netdev - Re: [PATCH 0/4] Implement persistent grant in xen-netfront/netback

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Date:	Fri, 16 Nov 2012 09:57:01 +0000
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Annie Li <annie.li@...cle.com>
CC:	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>
Subject: Re: [PATCH 0/4] Implement persistent grant in xen-netfront/netback

On Thu, 2012-11-15 at 07:03 +0000, Annie Li wrote:
> This patch implements persistent grants for xen-netfront/netback.

Hang on a sec. It has just occurred to me that netfront/netback in the
current mainline kernels don't currently use grant maps at all, they use
grant copy on both the tx and rx paths.

The supposed benefit of persistent grants is to avoid the TLB shootdowns
on grant unmap, but in the current code there should be exactly zero of
those.

If I understand correctly this patch goes from using grant copy
operations to persistently mapping frames and then using memcpy on those
buffers to copy in/out to local buffers. I'm finding it hard to think of
a reason why this should perform any better, do you have a theory which
explains it? (my best theory is that it has a beneficial impact on where
the cache locality of the data, but netperf doesn't typically actually
access the data so I'm not sure why that would matter)

Also AIUI this is also doing persistent grants for both Tx and Rx
directions?

For guest Rx does this mean it now copies twice, in dom0 from the DMA
buffer to the guest provided buffer and then again in the guest from the
granted buffer to a normal one?

For guest Tx how do you handle the lifecycle of the grant mapped pages
which are being sent up into the dom0 network stack? Or are you also now
copying twice in this case? (i.e. guest copies into a granted buffer and
dom0 copies out into a local buffer?)

Did you do measurement of the Tx and Rx cases independently? Do you know
that they both benefit from this change (rather than for example an
improvement in one direction masking a regression in the other). Were
the numbers you previously posted in one particular direction or did you
measure both?

Ian.

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