Re: [PATCH v2] dma: Fix max PFN arithmetic overflow on 32 bit systems

[Alexander Dahl <post@lespocky.de>]

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Hei hei,

I would like to kindly ask about the status of this patch.

On Wed, Apr 15, 2020 at 04:35:21PM +0200, Alexander Dahl wrote:
> now after v5.7-rc1 is out, I would kindly ask, if anyone had time to
> review this one line patch? Is anything wrong with that fix?

Did it maybe not reach the right maintainers? I used
scripts/get_maintainer.pl to get my recipient list.

> (I added the current fli4l kernel package maintainer Florian to Cc to
> let him know I'm still having an eye on this.)
> 
> Greets
> Alex
> 
> On Sat, Mar 21, 2020 at 07:28:23PM +0100, Alexander Dahl wrote:
> > For ARCH=x86 (32 bit) when you set CONFIG_IOMMU_INTEL since c5a5dc4cbbf4
> > ("iommu/vt-d: Don't switch off swiotlb if bounce page is used") there's
> > a dependency on CONFIG_SWIOTLB, which was not necessarily active before.
> > 
> > The init code for swiotlb in 'pci_swiotlb_detect_4gb()' compares
> > something against MAX_DMA32_PFN to decide if it should be active.
> > However that define suffers from an arithmetic overflow since
> > 1b7e03ef7570 ("x86, NUMA: Enable emulation on 32bit too") when it was
> > first made visible to x86_32.
> > 
> > The effect is at boot time 64 MiB (default size) were allocated for
> > bounce buffers now, which is a noticeable amount of memory on small
> > systems. We noticed this effect on the fli4l Linux distribution when
> > migrating from kernel v4.19 (LTS) to v5.4 (LTS) on boards like pcengines
> > ALIX 2D3 with 256 MiB memory for example:
> > 
> >   Linux version 5.4.22 (buildroot@buildroot) (gcc version 7.3.0 (Buildroot 2018.02.8)) #1 SMP Mon Nov 26 23:40:00 CET 2018
> >   …
> >   Memory: 183484K/261756K available (4594K kernel code, 393K rwdata, 1660K rodata, 536K init, 456K bss , 78272K reserved, 0K cma-reserved, 0K highmem)
> >   …
> >   PCI-DMA: Using software bounce buffering for IO (SWIOTLB)
> >   software IO TLB: mapped [mem 0x0bb78000-0x0fb78000] (64MB)
> > 
> > The initial analysis and the suggested fix was done by user 'sourcejedi'
> > at stackoverflow and explicitly marked as GPLv2 for inclusion in the
> > Linux kernel:
> > 
> >   https://unix.stackexchange.com/a/520525/50007
> > 
> > The actual calculation however is the same as for arch/mips now as
> > suggested by Robin Murphy.
> > 
> > Fixes: https://web.nettworks.org/bugs/browse/FFL-2560
> > Fixes: https://unix.stackexchange.com/q/520065/50007
> > Reported-by: Alan Jenkins <alan.christopher.jenkins@gmail.com>
> > Suggested-by: Robin Murphy <robin.murphy@arm.com>
> > Signed-off-by: Alexander Dahl <post@lespocky.de>
> > ---
> > 
> > Notes:
> >     v1 -> v2:
> >       - use the same calculation as with arch/mips (Robin Murphy)
> > 
> >  arch/x86/include/asm/dma.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/include/asm/dma.h b/arch/x86/include/asm/dma.h
> > index 00f7cf45e699..8e95aa4b0d17 100644
> > --- a/arch/x86/include/asm/dma.h
> > +++ b/arch/x86/include/asm/dma.h
> > @@ -74,7 +74,7 @@
> >  #define MAX_DMA_PFN   ((16UL * 1024 * 1024) >> PAGE_SHIFT)
> >  
> >  /* 4GB broken PCI/AGP hardware bus master zone */
> > -#define MAX_DMA32_PFN ((4UL * 1024 * 1024 * 1024) >> PAGE_SHIFT)
> > +#define MAX_DMA32_PFN (1UL << (32 - PAGE_SHIFT))
> >  
> >  #ifdef CONFIG_X86_32
> >  /* The maximum address that we can perform a DMA transfer to on this platform */
> > -- 
> > 2.20.1

Greets
Alex

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