From: Aditya Garg <gargaditya@linux.microsoft.com>
To: Amir Boussejra <amir@boussejra.com>,
linux-hyperv@vger.kernel.org, netdev@vger.kernel.org
Cc: amir.boussejra@gatewatcher.com
Subject: Re: hv_netvsc: user-configured RSS indirection table is zeroed on MTU change
Date: Wed, 4 Nov 2026 10:38:39 +0530 [thread overview]
Message-ID: <c3ec0e88-5077-4eee-8ed8-9b2f71f00b0a@linux.microsoft.com> (raw)
In-Reply-To: <40fb2352-4859-4334-849b-f6a4229605a1@boussejra.com>
On 29-10-2026 14:40, Amir Boussejra wrote:
> Hi,
>
> On Azure/Hyper-V, any MTU change on an hv_netvsc interface silently
> zeroes a
> user-configured RSS indirection table, so all RX traffic lands on ring 0.
>
> Reproducer in our case (any multi-queue hv_netvsc NIC, ethtool and ip
> only):
>
> ```
> ethtool -X eth0 hkey $SOME_KEY equal 4
> ethtool -x eth0 # See conf
> ip link set eth0 mtu 1600 # must differ from the current MTU
> ethtool -x eth0 # all zeros in indirection table
> ```
>
> With the table left to the driver (ethtool -X eth0 default) the same MTU
> change is harmless: the table comes back correctly spread.
>
> Confirmed with kprobes on 6.12.90 - these all fire on the MTU write:
>
> ```
> netvsc_change_mtu
> netvsc_detach
> rndis_filter_device_remove
> netvsc_attach
> rndis_filter_device_add
> ```
>
> [`netvsc_change_mtu()`](https://github.com/torvalds/linux/blob/v6.12/
> drivers/net/hyperv/netvsc_drv.c#L1209)
>
> An MTU change re-creates the device:
> [detach](https://github.com/torvalds/linux/blob/v6.12/drivers/net/
> hyperv/netvsc_drv.c#L1233), then
> [attach](https://github.com/torvalds/linux/blob/v6.12/drivers/net/
> hyperv/netvsc_drv.c#L1239).
>
> The detach [frees the RSS indirection table](https://github.com/
> torvalds/linux/blob/v6.12/drivers/net/hyperv/rndis_filter.c#L1622)
> (`rndis_filter_device_remove()`), and the attach
> [allocates it zeroed](https://github.com/torvalds/linux/blob/v6.12/
> drivers/net/hyperv/rndis_filter.c#L1557)
> with `kcalloc()` and refills it only if
> [`(!netif_is_rxfh_configured(net))`](https://github.com/torvalds/linux/
> blob/v6.12/drivers/net/hyperv/rndis_filter.c#L1572)
> (`rndis_filter_device_add()`).
>
>
Hi Amir,
Thanks for reporting this bug
I'll post a fix for this and notify here as well
Regards,
Aditya
prev parent reply other threads:[~2026-11-04 5:08 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-10-29 9:10 hv_netvsc: user-configured RSS indirection table is zeroed on MTU change Amir Boussejra
2026-11-04 5:08 ` Aditya Garg [this message]
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