Re: [Intel-wired-lan] [PATCH net] ice: fix VF queue configuration with low MTU values

[Michal Swiatkowski <michal.swiatkowski@linux.intel.com>]

On Tue, Apr 07, 2026 at 03:50:32PM -0700, Jacob Keller wrote:
> On 4/6/2026 7:56 AM, Jose Ignacio Tornos Martinez wrote:
> > The ice driver's VF queue configuration validation rejects
> > databuffer_size values below 1024 bytes, which prevents VFs from
> > using MTU values below 871 bytes.
> > 
> > The iavf driver calculates databuffer_size based on the MTU using:
> >   databuffer_size = ALIGN(MTU + LIBETH_RX_LL_LEN, 128)
> > 
> > where LIBETH_RX_LL_LEN = 26 (ETH_HLEN + 2*VLAN_HLEN + ETH_FCS_LEN).
> > 
> > For MTU values below 871:
> >   MTU 870: 870 + 26 = 896, aligned to 128 = 896 (< 1024, rejected)
> >   MTU 871: 871 + 26 = 897, aligned to 128 = 1024 (>= 1024, accepted)
> > 
> > The 1024-byte minimum seems unnecessarily restrictive, because the hardware
> > supports databuffer_size as low as 128 bytes (the alignment boundary),
> > which should allow MTU values down to the standard minimum of 68 bytes.
> > 
> > I haven't found the reason why the limit was configured in the commit
> > 9c7dd7566d18 ("ice: add validation in OP_CONFIG_VSI_QUEUES VF message"), so
> > with no more information and since it is working, change the minimum
> > databuffer_size validation from 1024 to 128 bytes to allow standard low
> > MTU values while still preventing invalid configurations.
> > 
> 
> I dug through some of our internal history and found that there was no
> justification on why 1024 was chosen.

I can't remember why I choose 1024 and didn't mention it in commit
message. I agree that it is unnecessarily restrictive.

Reviewed-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>

> 
> I agree with your assessment that the value of 128 makes the most sense
> as it is the actual hardware minimum. I wonder if we used to always use
> data buffer sizes of 1024 before the conversion to libeth. Either way, I
> think it makes sense to allow smaller buffers since the modern iAVF will
> request them.
> 
> Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
> 
> > Fixes: 9c7dd7566d18 ("ice: add validation in OP_CONFIG_VSI_QUEUES VF message")
> > cc: stable@vger.kernel.org
> > Signed-off-by: Jose Ignacio Tornos Martinez <jtornosm@redhat.com>
> > ---
> >  drivers/net/ethernet/intel/ice/virt/queues.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/net/ethernet/intel/ice/virt/queues.c b/drivers/net/ethernet/intel/ice/virt/queues.c
> > index f73d5a3e83d4..31be2f76181c 100644
> > --- a/drivers/net/ethernet/intel/ice/virt/queues.c
> > +++ b/drivers/net/ethernet/intel/ice/virt/queues.c
> > @@ -840,7 +840,7 @@ int ice_vc_cfg_qs_msg(struct ice_vf *vf, u8 *msg)
> >  
> >  			if (qpi->rxq.databuffer_size != 0 &&
> >  			    (qpi->rxq.databuffer_size > ((16 * 1024) - 128) ||
> > -			     qpi->rxq.databuffer_size < 1024))
> > +			     qpi->rxq.databuffer_size < 128))
> >  				goto error_param;
> >  
> >  			ring->rx_buf_len = qpi->rxq.databuffer_size;