Re: [PATCH v2] ARM64: mmu: implement mmu_disable completely in assembly

[Sascha Hauer <s.hauer@pengutronix.de>]

On Fri, Dec 12, 2025 at 12:10:19PM +0100, Ahmad Fatoum wrote:
> Splitting mmu_disable into two noinline function on a RK3568 leads to a
> barebox crash, because the code has the implicit assumption that the
> compiler won't generate memory accesses including spilling to stack.
> 
> We can't guarantee this in C code, so implement the procedure in
> assembly.
> 
> While at it, drop mmu_early_disable(), which is unused, superfluous and
> suffers from the same issue as old mmu_disable().
> 
> Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
> ---
> v1 -> v2:
>   - fix copy-paste problem in some ELs (Ulrich)
> ---
>  arch/arm/cpu/cache-armv8.S   | 54 ++++++++++++++++++++++++++++++++++++
>  arch/arm/cpu/mmu_64.c        | 27 +-----------------
>  arch/arm/include/asm/cache.h |  1 +
>  arch/arm/include/asm/mmu.h   |  1 -
>  4 files changed, 56 insertions(+), 27 deletions(-)
> 
> diff --git a/arch/arm/cpu/cache-armv8.S b/arch/arm/cpu/cache-armv8.S
> index 9d9e0fb585a1..130338d6034f 100644
> --- a/arch/arm/cpu/cache-armv8.S
> +++ b/arch/arm/cpu/cache-armv8.S
> @@ -9,6 +9,7 @@
>  
>  #include <linux/linkage.h>
>  #include <init.h>
> +#include <asm/system.h>
>  
>  /*
>   * void v8_flush_dcache_level(level)
> @@ -120,6 +121,59 @@ ENTRY(v8_invalidate_dcache_all)
>  	ret
>  ENDPROC(v8_invalidate_dcache_all)
>  
> +/*
> + * void v8_mmu_disable(void)
> + *
> + * Implements the equivalent of following C code:
> + *
> + *	set_cr(get_cr() & ~(CR_M | CR_C))
> + *	v8_flush_dcache_all();
> + *	tlb_invalidate();
> + *
> + *	dsb();
> + *	isb();
> + *
> + * As D$ needs to be cleaned after it was disabled, this procedure
> + * is not permitted to trigger memory access, including spilling
> + * locals to stack. It's thus necessarily needs to be implemented
> + * in assembly.
> + */
> +.section .text.v8_mmu_disable
> +ENTRY(v8_mmu_disable)
> +	mov	x14, lr
> +	mov	x1, #~(CR_C | CR_M)
> +	mrs	x0, currentel
> +	lsr	w0, w0, #2		/* w0 <- current exception level */
> +	cmp	w0, #0x1
> +	b.eq	1f
> +	cmp 	w0, #0x2
> +	b.eq	2f

The switch_el macro could be used here.

Sascha

> +3:	/* EL3 */
> +	mrs	x0, sctlr_el3
> +	and	x0, x0, x1
> +	msr	sctlr_el3, x0
> +	bl	v8_flush_dcache_all
> +	tlbi	alle3
> +	b	0f
> +2:	/* EL2 */
> +	mrs	x0, sctlr_el2
> +	and	x0, x0, x1
> +	msr	sctlr_el2, x0
> +	bl	v8_flush_dcache_all
> +	tlbi	alle2
> +	b	0f
> +1:	/* EL1 */
> +	mrs	x0, sctlr_el1
> +	and	x0, x0, x1
> +	msr	sctlr_el1, x0
> +	bl	v8_flush_dcache_all
> +	tlbi	vmalle1
> +0:	/* common prologue */
> +	dsb 	sy
> +	isb
> +	ret	x14
> +ENDPROC(v8_mmu_disable)
> +
>  /*
>   * void v8_flush_dcache_range(start, end)
>   *
> diff --git a/arch/arm/cpu/mmu_64.c b/arch/arm/cpu/mmu_64.c
> index f22fcb5f8ea4..56c6a21f2b2a 100644
> --- a/arch/arm/cpu/mmu_64.c
> +++ b/arch/arm/cpu/mmu_64.c
> @@ -344,17 +344,7 @@ void __mmu_init(bool mmu_on)
>  
>  void mmu_disable(void)
>  {
> -	unsigned int cr;
> -
> -	cr = get_cr();
> -	cr &= ~(CR_M | CR_C);
> -
> -	set_cr(cr);
> -	v8_flush_dcache_all();
> -	tlb_invalidate();
> -
> -	dsb();
> -	isb();
> +	v8_mmu_disable();
>  }
>  
>  void dma_inv_range(void *ptr, size_t size)
> @@ -436,18 +426,3 @@ void mmu_early_enable(unsigned long membase, unsigned long memsize, unsigned lon
>  
>  	mmu_enable();
>  }
> -
> -void mmu_early_disable(void)
> -{
> -	unsigned int cr;
> -
> -	cr = get_cr();
> -	cr &= ~(CR_M | CR_C);
> -
> -	set_cr(cr);
> -	v8_flush_dcache_all();
> -	tlb_invalidate();
> -
> -	dsb();
> -	isb();
> -}
> diff --git a/arch/arm/include/asm/cache.h b/arch/arm/include/asm/cache.h
> index dd022c1f23f2..ea78ae123aec 100644
> --- a/arch/arm/include/asm/cache.h
> +++ b/arch/arm/include/asm/cache.h
> @@ -15,6 +15,7 @@ void v8_flush_dcache_all(void);
>  void v8_invalidate_dcache_all(void);
>  void v8_flush_dcache_range(unsigned long start, unsigned long end);
>  void v8_inv_dcache_range(unsigned long start, unsigned long end);
> +void v8_mmu_disable(void);
>  
>  static inline void icache_invalidate(void)
>  {
> diff --git a/arch/arm/include/asm/mmu.h b/arch/arm/include/asm/mmu.h
> index eef6c53b5912..1fafbf9d77be 100644
> --- a/arch/arm/include/asm/mmu.h
> +++ b/arch/arm/include/asm/mmu.h
> @@ -65,6 +65,5 @@ void __dma_flush_range(unsigned long, unsigned long);
>  void __dma_inv_range(unsigned long, unsigned long);
>  
>  void mmu_early_enable(unsigned long membase, unsigned long memsize, unsigned long barebox_base);
> -void mmu_early_disable(void);
>  
>  #endif /* __ASM_MMU_H */
> -- 
> 2.47.3
> 
> 
> 

-- 
Pengutronix e.K.                           |                             |
Steuerwalder Str. 21                       | http://www.pengutronix.de/  |
31137 Hildesheim, Germany                  | Phone: +49-5121-206917-0    |
Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |