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

[Jisheng Zhang <jszhang@kernel.org>]

On Mon, Sep 07, 2026 at 05:21:11PM +0100, Conor Dooley wrote:
> 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.

The reason I put these two Kconfig options under NONPORTABLE is: all
distributions won't enable NONPORTABLE for their defconfig, and I guess
the distributions want to kept unified one kernel Image for all SV39,
SV48 and SV57.

I have no preference whether move them out of NONPORTABLE or not.
> 
> > +
> > +	/* 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
> >