netdev - Re: [RFC 1/2] net: Introduce recvmmsg socket syscall

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Message-Id: <20090520.205044.185308597.davem@davemloft.net>
Date:	Wed, 20 May 2009 20:50:44 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	nhorman@...driver.com
Cc:	acme@...hat.com, netdev@...r.kernel.org, vanhoof@...hat.com,
	williams@...hat.com
Subject: Re: [RFC 1/2] net: Introduce recvmmsg socket syscall

From: Neil Horman <nhorman@...driver.com>
Date: Wed, 20 May 2009 22:26:21 -0400

> I agree, your way of doing this definately lets you layer on top of
> the existing vetted implementation, which is nice, I just thought
> that avoiding the creation of another syscall might be worth a
> little extra work in the kernel.  Instead of arrays of msghdrs, We'd
> be looking at chains like this: msghdr->(struct msghdr
> *)msg_control[i].data->msghdr->etc
> 
> Not too hard to parse, I dont think.  But I'll defer to brighter
> minds than mine.  If the creation of another syscall isn't too
> difficult a barrier to overcome (assuming this is going to occur for
> sendmsg, and various other i/o ops as well), then your way here is
> probably the way to go.

Unfortunately you can't use msg flags for this.

We accept any message flag we don't understand without signalling
any errors.

So there is no way to determine if the kernel supports the flag
or not.  Whereas with a socket option, we'll always get an error
on older kernels for unsupported options.

I think the system call is the cleanest, because it's not only a
semantic change but also a data type change.  I also think the
socket option scheme is too cumbersome.  I think it would be
common for an application to want to use both modes of sending,
especially if that application uses lots of existing library
mode to compose some messages.  And extra setsockopt() around
every call down into that library?  Yikes, good luck getting
that right all the time.  It's way too error prone.
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