Messages in this thread | | | Date | Wed, 01 Jul 2020 11:27:03 -0700 (PDT) | Subject | Re: [PATCH v2 0/8] Introduce sv48 support | From | Palmer Dabbelt <> |
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On Wed, 03 Jun 2020 01:10:56 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. > > 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(-)
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