Re: [XEN v5 02/10] xen/arm: Typecast the DT values into paddr_t

[Ayan Kumar Halder <ayankuma@xxxxxxx>]

On 19/04/2023 16:18, Julien Grall wrote:
> Hi Ayan,
Hi Julien,
> On 19/04/2023 15:58, Ayan Kumar Halder wrote:
> > On 19/04/2023 14:54, Michal Orzel wrote:
> > > On 19/04/2023 15:19, Michal Orzel wrote:
> > > > Hi Ayan,
> > > >
> > > > On 13/04/2023 19:37, Ayan Kumar Halder wrote:
> > > > > The DT functions (dt_read_number(), device_tree_get_reg(), 
> > > > > fdt_get_mem_rsv())
> > > > > currently accept or return 64-bit values.
> > > > >
> > > > > In future when we support 32-bit physical address, these DT 
> > > > > functions are
> > > > > expected to accept/return 32-bit or 64-bit values (depending on 
> > > > > the width of
> > > > > physical address). Also, we wish to detect if any truncation has 
> > > > > occurred
> > > > > (i.e. while parsing 32-bit physical addresses from 64-bit values 
> > > > > read from DT).
> > > > > device_tree_get_reg() should now be able to return paddr_t. This 
> > > > > is invoked by
> > > > > various callers to get DT address and size.
> > > > >
> > > > > For fdt_get_mem_rsv(), we have introduced a wrapper named
> > > > > fdt_get_mem_rsv_paddr() which will invoke fdt_get_mem_rsv() and 
> > > > > translate
> > > > > uint64_t to paddr_t. The reason being we cannot modify 
> > > > > fdt_get_mem_rsv() as it
> > > > > has been imported from external source.
> > > > >
> > > > > For dt_read_number(), we have also introduced a wrapper named 
> > > > > dt_read_paddr()
> > > > > dt_read_paddr() to read physical addresses. We chose not to modify 
> > > > > the original
> > > > > function as it is used in places where it needs to specifically 
> > > > > read 64-bit
> > > > > values from dt (For e.g. dt_property_read_u64()).
> > > > >
> > > > > Xen prints warning when it detects truncation in cases where it is 
> > > > > not able to
> > > > > return error.
> > > > >
> > > > > Also, replaced u32/u64 with uint32_t/uint64_t in the functions 
> > > > > touched
> > > > > by the code changes.
> > > > >
> > > > > Also, initialized variables to fix the warning 
> > > > > "-Werror=maybe-uninitialized".
> > > > I can see that now you explicitly set to 0 variables passed to 
> > > > fdt_get_mem_rsv_paddr()
> > > > which haven't been initialized before being passed to 
> > > > fdt_get_mem_rsv(). Is this what
> > > > you are reffering to? I cannot reproduce it, hence my question.
> > > I can see why did you get this error.
> > > Before your change, we always checked for an error from 
> > > fdt_get_mem_rsv() by checking if < 0.
> > > In your wrapper fdt_get_mem_rsv_paddr(), you switched (not sure why) 
> > > to checking if not zero.
> > > Becasue of this, you got an error and tried to fix it by 
> > > initializing the variables to 0.
> > I actually wanted to return the error code obtained from 
> > fdt_get_mem_rsv() to the caller.
> > In this case, it returns a single error code.
> I would rather not rely on this.
>
> > So does this look sane to you ?
> >
> > diff --git a/xen/include/xen/libfdt/libfdt-xen.h 
> > b/xen/include/xen/libfdt/libfdt-xen.h
> > index 3296a368a6..1da87d6668 100644
> > --- a/xen/include/xen/libfdt/libfdt-xen.h
> > +++ b/xen/include/xen/libfdt/libfdt-xen.h
> > @@ -22,9 +22,8 @@ static inline int fdt_get_mem_rsv_paddr(const void 
> > *fdt, int n,
> >       uint64_t dt_size;
> >       int ret = 0;
> >
> > -    ret = fdt_get_mem_rsv(fdt, n, &dt_addr, &dt_size);
> > -    if ( ret )
> > -        return ret;
> > +    if ( fdt_get_mem_rsv(fdt, n, &dt_addr, &dt_size) < 0 )
> > +        return -FDT_ERR_BADOFFSET;
> So the problem if you check for ret to be non-zero. But the caller of 
> fdt_get_mem_rsv_paddr() check for < 0.
> Given that fdt_get_mem_rsv() is not inline, the compiler doesn't know 
> that it will not return a positive value (other than 0). Hence why I 
> think you get an unitialize value.
> The snippet below should work:
>
> if ( ret < 0 )
>   return ret;
Awesome, thanks for the explanation. This works. :)

- Ayan

> Cheers,