[PATCH net-next] virtio_net: Increase RSS max key size to match NETDEV_RSS_KEY_LEN

[PATCH net-next] virtio_net: Increase RSS max key size to match NETDEV_RSS_KEY_LEN

[Srujana Challa]

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. 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.

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@marvell.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

Re: [PATCH net-next] virtio_net: Increase RSS max key size to match NETDEV_RSS_KEY_LEN

[Michael S. Tsirkin]

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 <.


Can't we just do:
min(vi->rss_key_size, VIRTIO_NET_RSS_MAX_KEY_SIZE)


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@marvell.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

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

[Srujana Challa]

> 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@marvell.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

Re: [PATCH net-next] virtio_net: Increase RSS max key size to match NETDEV_RSS_KEY_LEN

[Jakub Kicinski]

On Thu, 22 Jan 2026 14:45:27 +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.

I've been meaning to bump NETDEV_RSS_KEY_LEN to 128 or some such.
Most modern drivers can't use netdev_rss_key_fill() either because
core generates a tiny key. And the devices support hashing over two
layers of IPv6 for tunnels.

Willem, WDYT? Is there a reason to keep the core key small?
Or deprecate netdev_rss_key_fill()?

> The virtio specification requires devices to support at least 40 bytes
> for the RSS key size. 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.
> 
> 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.

> -#define VIRTIO_NET_RSS_MAX_KEY_SIZE     40
> +#define VIRTIO_NET_RSS_MAX_KEY_SIZE     52

Re: [PATCH net-next] virtio_net: Increase RSS max key size to match NETDEV_RSS_KEY_LEN

[Eric Dumazet]

On Thu, Jan 22, 2026 at 4:52 PM Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Thu, 22 Jan 2026 14:45:27 +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.
>
> I've been meaning to bump NETDEV_RSS_KEY_LEN to 128 or some such.
> Most modern drivers can't use netdev_rss_key_fill() either because
> core generates a tiny key. And the devices support hashing over two
> layers of IPv6 for tunnels.

Seems fine to me. Back in 2014, RSS was limited to 52 bytes on all known NIC.

>
> Willem, WDYT? Is there a reason to keep the core key small?
> Or deprecate netdev_rss_key_fill()?

What do you mean by deprecating it ?

One of the ideas was to make sure multiple NIC would share the same key,
for some bonding setups.

commit 960fb622f85180f36d3aff82af53e2be3db2f888    net: provide a per
host RSS key generic infrastructure



>
> > The virtio specification requires devices to support at least 40 bytes
> > for the RSS key size. 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.
> >
> > 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.
>
> > -#define VIRTIO_NET_RSS_MAX_KEY_SIZE     40
> > +#define VIRTIO_NET_RSS_MAX_KEY_SIZE     52

Re: [PATCH net-next] virtio_net: Increase RSS max key size to match NETDEV_RSS_KEY_LEN

[Eric Dumazet]

On Thu, Jan 22, 2026 at 5:13 PM Eric Dumazet <edumazet@google.com> wrote:
>
> On Thu, Jan 22, 2026 at 4:52 PM Jakub Kicinski <kuba@kernel.org> wrote:
> >
> > On Thu, 22 Jan 2026 14:45:27 +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.
> >
> > I've been meaning to bump NETDEV_RSS_KEY_LEN to 128 or some such.
> > Most modern drivers can't use netdev_rss_key_fill() either because
> > core generates a tiny key. And the devices support hashing over two
> > layers of IPv6 for tunnels.
>
> Seems fine to me. Back in 2014, RSS was limited to 52 bytes on all known NIC.
>

BTW make it 256 bytes. 128 is already too small.

