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

[Ard Biesheuvel <ardb@kernel.org>]

On Thu, 4 Nov 2021 at 13:31, Walter Wu <walter-zh.wu@mediatek.com> wrote:
>
> On Thu, 2021-11-04 at 09:57 +0100, Ard Biesheuvel wrote:
> > On Thu, 4 Nov 2021 at 09:53, Christoph Hellwig <hch@lst.de> 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.
>

Fix which behavior? Please explain

1) the current behavior
2) why the current behavior is problematic for you
3) how this patch changes the current behavior
4) why the new behavior fixes your problem.

There is no penalty for using too many words.