On-stack allocation of DMA S/G lists
John Baldwin
jhb at freebsd.org
Mon Aug 6 20:13:27 UTC 2012
On Monday, August 06, 2012 3:41:50 pm Stanislav Sedov wrote:
> On Mon, 6 Aug 2012 10:26:06 -0400
> John Baldwin <jhb at freebsd.org> mentioned:
>
> > On Thursday, July 12, 2012 8:26:05 am John Baldwin wrote:
> > > On Sunday, July 08, 2012 7:05:16 am Peter Jeremy wrote:
> > > > BTW(2): Whilst studying busdma_machdep.c for arm and mips, I've
> > > > noticed they appear to potentially allocate substantial kernel stack
> > > > under some conditions as several bus_dma(9) functions include:
> > > > bus_dma_segment_t dm_segments[dmat->nsegments];
> > > > What prevents this overflowing the kernel stack?
> > >
> > > That does seem dubious. x86 stores the array in the tag instead.
> >
> > I have an untested patch to change bus-dma on arm and mips to allocate a
> > dynamic S/G list in each DMA tag on first use instead of using on-stack
> > allocation (which I think is rather bogus). Can folks review and test
this
> > patch please? Thanks.
> >
> > http://www.FreeBSD.org/~jhb/patches/arm_mips_dynamic_dma_segs.patch
> >
>
> Seems to work fine for me on ARM.
> I had to initialize mflags to 0 in one place to get it compiling though.
Ah, yes. That's why x86 did that. :) I've updated it to fix that, thanks
for testing!
--
John Baldwin
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