Re: [PATCH] Make PFN_PHYS return a properly-formed physical address

Jeremy Fitzhardinge
2008-08-07T23:45:36Z

Andrew Morton wrote:
> Yes, but resource_size_t is for IO addressing, not for memory addressing.
>
> Lots of X86_32 machines can happily support 32-bit physical addresses
> for IO while needing >32 bit addresses for physical memory.
>

Really? The resource tree treats normal memory as just another
resource. Is it expected that you could have usable memory not
represented by /proc/iomem?

Hm, looks like memory hotplug assumes that resource_size_t is always
64-bits, but the e820->resource conversion simply skips over-large
addresses.

>>>> #define PFN_ALIGN(x) (((unsigned long)(x) + (PAGE_SIZE - 1)) & PAGE_MASK)
>>>> #define PFN_UP(x) (((x) + PAGE_SIZE-1) >> PAGE_SHIFT)
>>>> #define PFN_DOWN(x) ((x) >> PAGE_SHIFT)
>>>> -#define PFN_PHYS(x) ((x) << PAGE_SHIFT)
>>>> +#define PFN_PHYS(x) ((resource_size_t)(x) << PAGE_SHIFT)
>>>>
>>>>
>>> Busted on PAE with CONFIG_RESOURCES_64BIT=n, surely?
>>>
>>>
>> Not an option:
>>
>> config X86_PAE
>> def_bool n
>> prompt "PAE (Physical Address Extension) Support"
>> depends on X86_32 && !HIGHMEM4G
>> select RESOURCES_64BIT
>>
>>
>
> err, OK, that was a bit arbitrary of us.
>
> Oh well, scrub the above assertion.
>
> Then again, do all architectures disallow 32-bit resource_size_t on
> 64-bit? And there's ppc32's CONFIG_HIGHMEM option to think about.
>

x86 and ppc were the only archs to touch it; they otherwise use the
default of "default 64BIT".

I didn't look at the ppc use case. I wasn't terribly concerned about
current users of PFN_PHYS, because it presumably works OK for them.

>> "Properly" would be to define a phys_addr_t which can always represent a
>> physical address. We have one in x86-land, but I hesitate to add it for
>> everyone else.
>>
>
> hm. It is a distinct and singular concept - it makes sense to have a
> specific type to represet "a physical address for memory".
>

Yes. We had to be particularly careful with it on x86 because of all
the problems it's caused, but it is a generally useful thing to be able
to talk about.

Shall we go with just using plain u64 (or unsigned long long if we want
a really consistent type) in the meantime, and then waffle about
introducing a new type everywhere?

Or we could redefine resource_size_t to be big enough to refer to any
resource, including all memory. It's close to being that anyway.

> nope ;) We don't know what type u64 has - some architectures use
> `unsigned long' (we might fix this soon).
>
> For now, a full cast to `unsigned long long' is needed.
>

Yep.

J