linux-kernel - Re: [PATCH] tracing/events: Add bounce tracing to swiotbl-xen

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Date:	Tue, 3 Sep 2013 08:42:38 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Zoltan Kiss <zoltan.kiss@...rix.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	xen-devel@...ts.xensource.com,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH] tracing/events: Add bounce tracing to swiotbl-xen

On Mon, Sep 02, 2013 at 06:54:30PM +0100, Zoltan Kiss wrote:
> On 23/08/13 13:55, Konrad Rzeszutek Wilk wrote:
> >On Thu, Aug 22, 2013 at 10:47:28PM +0100, Zoltan Kiss wrote:
> >>Ftrace is currently not able to detect when SWIOTLB has to do double buffering
> >>under Xen. You can only see it indirectly in function_graph, when
> >>xen_swiotlb_map_page() doesn't stop after range_straddles_page_boundary(), but
> >>calls spinlock functions, memcpy() and xen_phys_to_bus() as well. This patch
> >>introduces the swiotlb-xen:bounced event, which also prints out the following
> >>informations to help you find out why bouncing happened:
> >>
> >>dev_name: 0000:08:00.0 dma_mask=ffffffffffffffff dev_addr=9149f000 size=32768
> >>swiotlb_force=0
> >>
> >>If (dev_addr + size + 1) > dma_mask, the buffer is out of the device's DMA
> >>range. If swiotlb_force == 1, you should really change the kernel parameters.
> >>Otherwise, the buffer is not contiguous in mfn space.
> >
> >Could this be in the lib/swiotlb.c instead?
> 
> You mean instead of drivers/xen/swiotlb-xen.c ? This is a Xen
> SWIOTLB specific thing, it will hit exactly at the point when double
> buffering becomes sure - under Xen.

Correct. The double buffering code is being run in lib/swiotlb.c not the
xen-swiotlb.c. Hence the question of why not move the tracing in there.

> But I can rename the tracer and call trace_bounced in
> swiotlb_map_page as well, so it can be used for normal SWIOTLB
> bounce tracing as well. Is it OK for you?

Yes. And please (if it adds a benefit) also for unmap/sync which can trigger
the bounce buffer.

Or if it makes sense just for the bounce buffer copying - then just
leave it at that. Thanks.
> 
> Regards,
> 
> Zoli
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