netdev - Re: [PATCH v2 17/18] vhost: don't bother copying iovecs in handle_rx(), kill memcpy

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Message-ID: <20150203221311.GD29656@ZenIV.linux.org.uk>
Date:	Tue, 3 Feb 2015 22:13:11 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	kvm@...r.kernel.org
Subject: Re: [PATCH v2 17/18] vhost: don't bother copying iovecs in
 handle_rx(), kill memcpy_toiovecend()

On Tue, Feb 03, 2015 at 04:21:54PM +0100, Michael S. Tsirkin wrote:
> > Hmm having second thoughts here.
> > Will this modify the iov in vq->iov?
> > If yes, how will recvmsg fill it?
> 
> OK that was just me misunderstanding what the
> function does. As it doesn't modify the iovec itself,
> I think there's no issue, my ack stands.

That, BTW, was the point of switching ->sendmsg() and ->recvmsg() to
iov_iter primitives - not only do memcpy_toiovec() et.al. change the
iovec, the way it's done was protocol-dependent, up to and including
the things like "have sent 60 caller-supplied bytes, iovec modified
with only 6 bytes consumed".  What's worse, on TCP sendmsg we usually
left iovec unchanged, but sometimes it got drained.  Granted, that
happened only after setsockopt() playing caller with TCP_REPAIR (i.e.
checkpoint/restart stuff), but...
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