Re: [PATCH v2] dma-direct: improve DMA_ATTR_NO_KERNEL_MAPPING

[Walter Wu]

On Thu, 2021-11-04 at 09:57 +0100, Ard Biesheuvel wrote:
> On Thu, 4 Nov 2021 at 09:53, Christoph Hellwig <hch@xxxxxx> wrote:
> > 
> > On Thu, Nov 04, 2021 at 10:32:21AM +0800, Walter Wu wrote:
> > > diff --git a/include/linux/set_memory.h
> > > b/include/linux/set_memory.h
> > > index f36be5166c19..6c7d1683339c 100644
> > > --- a/include/linux/set_memory.h
> > > +++ b/include/linux/set_memory.h
> > > @@ -7,11 +7,16 @@
> > > 
> > >  #ifdef CONFIG_ARCH_HAS_SET_MEMORY
> > >  #include <asm/set_memory.h>
> > > +
> > > +#ifndef CONFIG_RODATA_FULL_DEFAULT_ENABLED
> > 
> > This is an arm64-specific symbol, and one that only controls a
> > default.  I don't think it is suitable to key off stubs in common
> > code.
> > 
> > > +static inline int set_memory_valid(unsigned long addr, int
> > > numpages, int enable) { return 0; }
> > 
> > Pleae avoid overly long lines.
> > 
> > > +             if (IS_ENABLED(CONFIG_RODATA_FULL_DEFAULT_ENABLED))
> > > {
> > > +                     kaddr = (unsigned
> > > long)phys_to_virt(dma_to_phys(dev, *dma_handle));
> > 
> > This can just use page_address.
> > 
> > > +                     /* page remove kernel mapping for arm64 */
> > > +                     set_memory_valid(kaddr, size >> PAGE_SHIFT,
> > > 0);
> > > +             }
> > 
> > But more importantly:  set_memory_valid only exists on arm64, this
> > will break compile everywhere else.  And this API is complete crap.
> > Passing kernel virtual addresses as unsigned long just sucks, and
> > passing an integer argument for valid/non-valid also is a horrible
> > API.
> > 
> 
> ... and as I pointed out before, you can still pass rodata=off on
> arm64, and get the old behavior, in which case bad things will happen
> if you try to use an API that expects to operate on page mappings
> with
> a 1 GB block mapping.
> 

Thanks for your suggestion.


> And you still haven't explained what the actual problem is: is this
> about CPU speculation corrupting non-cache coherent inbound DMA?

No corrupiton, only cpu read it, we hope to fix the behavior.


Thanks.
Walter