Messages in this thread | | | Date | Fri, 01 Apr 2011 12:32:07 -0700 | From | Yinghai Lu <> | Subject | Re: 2.6.38.2 breaks suspend to disk |
| |
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
| |