Re: [PATCH v2 4/5] riscv: RISCV_ISA_SV48 and RISCV_ISA_SV57 under NONPORTABLE

[Conor Dooley <conor@kernel.org>]

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On Mon, Sep 07, 2026 at 11:14:36PM +0800, Jisheng Zhang wrote:
> In last commit, we have optimized the pgtable_l4|l5_enabled with isa
> extension alternative mechanism, thus a typical access_ok(addr, 1);
> is compiled as:
> 
> ffffffff8001dd30:	1141                	addi	sp,sp,-16
> ffffffff8001dd32:	e022                	sd	s0,0(sp)
> ffffffff8001dd34:	e406                	sd	ra,8(sp)
> ffffffff8001dd36:	0800                	addi	s0,sp,16
> ffffffff8001dd38:	00000013          	nop
> ffffffff8001dd3c:	02a0006f          	j	ffffffff8001dd66 <foo+0x3e>
> ffffffff8001dd40:	60a2                	ld	ra,8(sp)
> ffffffff8001dd42:	6402                	ld	s0,0(sp)
> ffffffff8001dd44:	57fd                	li	a5,-1
> ffffffff8001dd46:	83a1                	srli	a5,a5,0x8
> ffffffff8001dd48:	00a7b533          	sltu	a0,a5,a0
> ffffffff8001dd4c:	00154513          	xori	a0,a0,1
> ffffffff8001dd50:	0141                	addi	sp,sp,16
> ffffffff8001dd52:	8082                	ret
> ...
> ffffffff8001dd64:	bfe1                	j	ffffffff8001dd3c <foo+0x14>
> ffffffff8001dd66:	0180006f          	j	ffffffff8001dd7e <foo+0x56>
> ffffffff8001dd6a:	60a2                	ld	ra,8(sp)
> ffffffff8001dd6c:	6402                	ld	s0,0(sp)
> ffffffff8001dd6e:	57fd                	li	a5,-1
> ffffffff8001dd70:	83c5                	srli	a5,a5,0x11
> ffffffff8001dd72:	00a7b533          	sltu	a0,a5,a0
> ffffffff8001dd76:	00154513          	xori	a0,a0,1
> ffffffff8001dd7a:	0141                	addi	sp,sp,16
> ffffffff8001dd7c:	8082                	ret
> ffffffff8001dd7e:	60a2                	ld	ra,8(sp)
> ffffffff8001dd80:	6402                	ld	s0,0(sp)
> ffffffff8001dd82:	57fd                	li	a5,-1
> ffffffff8001dd84:	83e9                	srli	a5,a5,0x1a
> ffffffff8001dd86:	00a7b533          	sltu	a0,a5,a0
> ffffffff8001dd8a:	00154513          	xori	a0,a0,1
> ffffffff8001dd8e:	0141                	addi	sp,sp,16
> ffffffff8001dd90:	8082                	ret
> 
> As can be seen, different branches for SV39/SV48/SV57 are still there,
> since access_ok() sits at hot code path, why not reduce the code size
> to optimize to make the instruction cache happy?
> 
> Introduce RISCV_ISA_SV48 and RISCV_ISA_SV57 under NONPORTABLE, so that
> the embedded platforms can choose the best option themselves, while
> still keep the feature of unified one kernel Image for all SV39, SV48
> and SV57.
> 
> Before the patch, vmlinux built with pure RISCV64 defconfig
>    text    data     bss     dec     hex filename
> 13439437   6790446  483221 20713104 13c0e90 /tmp/old/vmlinux
> 
> After the patch, vmlinux built with pure RISCV64 defconfig:
>    text    data     bss     dec     hex filename
> 13314445   6787934  466837 20569216 139dc80 /tmp/new/vmlinux
> 
> .text section is reduced by 122KB!
> 
> And after the patch, typical access_ok(addr, 1) is compiled as:
> ffffffff8001809a:	1141                	addi	sp,sp,-16
> ffffffff8001809c:	e022                	sd	s0,0(sp)
> ffffffff8001809e:	e406                	sd	ra,8(sp)
> ffffffff800180a0:	0800                	addi	s0,sp,16
> ffffffff800180a2:	00000013          	nop
> ffffffff800180a6:	60a2                	ld	ra,8(sp)
> ffffffff800180a8:	6402                	ld	s0,0(sp)
> ffffffff800180aa:	4785                	li	a5,1
> ffffffff800180ac:	179a                	slli	a5,a5,0x26
> ffffffff800180ae:	00f53533          	sltu	a0,a0,a5
> ffffffff800180b2:	0141                	addi	sp,sp,16
> ffffffff800180b4:	8082                	ret
> 
> only 12 instruction!! For refernece, before the patch, the access(addr,
> 1) needs 32 instructions.
> 
> Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
> ---
>  arch/riscv/Kconfig                  | 24 ++++++++++++++++++++++++
>  arch/riscv/include/asm/pgtable-64.h | 12 ++++++++++++
>  arch/riscv/mm/init.c                | 10 ++++++++++
>  3 files changed, 46 insertions(+)
> 
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index e9476b8cbeb0..4cdb1e3f34be 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -602,6 +602,30 @@ config RISCV_ISA_SSQOSID
>  	  cache partitioning scheme and use the MCID to track how much
>  	  cache a process, or a group of processes, is using.
>  
> +config RISCV_ISA_SV48
> +	bool "Page-Based 48-bit Virtual-Memory System"
> +	depends on 64BIT && NONPORTABLE
> +	default y

I don't understand how you've got this set up here.
If this is genuinely non-portable, then you should not set the default to
enabled. However, I'm not really sure that this is actually non-portable
at all....

