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

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Message-ID: <489B72C3.30603@goop.org>
Date:	Thu, 07 Aug 2008 15:10:11 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	ehabkost@...hat.com, x86@...nel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Make PFN_PHYS return a properly-formed physical address

Andrew Morton wrote:
> On Thu, 07 Aug 2008 14:38:08 -0700
> Jeremy Fitzhardinge <jeremy@...p.org> wrote:
>
>   
>> PFN_PHYS, as its name suggests, turns a pfn into a physical address.
>> However, it is a macro which just operates on its argument without
>> modifying its type.  pfns are typed unsigned long, but an unsigned
>> long may not be long enough to hold a physical address (32-bit systems
>> with more than 32 bits of physcial address).  This means that the
>> resulting address could be truncated if it doesn't fit within an
>> unsigned long.  This isn't generally a problem because most users end
>> up using it for "low" memory, but there's no reason why PFN_PHYS
>> couldn't be used for any possible pfn.
>>     
>
> Please copy a mailing list on patches.  So you can get your titties
> toasted off ;)
>   

Oops.  Forgot.

>> Fortunately, resource_size_t is the right size, and has approximately
>> the right meaning.  It's 64-bits on platforms where that's
>> appropriate, but 32-bits where the extra bits are not needed.
>>     
>
> aww maaan.  Hack or what?
>   

I don't know.  Is it?  It's what linux/ioport.h:struct resource uses to 
hold "start" and "end", which presumably means its intended to hold 
arbitrary physical 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


And if you don't enable RESOURCES_64BIT, then I guess it's reasonable 
for PFN_PHYS to discount the possibility of high pages?

> Can we please do this properly, whatever that is?  Even a dumb
> always-return-u64 would be better?
>   

I had that originally, but someone (hpa?) suggested resource_size_t.  
The sad thing is that most users don't really care;  they're either 
64-bit anyway, or immediately truncate the result to 32-bit.

"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.

>> 			printk("initrd extends beyond end of memory "
>> -				"(0x%08lx > 0x%08lx)\ndisabling initrd\n",
>> +				"(0x%08lx > 0x%08llx)\ndisabling initrd\n",
>> 				INITRD_START + INITRD_SIZE,
>> 				PFN_PHYS(max_low_pfn));
>>     
>
> that'll generate a compile warning if m32r can set CONFIG_RESOURCES_64BIT=n.
>   

(u64) cast, I guess.

    J
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