netdev - Re: [PATCH 2/3] partial checksum and GSO support for tun/tap.

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Date:	Tue, 04 Mar 2008 12:08:58 -0800
From:	Max Krasnyanskiy <maxk@...lcomm.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	netdev@...r.kernel.org, Herbert Xu <herbert@...dor.apana.org.au>,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 2/3] partial checksum and GSO support for tun/tap.

Rusty Russell wrote:
> On Tuesday 04 March 2008 16:08:00 Max Krasnyansky wrote:
>>> The problem with this approach is that for what I'm doing, the packets
>>> aren't nicely arranged somewhere; they're in random process memory.
>> That's fine. RX/TX descriptors would not contain the data itself. They'd
>> contain pointers to actual packets (ie just like the NIC takes physical
>> memory address and DMAs data in/out).
>> The allows for sending/receiving packets without syscalls and fits nicely
>> with the async schemes like GSO.
> 
> Yes, yes it does.  That would be a very nice extension (it's orthogonal to 
> this patch though, so should we get Dave to take these for 2.6.25?).
It's orthogonal in general I agree. The only concern is whether we should keep 
extending existing driver API (ie adding more flags like TUN_NO_PI, etc) or go 
straight to the new/better ring based API. I do not have a strong preference 
one way or the other. So I guess I'm saying I'd be ok with Dave taking them 
for 2.6.25, but that API may be obsolete when the ring based thing comes out.

> And as it happens, virtio already has such a structure: virtio_ring.  See 
> linux/virtio_ring.h.
I'll take a look.

>>> The structure is for virtio, I'm just borrowing it for tap because it's
>>> already there.  We could rename it and move it out to its own header, but
>>> if so we should do that before 2.6.25 is released.
>> If we do the whole enchilada with the RX/TX rings then we probably do not
>> even need it. I'm thinking that RX/TX descriptor would include everything
>> you need for the GSO and stuff.
>> I meant do not need it for the TUN/TAP driver that is. Is it used anywhere
>> else ?
> 
> Just for the linux virtio drivers.  Reusing it for tun/tap was an 
> afterthought.  It just meant I could pass the same structure straight thru, 
> though, which is nice.
> 
> The userspace->kernel problem is very similar to the guest->host problem, so 
> it doesn't surprise me if we end up with very similar (identical?) 
> interfaces.
> 
> Take a look at virtio_ring.h, virtio_ring.c and Documentation/lguest/lguest.c 
> to see how we use it...
Thanx for the pointers.

Max

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