Re: [PATCH for-4.14 1/3] xen/arm: Allow a platform to override the DMA width

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 18 May 2020, Volodymyr Babchuk wrote:
> Hi Julien,
> 
> On Mon, 2020-05-18 at 12:30 +0100, Julien Grall wrote:
> > From: Julien Grall <jgrall@xxxxxxxxxx>
> > 
> > At the moment, Xen is assuming that all the devices are at least 32-bit
> > DMA capable. However, some SoC have devices that may be able to access
> > a much restricted range. For instance, the RPI has devices that can
> > only access the first 1GB of RAM.
> > 
> > The structure platform_desc is now extended to allow a platform to
> > override the DMA width. The new is used to implement
> > arch_get_dma_bit_size().
> > 
> > The prototype is now moved in asm-arm/mm.h as the function is not NUMA
> > specific. The implementation is done in platform.c so we don't have to
> > include platform.h everywhere. This should be fine as the function is
> > not expected to be called in hotpath.
> > 
> > Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
> Reviewed-by: Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


> > ---
> > 
> > Cc: Jan Beulich <jbeulich@xxxxxxxx>
> > Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > Cc: George Dunlap <george.dunlap@xxxxxxxxxx>
> > 
> > I noticed that arch_get_dma_bit_size() is only called when there is more
> > than one NUMA node. I am a bit unsure what is the reason behind it.
> > 
> > The goal for Arm is to use arch_get_dma_bit_size() when deciding how low
> > the first Dom0 bank should be allocated.
> > ---
> >  xen/arch/arm/platform.c        | 5 +++++
> >  xen/include/asm-arm/mm.h       | 2 ++
> >  xen/include/asm-arm/numa.h     | 5 -----
> >  xen/include/asm-arm/platform.h | 2 ++
> >  4 files changed, 9 insertions(+), 5 deletions(-)
> > 
> > diff --git a/xen/arch/arm/platform.c b/xen/arch/arm/platform.c
> > index 8eb0b6e57a5a..4db5bbb4c51d 100644
> > --- a/xen/arch/arm/platform.c
> > +++ b/xen/arch/arm/platform.c
> > @@ -155,6 +155,11 @@ bool platform_device_is_blacklisted(const struct 
> > dt_device_node *node)
> >      return (dt_match_node(blacklist, node) != NULL);
> >  }
> >  
> > +unsigned int arch_get_dma_bitsize(void)
> > +{
> > +    return ( platform && platform->dma_bitsize ) ? platform->dma_bitsize : 
> > 32;
> > +}
> > +
> >  /*
> >   * Local variables:
> >   * mode: C
> > diff --git a/xen/include/asm-arm/mm.h b/xen/include/asm-arm/mm.h
> > index 7df91280bc77..f8ba49b1188f 100644
> > --- a/xen/include/asm-arm/mm.h
> > +++ b/xen/include/asm-arm/mm.h
> > @@ -366,6 +366,8 @@ int arch_acquire_resource(struct domain *d, unsigned 
> > int type, unsigned int id,
> >      return -EOPNOTSUPP;
> >  }
> >  
> > +unsigned int arch_get_dma_bitsize(void);
> > +
> >  #endif /*  __ARCH_ARM_MM__ */
> >  /*
> >   * Local variables:
> > diff --git a/xen/include/asm-arm/numa.h b/xen/include/asm-arm/numa.h
> > index 490d1f31aa14..31a6de4e2346 100644
> > --- a/xen/include/asm-arm/numa.h
> > +++ b/xen/include/asm-arm/numa.h
> > @@ -25,11 +25,6 @@ extern mfn_t first_valid_mfn;
> >  #define node_start_pfn(nid) (mfn_x(first_valid_mfn))
> >  #define __node_distance(a, b) (20)
> >  
> > -static inline unsigned int arch_get_dma_bitsize(void)
> > -{
> > -    return 32;
> > -}
> > -
> >  #endif /* __ARCH_ARM_NUMA_H */
> >  /*
> >   * Local variables:
> > diff --git a/xen/include/asm-arm/platform.h b/xen/include/asm-arm/platform.h
> > index ed4d30a1be7c..997eb2521631 100644
> > --- a/xen/include/asm-arm/platform.h
> > +++ b/xen/include/asm-arm/platform.h
> > @@ -38,6 +38,8 @@ struct platform_desc {
> >       * List of devices which must not pass-through to a guest
> >       */
> >      const struct dt_device_match *blacklist_dev;
> > +    /* Override the DMA width (32-bit by default). */
> > +    unsigned int dma_bitsize;
> >  };
> >  
> >  /*
>