Re: [Xen-devel] [PATCH] Revert "swiotlb: remove SWIOTLB_MAP_ERROR"

[Julien Grall]

Hi Arnd,

On 3/5/19 8:16 AM, Arnd Bergmann wrote:
> On Tue, Mar 5, 2019 at 12:56 AM Robin Murphy <robin.murphy@xxxxxxx> wrote:
> > On 2019-03-04 7:59 pm, Arnd Bergmann wrote:
> > > This reverts commit b907e20508d0 ("swiotlb: remove SWIOTLB_MAP_ERROR"), which
> > > introduced an overflow warning in configurations that have a larger
> > > dma_addr_t than phys_addr_t:
> > >
> > > In file included from include/linux/dma-direct.h:5,
> > >                    from kernel/dma/swiotlb.c:23:
> > > kernel/dma/swiotlb.c: In function 'swiotlb_tbl_map_single':
> > > include/linux/dma-mapping.h:136:28: error: conversion from 'long long unsigned int' to 'phys_addr_t' {aka 'unsigned int'} changes value from '18446744073709551615' to '4294967295' [-Werror=overflow]
> > >    #define DMA_MAPPING_ERROR  (~(dma_addr_t)0)
> > >                               ^
> > > kernel/dma/swiotlb.c:544:9: note: in expansion of macro 'DMA_MAPPING_ERROR'
> > >     return DMA_MAPPING_ERROR;
> > >
> > > The configuration that caused this is on 32-bit ARM, where the DMA address
> > > space depends on the enabled hardware platforms, while the physical
> > > address space depends on the type of MMU chosen (classic vs LPAE).
> >
> > Are these real platforms, or random configs? Realistically I don't see a
> > great deal of need to support DMA_ADDR_T_64BIT for non-LPAE.
> > Particularly in this case since AFAIK the only selector of SWIOTLB on
> > Arm is Xen, and that by definition is never going to be useful on
> > non-LPAE hardware.
> ...
> On Mon, Mar 4, 2019 at 11:00 PM Konrad Rzeszutek Wilk
> <konrad.wilk@xxxxxxxxxx> wrote:
> > What about making the phys_addr_t and dma_addr_t have the same
> > width with some magic #ifdef hackery?
>
> As far as I can tell, only randconfig builds see this problem, in
> real systems phys_addr_t is normally the same as dma_addr_t,
> and you could reasonably have a machine with a larger phys_addr_t
> than dma_addr_t but wouldn't need to bother.

On Xen, dma_addr_t will always be 64-bit while the phys_addr_t will 
depend on the MMU type. So we may have phys_addr_t smaller than 
dma_addr_t from the kernel point of view.

Cheers,

--
Julien Grall