> >
> > Willem, WDYT? Is there a reason to keep the core key small?
> > Or deprecate netdev_rss_key_fill()?
>
> What do you mean by deprecating it ?
>
> One of the ideas was to make sure multiple NIC would share the same key,
> for some bonding setups.
>
> commit 960fb622f85180f36d3aff82af53e2be3db2f888    net: provide a per
> host RSS key generic infrastructure
>
>
>
> >
> > > The virtio specification requires devices to support at least 40 bytes
> > > for the RSS key size. 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.
> > >
> > > 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.
> >
> > > -#define VIRTIO_NET_RSS_MAX_KEY_SIZE     40
> > > +#define VIRTIO_NET_RSS_MAX_KEY_SIZE     52

Re: [PATCH net-next] virtio_net: Increase RSS max key size to match NETDEV_RSS_KEY_LEN

[Jakub Kicinski]

On Thu, 22 Jan 2026 17:13:55 +0100 Eric Dumazet wrote:
> > Willem, WDYT? Is there a reason to keep the core key small?
> > Or deprecate netdev_rss_key_fill()?  
> 
> What do you mean by deprecating it ?
> 
> One of the ideas was to make sure multiple NIC would share the same key,
> for some bonding setups.
> 
> commit 960fb622f85180f36d3aff82af53e2be3db2f888    net: provide a per
> host RSS key generic infrastructure

Just wanted to put that option on the table, it wasn't clear to me what
the exact use case for the sysfs file was. Good to know that someone
cares :) thanks for the reference!

Re: [PATCH net-next] virtio_net: Increase RSS max key size to match NETDEV_RSS_KEY_LEN

[Willem de Bruijn]

Eric Dumazet wrote:
> On Thu, Jan 22, 2026 at 4:52 PM Jakub Kicinski <kuba@kernel.org> wrote:
> >
> > On Thu, 22 Jan 2026 14:45:27 +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.
> >
> > I've been meaning to bump NETDEV_RSS_KEY_LEN to 128 or some such.
> > Most modern drivers can't use netdev_rss_key_fill() either because
> > core generates a tiny key. And the devices support hashing over two
> > layers of IPv6 for tunnels.
> 
> Seems fine to me. Back in 2014, RSS was limited to 52 bytes on all known NIC.

What is the purpose of extending?

The current length is sufficient for standard Toeplitz 4-tuple hashing.

Re: [PATCH net-next] virtio_net: Increase RSS max key size to match NETDEV_RSS_KEY_LEN

[Willem de Bruijn]

Willem de Bruijn wrote:
> Eric Dumazet wrote:
> > On Thu, Jan 22, 2026 at 4:52 PM Jakub Kicinski <kuba@kernel.org> wrote:
> > >
> > > On Thu, 22 Jan 2026 14:45:27 +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.
> > >
> > > I've been meaning to bump NETDEV_RSS_KEY_LEN to 128 or some such.
> > > Most modern drivers can't use netdev_rss_key_fill() either because
> > > core generates a tiny key. And the devices support hashing over two
> > > layers of IPv6 for tunnels.
> > 
> > Seems fine to me. Back in 2014, RSS was limited to 52 bytes on all known NIC.
> 
> What is the purpose of extending?
> 
> The current length is sufficient for standard Toeplitz 4-tuple hashing.

Never mind, I should read better:

> And the devices support hashing over two layers of IPv6 for tunnels.

I'm hard pressed to see over 128 B of entropy. But if extending, might
as well go to 256 B right away.

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

[Srujana Challa]

> On Thu, 22 Jan 2026 14:45:27 +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.
> 
> I've been meaning to bump NETDEV_RSS_KEY_LEN to 128 or some such.
> Most modern drivers can't use netdev_rss_key_fill() either because core
> generates a tiny key. And the devices support hashing over two layers of IPv6
> for tunnels.
Instead of hardcoding 52, we can use:

#define VIRTIO_NET_RSS_MAX_KEY_SIZE  NETDEV_RSS_KEY_LEN

This automatically stays in sync with NETDEV_RSS_KEY_LEN and will adapt 
when it's increased to 128+. Please confirm.
Thanks.
> 
> Willem, WDYT? Is there a reason to keep the core key small?
> Or deprecate netdev_rss_key_fill()?
> 
> > The virtio specification requires devices to support at least 40 bytes
> > for the RSS key size. 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.
> >
> > 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.
> 
> > -#define VIRTIO_NET_RSS_MAX_KEY_SIZE     40
> > +#define VIRTIO_NET_RSS_MAX_KEY_SIZE     52