> +	help
> +	  Add support for the Page-Based 48-bit Virtual-Memory System.
> +
> +	  Enable this option for systems for which a 39-bit virtual address
> +	  space is insufficient. But if 39-bit is enough, disabling this
> +	  option will result in smaller kernel size and slighly better
> +	  performance.
> +
> +config RISCV_ISA_SV57
> +	bool "Page-Based 57-bit Virtual-Memory System"
> +	depends on 64BIT && NONPORTABLE
> +	default y
> +	help
> +	  Add support for the Page-Based 57-bit Virtual-Memory System.
> +
> +	  Enable this option for systems for which a 48-bit virtual address
> +	  space is insufficient. But if 48-bit is enough, disabling this
> +	  option will result in smaller kernel size and slighly better
> +	  performance.
> +
>  config RISCV_ISA_SVPBMT
>  	bool "Svpbmt extension support for supervisor mode page-based memory types"
>  	depends on 64BIT && MMU
> diff --git a/arch/riscv/include/asm/pgtable-64.h b/arch/riscv/include/asm/pgtable-64.h
> index 7e30ec0e9f6c..f2d29da93eea 100644
> --- a/arch/riscv/include/asm/pgtable-64.h
> +++ b/arch/riscv/include/asm/pgtable-64.h
> @@ -16,21 +16,33 @@ extern bool _pgtable_l5_enabled;
>  #ifdef USE_EARLY_PGTABLE_LEVELS
>  static __always_inline bool pgtable_l5_enabled(void)
>  {
> +	if (IS_ENABLED(CONFIG_NONPORTABLE) && !IS_ENABLED(CONFIG_RISCV_ISA_SV57))
> +		return false;
> +
>  	return _pgtable_l5_enabled;
>  }
>  
>  static __always_inline bool pgtable_l4_enabled(void)
>  {
> +	if (IS_ENABLED(CONFIG_NONPORTABLE) && !IS_ENABLED(CONFIG_RISCV_ISA_SV48))
> +		return false;
> +
>  	return _pgtable_l4_enabled;
>  }
>  #else
>  static __always_inline bool pgtable_l4_enabled(void)
>  {
> +	if (IS_ENABLED(CONFIG_NONPORTABLE) && !IS_ENABLED(CONFIG_RISCV_ISA_SV48))
> +		return false;
> +
>  	return riscv_has_extension_likely(RISCV_ISA_EXT_SV48);
>  }
>  
>  static __always_inline bool pgtable_l5_enabled(void)
>  {
> +	if (IS_ENABLED(CONFIG_NONPORTABLE) && !IS_ENABLED(CONFIG_RISCV_ISA_SV57))
> +		return false;
> +
>  	return riscv_has_extension_likely(RISCV_ISA_EXT_SV57);
>  }
>  #endif

...because the code here won't enable sv57 etc if not supported...

> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index 2a2402d79bc4..90bb85c5f167 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -844,6 +844,16 @@ static __init void set_satp_mode(uintptr_t dtb_pa)
>  
>  	kernel_map.page_offset = PAGE_OFFSET_L5;
>  
> +	/* Fall back to Sv48 if Sv57 is unsupported on non-portable systems */
> +	if (IS_ENABLED(CONFIG_NONPORTABLE) && !IS_ENABLED(CONFIG_RISCV_ISA_SV57) &&
> +			satp_mode_limit == 0)
> +		satp_mode_limit = SATP_MODE_48;

...and neither does the code here.

> Introduce RISCV_ISA_SV48 and RISCV_ISA_SV57 under NONPORTABLE, so that
> the embedded platforms can choose the best option themselves, while
> still keep the feature of unified one kernel Image for all SV39, SV48
> and SV57.

NONPORTABLE is what you're meant to depend on when the kernel that gets
built cannot run on platforms without the feature.

> +
> +	/* Fall back to Sv39 if Sv48 is unsupported on non-portable systems */
> +	if (IS_ENABLED(CONFIG_NONPORTABLE) && !IS_ENABLED(CONFIG_RISCV_ISA_SV48) &&
> +			satp_mode_limit == SATP_MODE_48)
> +		satp_mode_limit = SATP_MODE_39;
> +
>  	if (satp_mode_limit == SATP_MODE_48) {
>  		disable_pgtable_l5();
>  	} else if (satp_mode_limit == SATP_MODE_39) {
> -- 
> 2.53.0
> 

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