Re: 2.6.38.2 breaks suspend to disk

[Yinghai Lu <yinghai@kernel.org>]

On 04/01/2011 11:55 AM, H. Peter Anvin wrote:
> On 04/01/2011 11:14 AM, Stefano Stabellini wrote:
>>> Why the heck should we save and restore CR4 only for x86-64?
>>
>> AFAICT it has always been the case since the beginning of time (Initial
>> git repository build).
>
> Doesn't change the fact that at least conceptually it's wrong; consider
> bits like NX.  Rafael, do you have any thoughts on this?
>
more info:

in arch/x86/kernel/setup.c  we have
#if !defined(CONFIG_X86_PAE) || defined(CONFIG_X86_64)
unsigned long mmu_cr4_features;
#else
unsigned long mmu_cr4_features = X86_CR4_PAE;
#endif


in arch/x86/kernel/head_32.S

/*
  *      New page tables may be in 4Mbyte page mode and may
  *      be using the global pages.
  *
  *      NOTE! If we are on a 486 we may have no cr4 at all!
  *      So we do not try to touch it unless we really have
  *      some bits in it to set.  This won't work if the BSP
  *      implements cr4 but this AP does not -- very unlikely
  *      but be warned!  The same applies to the pse feature
  *      if not equally supported. --macro
  *
  *      NOTE! We have to correct for the fact that we're
  *      not yet offset PAGE_OFFSET..
  */
#define cr4_bits pa(mmu_cr4_features)
         movl cr4_bits,%edx
         andl %edx,%edx
         jz 6f
         movl %cr4,%eax          # Turn on paging options (PSE,PAE,..)
         orl %edx,%eax
         movl %eax,%cr4
...
6:

So for 32 bit, PAE support is not compiled in for old 486 cpus.
mmu_cr4_feautres will be used to make sure head_32.S will not access cr4.

that could be the reason why 32 bit does not do read back at beginning.

Thanks

Yinghai