Re: [RFC PATCH 0/1] vmxnet3: Adjust maximum Rx ring buffer size

Aaron Tomlin
2025-01-08T21:05:20Z

On Tue, 7 Jan 2025, Jakub Kicinski wrote:
> This is a bit of a weird driver. But we should distinguish the default
> ring size, which yes, should not be too large, and max ring size which
> can be large but user setting a large size risks the fact the
> allocations will fail and device will not open.
>
> This driver seems to read the default size from the hypervisor, is that
> the value that is too large in your case? Maybe we should min() it with
> something reasonable? The max allowed to be set via ethtool can remain
> high IMO
>

See vmxnet3_get_ringparam(). If I understand correctly, since commit
50a5ce3e7116a ("vmxnet3: add receive data ring support"), if the specified
VMXNET3 adapter has support for the Rx Data ring feature then the maximum
Rx Data buffer size is reported as VMXNET3_RXDATA_DESC_MAX_SIZE (i.e. 2048)
by 'ethtool'. Furthermore, See vmxnet3_set_ringparam(). A user specified Rx
mini value cannot be more than VMXNET3_RXDATA_DESC_MAX_SIZE. Indeed the Rx
mini value in the context of VMXNET3 would be the size of the Rx Data ring
buffer. See the following excerpt from vmxnet3_set_ringparam(). As far as I
can tell, the Rx Data buffer cannot be more than
VMXNET3_RXDATA_DESC_MAX_SIZE:

686 static int
687 vmxnet3_set_ringparam(struct net_device *netdev,
688 struct ethtool_ringparam *param,
689 struct kernel_ethtool_ringparam *kernel_param,
690 struct netlink_ext_ack *extack)
691 {
:
760 new_rxdata_desc_size =
761 (param->rx_mini_pending + VMXNET3_RXDATA_DESC_SIZE_MASK) &
762 ~VMXNET3_RXDATA_DESC_SIZE_MASK;
763 new_rxdata_desc_size = min_t(u16, new_rxdata_desc_size,
764 VMXNET3_RXDATA_DESC_MAX_SIZE);

Have I missed something?

--
Aaron Tomlin