netdev - Re: [Pv-drivers] [PATCH] vmxnet3: Enable GRO support.

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Date:	Fri, 24 Jun 2011 15:47:41 -0700
From:	Jesse Gross <jesse@...ira.com>
To:	Scott Goldman <scottjg@...are.com>
Cc:	David Miller <davem@...emloft.net>,
	Shreyas Bhatewara <sbhatewara@...are.com>,
	VMware PV-Drivers <pv-drivers@...are.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [Pv-drivers] [PATCH] vmxnet3: Enable GRO support.

On Fri, Jun 24, 2011 at 3:02 PM, Scott Goldman <scottjg@...are.com> wrote:
>> -                     netif_receive_skb(skb);
>> +                     napi_gro_receive(&rq->napi, skb);
>
> So... this doesn't discriminate between if LRO is off or on.  The last time I tried using GRO on top of our hardware LRO, there was actually some minor performance penalty. Do you have any benchmarks showing that this is ok? If not, do you think it might make sense to just do gro only if(unlikely(lro is off))?

I ran some benchmarks and do see a slight performance drop with GRO
when LRO is also on, so it seems reasonable to avoid it in that
situation.  I can resubmit with that change.

As an aside, in many cases the hypervisor actually has all of the
information that is necessary to keep LRO on but does not provide it
to the guest.  For example, in the VM-to-VM case the MSS is provided
by the sender as part of the TSO descriptor and if given to the
receiver we could generate a GSO frame and avoid the need to do GRO in
the first place.  Do you know if it is possible to do this?
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