On-stack allocation of DMA S/G lists

[John Baldwin]

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