linux-kernel - Re: [RFC PATCH 0/8] virtio: new API for addition of buffers, scatterlist changes

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Message-ID: <5113AC8E.5030508@redhat.com>
Date:	Thu, 07 Feb 2013 14:30:54 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	linux-kernel@...r.kernel.org,
	Wanlong Gao <gaowanlong@...fujitsu.com>, asias@...hat.com,
	Rusty Russell <rusty@...tcorp.com.au>, kvm@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [RFC PATCH 0/8] virtio: new API for addition of buffers, scatterlist
 changes

Il 07/02/2013 14:31, Michael S. Tsirkin ha scritto:
> > Single means *this piece* (for example a request header) is single.  It
> > could still end up in an indirect buffer because QEMU does not support
> > mixed direct/indirect buffers.
> 
> Yes but why is the optimization worth it?
> It makes sense if all we want to do is add a single buffer
> in one go, this would give us virtqueue_add_buf_single.
> 
> But if we are building up an s/g list anyway,
> speeding up one of the entries a tiny bit
> seems very unlikely to be measureable.
> No?

There is some optimization potential even in unrolling the loop, but
yes, it looks like I misunderstood.  I'll add virtqueue_add_buf_single
instead.

Paolo

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