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SubjectRe: [RFC PATCH 0/7] Introduce sv48 support
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On Sun, 22 Mar 2020 04:00:21 PDT (-0700), alex@ghiti.fr wrote:
> This patchset implements sv48 support at runtime. The kernel will try to
> boot with 4-level page table and will fallback to 3-level if the HW does not
> support it.
>
> The biggest advantage is that we only have one kernel for 64bit, which
> is way easier to maintain.

Thanks, that's great! This is something we've been missing for a long time
now.

> Folding the 4th level into a 3-level page table has almost no cost at
> runtime.
>
> At the moment, there is no way to enforce 3-level if the HW supports
> 4-level page table: early parameters are parsed after the choice must be
> made.

This is different than how I'd been considering doing it -- specifically, my
worry was that 4-level paging would have a meaningful performance hit and
therefor we'd want to allow users to select 3-level paging somehow. I'd been
thinking of having a Kconfig option, so the options would be "3-level only" or
"3 or 4 level". That came with a bunch of drawbacks, so I'd be much happier to
have a single kernel.

Where did you get your performance numbers from? Appologies in advance if
there's more info in the patches, I'll look at those now...

> It is based on my relocatable patchset v3 that I have not posted yet,
> you can try the sv48 support by using the branch
> int/alex/riscv_sv48_runtime_v1 here:
>
> https://github.com/AlexGhiti/riscv-linux
>
> Any feedback appreciated,
>
> Thanks,
>
> Alexandre Ghiti (7):
> riscv: Get rid of compile time logic with MAX_EARLY_MAPPING_SIZE
> riscv: Allow to dynamically define VA_BITS
> riscv: Simplify MAXPHYSMEM config
> riscv: Implement sv48 support
> riscv: Use pgtable_l4_enabled to output mmu type in cpuinfo
> dt-bindings: riscv: Remove "riscv,svXX" property from device-tree
> riscv: Explicit comment about user virtual address space size
>
> .../devicetree/bindings/riscv/cpus.yaml | 13 --
> arch/riscv/Kconfig | 34 ++---
> arch/riscv/boot/dts/sifive/fu540-c000.dtsi | 4 -
> arch/riscv/include/asm/csr.h | 3 +-
> arch/riscv/include/asm/fixmap.h | 1 +
> arch/riscv/include/asm/page.h | 15 +-
> arch/riscv/include/asm/pgalloc.h | 36 +++++
> arch/riscv/include/asm/pgtable-64.h | 98 +++++++++++-
> arch/riscv/include/asm/pgtable.h | 24 ++-
> arch/riscv/include/asm/sparsemem.h | 2 +-
> arch/riscv/kernel/cpu.c | 24 +--
> arch/riscv/kernel/head.S | 37 ++++-
> arch/riscv/mm/context.c | 4 +-
> arch/riscv/mm/init.c | 142 +++++++++++++++---
> 14 files changed, 341 insertions(+), 96 deletions(-)

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