using identity_mapping_add & switching MMU state - how ?

[Dave Martin]

On Thu, Jun 02, 2011 at 04:44:02PM +0100, Frank Hofmann wrote:
> Hi,
> 
> 
> I'm trying to find a way to do an MMU off / on transition.
> 
> What I want to do is to call cpu_do_resume() from the hibernation
> restore codepath.
> 
> I've succeeded to make this work by adding a test whether the MMU is
> already on when cpu_resume_mmu() is called.
> 
> I'm not sure that sort of thing is proper; hence I've been trying to
> find a way to disable the MMU before calling cpu_do_resume().
> 
> Can't seem to get this to work though; even though I'm creating a
> separate MMU context that's given 1:1 mappings for all of kernel
> code/data, execution still hangs as soon as I enable the code
> section below that switches the MMU off.

I think Per Fransson has been working on this area for kexec.

Given the intrinsic scariness of code for turning MMUs off, I think
it would be best if there's a single common implementation which is
usable for hibernation and kexec, and any other code which needs
to do this (CPU hotplug?)

Maybe Per has some ideas on how to do that...

Cheers
---Dave

> 
> 
> What I have at the moment is code that looks like this:
> 
> ==============================================================================
> [ ... ]
> unsigned long notrace __swsusp_arch_restore_image(void)
> {
> 	extern struct pbe *restore_pblist;
> 	struct pbe *pbe;
> 
> 	/* __swsusp_pg_dir has been created using pgd_alloc(&init_mm); */
> 
> 	cpu_switch_mm(__swsusp_pg_dir, &init_mm);
> 
> 	for (pbe = restore_pblist; pbe; pbe = pbe->next)
> 		copy_page(pbe->orig_address, pbe->address);
> 
> 	flush_tlb_all();
> 	flush_cache_all();
> 
> 	identity_mapping_add(__swsusp_pg_dir, __pa(_stext), __pa(_etext));
> 	identity_mapping_add(__swsusp_pg_dir, __pa(_sdata), __pa(_edata));
> 
> 	cpu_switch_mm(__swsusp_pg_dir, &init_mm);
> 
> 	flush_tlb_all();
> 	flush_cache_all();
> 
> 	cpu_proc_fin();		/* turns caches off */
> 
> 	/* caller requires v:p offset to calculate physical addresses */
> 	return (unsigned long)(PHYS_OFFSET - PAGE_OFFSET);
> }
> 
> [ ... ]
> ENTRY(swsusp_arch_resume)
> 	mov	r2, #PSR_I_BIT | PSR_F_BIT | SVC_MODE
> 	msr	cpsr_c, r2
> 	/*
> 	 * Switch stack to a nosavedata region to make sure image restore
> 	 * doesn't clobber it underneath itself.
> 	 */
> 	ldr	sp, =(__swsusp_resume_stk + PAGE_SIZE / 2)
> 	bl	__swsusp_arch_restore_image
> 
> 	/*
> 	 * Restore the CPU registers.
> 	 */
> 	mov	r1, r0
> 	ldr	r0, =(__swsusp_arch_ctx + (NREGS * 4))
> /*
>  * This is what I'm trying to switch off; yet, doing so makes things hang
>  */
> #if 0
> 	ldr	r2, =cpu_do_resume
> 	sub	r2, r1			@ __pa()
> 	ldr	r3, =.Lmmu_is_off
> 	sub	r3, r1			@ __pa()
> 	sub	r0, r1			@ __pa()
> 	ldr	lr, =.Lpost_mmu
> 	mrc	p15, 0, r1, c1, c0, 0
> 	bic	r1, #CR_M
> 	mcr	p15, 0, r1, c1, c0, 0	@ MMU OFF
> 
> 	mrc	p15, 0, r1, c2, c0, 0	@ queue a dependency on CP15
>         sub     pc, r3, r1, lsr #32	@ to local label, phys addr
> .ltorg
> .align 5
> .Lmmu_is_off:
> 	mov	pc, r2			@ jump to phys cpu_v6_do_resume
> .Lpost_mmu:
> #else
> 	bl	cpu_do_resume
> #endif
> 	ldr	r0, =__swsusp_arch_ctx
> 	ldmia	r0!, {r1-r11,lr}	@ nonvolatile regs
> 	ldr	sp, [r0]		@ stack
> 	msr	cpsr, r1
> 	msr	spsr, r2
> 
> 	mov	r0, #0
> 	stmfd	sp!, {r0, lr}
> 	bl	cpu_init		@ reinitialize other modes
> 	ldmfd	sp!, {r0, pc}
> ENDPROC(swsusp_arch_resume)
> ==============================================================================
> 
> I.e. it performs the steps:
> 
> 	- flush all caches, tlbs
> 	- setup identity mappings for all kernel code & data
> 	- switch to a self-contained pagedir
> 	- flush again
> 	- finish cpu (disable caches)
> 	- switch MMU off, re-read config reg to force necessary wait,
> 	  and jump to physical address of "self".
> 
> 
> As said, things work just fine if simply doing the "bl
> cpu_do_resume" and adding a "MMU already on" check to
> cpu_resume_mmu().
> 
> 
> I must be missing something there; I've been reading the ARM kexec
> postings,
> 
> http://lists.infradead.org/pipermail/linux-arm-kernel/2010-July/020183.html
> 
> for the basic idea, and used the style from smp.c (alloc a temporary
> pagedir, create identity mappings there). Still, there's something
> not quite right ...
> 
> 
> Any ideas what I'm missing ?
> Thanks,
> 
> FrankH.
> 
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