.microsoft.com> Sent: Wednesday,
> August 14, 2024 9:59 AM
> >
> > Change VMBus channels macro (VRSS_CHANNEL_DEFAULT) in
> > Linux netvsc from 8 to 16 to align with Azure Windows VM
> > and improve networking throughput.
> >
> > For VMs having less than 16 vCPUS, the channels depend
> > on number of vCPUs. Between 16 to 32 vCPUs, the channels
> > default to VRSS_CHANNEL_DEFAULT. For greater than 32 vCPUs,
> > set the channels to number of physical cores / 2 as a way
> > to optimize CPU resource utilization and scale for high-end
> > processors with many cores.
> > Maximum number of channels are by default set to 64.
>
> Where in the code is this enforced? It's not part of this patch. It
> might be in rndis_set_subchannel(), where a value larger than
> 64 could be sent to the Hyper-V host, expecting that the Hyper-V
> host will limit it to 64. But netvsc driver code is declaring an array
> of size VRSS_CHANNEL_MAX, and there's nothing that guarantees
> that Hyper-V will always limit the channel count to 64. But maybe
> the netvsc driver enforces the limit of VRSS_CHANNEL_MAX in a
> place that I didn't immediately see in a quick look at the code.
Yes, netvsc driver limits the num_chn to be <=64:
#define VRSS_CHANNEL_MAX 64
/* This guarantees that num_possible_rss_qs <= num_online_cpus */
num_possible_rss_qs = min_t(u32, num_online_cpus(),
rsscap.num_recv_que);
net_device->max_chn = min_t(u32, VRSS_CHANNEL_MAX, num_possible_rss_qs);
/* We will use the given number of channels if available. */
net_device->num_chn = min(net_device->max_chn, device_info->num_chn);
Thanks,
- Haiyang
