Re: breaks suspend to disk

From: Yinghai Lu
Date: Fri Apr 01 2011 - 15:32:49 EST

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;
unsigned long mmu_cr4_features = X86_CR4_PAE;

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

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.


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