linux-kernel - Re: [PATCH] swiotlb: iommu/dma: Move trace_swiotlb_bounced() into swiotlb_tbl_map

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Message-ID: <20250611115640.5e79bde8@batman.local.home>
Date: Wed, 11 Jun 2025 11:56:40 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Robin Murphy <robin.murphy@....com>
Cc: LKML <linux-kernel@...r.kernel.org>, Linux trace kernel
 <linux-trace-kernel@...r.kernel.org>, iommu@...ts.linux.dev, Christoph
 Hellwig <hch@...radead.org>, Joerg Roedel <joro@...tes.org>, Will Deacon
 <will@...nel.org>
Subject: Re: [PATCH] swiotlb: iommu/dma: Move trace_swiotlb_bounced() into
 swiotlb_tbl_map_single()

On Wed, 11 Jun 2025 15:28:52 +0100
Robin Murphy <robin.murphy@....com> wrote:

> On 2025-06-11 3:01 pm, Steven Rostedt wrote:
> > From: Steven Rostedt <rostedt@...dmis.org>
> > 
> > I'm working on code that will warn when a tracepoint is defined but not
> > used. As the TRACE_EVENT() logic still creates all the code regardless if
> > something calls the trace_<event>() function. It wastes around 5K per trace
> > event (less for tracepoints).
> > 
> > But it seems that the code in drivers/iommu/dma-iommu.c does the opposite.
> > It calls the trace_swiotlb_bounced() tracepoint without it being defined.
> > The tracepoint is defined in kernel/dma/swiotlb.c when CONFIG_SWIOTLB is
> > defined, but this code exists when that config is not defined.
> > 
> > This now fails with my work because I have all the callers reference the
> > tracepoint that they will call.
> > 
> > Thanks to the kernel test robot, it found this:
> > 
> >    https://lore.kernel.org/all/202506091015.7zd87kI7-lkp@intel.com/
> > 
> > Instead of calling trace_swiotlb_bounced() from drivers/iommu/dma-iommu.c
> > where it is useless when CONFIG_SWIOTLB is not defined, move the
> > tracepoint into swiotlb_tbl_map_single(). This also makes it consistent
> > with which memory is being traced (physical as supposed to dma address).  
> 
> Consistent with what? Certainly not the other callers which invoke the 
> tracepoint with phys_to_dma(orig_addr), but will now do so twice with 
> potentially different values. Arguably iommu-dma is already consistent 
> as things stand, since it does not support static address offsets 
> underneath IOMMU translation, and therefore orig_addr will always be 
> equal to phys_to_dma(orig_addr) anyway. The point of interest really is 
> in those other cases, where if there *were* a static DMA offset then 
> phys_to_dma(orig_addr) would actually be a pretty meaningless value 
> which is not used anywhere else and therefore not overly helpful to 
> trace - neither the recognisable PA of the underlying memory itself, nor 
> the actual DMA address which will be used subsequently, since at this 
> point that's yet to be allocated.

I only added that line because of what Christoph said. I'll let you
folks hammer that point.

> 
> > Fixes: ed18a46262be4 ("iommu/dma: Factor out a iommu_dma_map_swiotlb helper")  
> 
> It's a fair bit older than that, try a63c357b9fd5 ("iommu/dma: Trace 
> bounce buffer usage when mapping buffers") - looks like it's always just 
> been hopeful of being DCE'd.

Ah, yeah, that's the real culprit. Thanks!

Well, it may be some time before I push my code that will cause this to
fail to build, as I want to clean up the unused events before I add it
(otherwise there's going to be lots of warnings whenever it's enabled).

-- Steve