Re: [PATCH v2 0/8] Introduce sv48 support

[Palmer Dabbelt]

On Wed, 03 Jun 2020 01:10:56 PDT (-0700), alex@xxxxxxxx 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.
>
> Folding the 4th level into a 3-level page table has almost no cost at
> runtime. But as mentioned Palmer, the relocatable code generated is less
> performant.
>
> At the moment, there is no way to build a 3-level page table non-relocatable
> 64bit kernel. We agreed that distributions will use this runtime configuration
> anyway, but Palmer proposed to introduce a new Kconfig, which I will do later
> as sv48 support was asked for 5.8.

Sorry I wasn't clear last time, but this still has the same fundamental issue:
it forces 64-bit kernels to be relocatable, which imposes a performance
penalty.  We don't have any hardware that can actually take advantage of sv48,
so I don't want to take anything that penalizes what people are actually using
in order to add a feature people can't use.

I'd be OK taking this if sv48 support simply depended on a relocatable kernel,
as then users who want the faster kernel could still build one.  I don't want
to take something that forces all 64-bit kernels to be relocatable.

> Finally, the user can now ask for sv39 explicitly by using the device-tree
> which will reduce memory footprint and reduce the number of memory accesses
> in case of TLB miss.
>
> Changes in v2:
>   * Move variable declarations to pgtable.h in patch 5/7 as suggested by Anup
>   * Restore mmu-type properties in patch 6 as suggested by Anup
>   * Fix unused variable in patch 5 that was used in patch 6
>   * Fix SPARSEMEM build (patch 2 was modified so I dropped the Reviewed-by)
>   * Applied various Reviewed-by
>
> Alexandre Ghiti (8):
>   riscv: Get rid of compile time logic with MAX_EARLY_MAPPING_SIZE
>   riscv: Allow to dynamically define VA_BITS
>   riscv: Simplify MAXPHYSMEM config
>   riscv: Prepare ptdump for vm layout dynamic addresses
>   riscv: Implement sv48 support
>   riscv: Allow user to downgrade to sv39 when hw supports sv48
>   riscv: Use pgtable_l4_enabled to output mmu type in cpuinfo
>   riscv: Explicit comment about user virtual address space size
>
>  arch/riscv/Kconfig                  |  34 ++---
>  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 |  97 +++++++++++++-
>  arch/riscv/include/asm/pgtable.h    |  31 ++++-
>  arch/riscv/include/asm/sparsemem.h  |   6 +-
>  arch/riscv/kernel/cpu.c             |  23 ++--
>  arch/riscv/kernel/head.S            |   3 +-
>  arch/riscv/mm/context.c             |   2 +-
>  arch/riscv/mm/init.c                | 194 ++++++++++++++++++++++++----
>  arch/riscv/mm/ptdump.c              |  49 +++++--
>  13 files changed, 412 insertions(+), 82 deletions(-)