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

From: Conor Dooley

Date: Wed Sep 09 2026 - 07:28:22 EST

On Wed, Sep 09, 2026 at 07:48:57AM +0800, Jisheng Zhang wrote:
> On Tue, Sep 08, 2026 at 02:07:27PM +0100, Conor Dooley wrote:
> > On Tue, Sep 08, 2026 at 11:49:10AM +0800, Jisheng Zhang wrote:
> > > 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@xxxxxxxxxx>
> > > > > ---
> > > > > 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.
> >
> > Oh right I think I made a mistake in my understanding of what's going on
> > here.
> >
> > If you turn on NONPORTABLE (but not these new options) then you only get
> > sv39 support. I think what you should do to make the behaviour of these
> > options clearer is invert them.
>
> hmm, other ISA ext Kconfig options are not inverted, so I prefer moving
> SV48|57 out of NONPORTABLE and make them y by default.

Sure, that works for me.

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