netdev - RE: [EXTERNAL] Re: [PATCH net-next] virtio_net: Increase RSS max key size to match NETDEV_RSS_KEY

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Message-ID:
 <CH3PR18MB6379AB2F09B861C3F2B3EC9EA094A@CH3PR18MB6379.namprd18.prod.outlook.com>
Date: Fri, 23 Jan 2026 11:37:44 +0000
From: Srujana Challa <schalla@...vell.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "virtualization@...ts.linux.dev" <virtualization@...ts.linux.dev>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "jasowang@...hat.com"
	<jasowang@...hat.com>,
        "xuanzhuo@...ux.alibaba.com"
	<xuanzhuo@...ux.alibaba.com>,
        "eperezma@...hat.com" <eperezma@...hat.com>,
        "andrew+netdev@...n.ch" <andrew+netdev@...n.ch>,
        "davem@...emloft.net"
	<davem@...emloft.net>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "kuba@...nel.org" <kuba@...nel.org>,
        Nithin Kumar Dabilpuram
	<ndabilpuram@...vell.com>,
        Shiva Shankar Kommula <kshankar@...vell.com>
Subject: RE: [EXTERNAL] Re: [PATCH net-next] virtio_net: Increase RSS max key
 size to match NETDEV_RSS_KEY_LEN

> On Thu, Jan 22, 2026 at 02:45:27PM +0530, Srujana Challa wrote:
> > Increase VIRTIO_NET_RSS_MAX_KEY_SIZE from 40 to 52 bytes to align with
> > the kernel's standard RSS key length defined by NETDEV_RSS_KEY_LEN.
> >
> > The virtio specification requires devices to support at least 40 bytes
> > for the RSS key size.
> 
> Indeed:
> 	The device MUST set \field{rss_max_key_size} to at least 40, if it offers
> 	VIRTIO_NET_F_RSS or VIRTIO_NET_F_HASH_REPORT.
> 
> 
> 
> > However, devices may support larger key sizes up to 52 bytes (as
> > reported by the device's rss_max_key_size config field). This change
> > allows such devices to work properly.
> > Previously, devices reporting rss_max_key_size > 40 would fail with an
> > error during initialization.
> 
> This one, yes?
> 
>                 if (vi->rss_key_size > VIRTIO_NET_RSS_MAX_KEY_SIZE) {
>                         dev_err(&vdev->dev, "rss_max_key_size=%u exceeds the limit
> %u.\n",
>                                 vi->rss_key_size, VIRTIO_NET_RSS_MAX_KEY_SIZE);
>                         err = -EINVAL;
>                         goto free;
>                 }
> 
> 
> Hmm but why do we do it like this? I suspect we just got confused with the
> spec: > vs <.
I believe the check was added to prevent out-of-bounds errors when 
accessing the hash key buffer.
> 
> 
> Can't we just do:
> min(vi->rss_key_size, VIRTIO_NET_RSS_MAX_KEY_SIZE)
Our device requires hash_key_length to exactly match rss_max_key_size 
for symmetric bidirectional flow hashing. When rss_max_key_size > 
VIRTIO_NET_RSS_MAX_KEY_SIZE, clamping isn't possible. Should we 
disable RSS instead of failing initialization?
Thanks.
> 
> 
> If yes I'd like that fix in addition to the increase.
> 
> 
> 
> If we are validating rss_key_size  against 40, I think we should clear the feature
> rather than failing init completely.
> 
> 
> 
> 
> 
> 
> > The driver already handles variable key sizes dynamically through
> > vi->rss_key_size, so increasing the maximum limit is safe and
> > maintains backward compatibility with devices that support smaller key
> > sizes.
> >
> > Signed-off-by: Srujana Challa <schalla@...vell.com>
> > ---
> >  drivers/net/virtio_net.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c index
> > db88dcaefb20..5f06cbc058d7 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -381,7 +381,7 @@ struct receive_queue {
> >  	struct xdp_buff **xsk_buffs;
> >  };
> >
> > -#define VIRTIO_NET_RSS_MAX_KEY_SIZE     40
> > +#define VIRTIO_NET_RSS_MAX_KEY_SIZE     52
> >
> >  /* Control VQ buffers: protected by the rtnl lock */  struct
> > control_buf {
> > --
> > 2.25.1