[RFC PATCH V1 00/11] riscv: Introduce 64K base page

[Xu Lu]

A gentle ping.

On Thu, Nov 23, 2023 at 2:57 PM Xu Lu <luxu.kernel at bytedance.com> wrote:
>
> Some existing architectures like ARM supports base page larger than 4K
> as their MMU supports more page sizes. Thus, besides hugetlb page and
> transparent huge page, there is another way for these architectures to
> enjoy the benefits of fewer TLB misses without worrying about cost of
> splitting and merging huge pages. However, on architectures with only
> 4K MMU, larger base page is unavailable now.
>
> This patch series attempts to break through the limitation of MMU and
> supports larger base page on RISC-V, which only supports 4K page size
> now.
>
> The key idea to implement larger base page based on 4K MMU is to
> decouple the MMU page from the base page in view of kernel mm, which we
> denote as software page. In contrary to software page, we denote the MMU
> page as hardware page. Below is the difference between these two kinds
> of pages.
>
> 1. Kernel memory management module manages, allocates and maps memory at
> a granularity of software page, which should not be restricted by
> MMU and can be larger than hardware page.
>
> 2. Architecture page table operations should be carried out from MMU's
> perspective and page table entries are encoded at a granularity of
> hardware page, which is 4K on RISC-V MMU now.
>
> The main work to decouple these two kinds of pages lies in architecture
> code. For example, we turn the pte_t struct to an array of page table
> entries to match it with software page which can be larger than hardware
> page, and adapt the page table operations accordingly. For 64K software
> base page, the pte_t struct now contains 16 contiguous page table
> entries which point to 16 contiguous 4K hardware pages.
>
> To achieve the benefits of large base page, we applies Svnapot for each
> base page's mapping. The Svnapot extension on RISC-V is like contiguous
> PTE on ARM64. It allows ptes of a naturally aligned power-of 2 size
> memory range be encoded in the same format to save the TLB space.
>
> This patch series is the first version and is based on v6.7-rc1. This
> version supports both bare metal and virtualization scenarios.
>
> In the next versions, we will continue on the following works:
>
> 1. Reduce the memory usage of page table page as it only uses 4K space
> while costs a whole base page.
>
> 2. When IMSIC interrupt file is smaller than 64K, extra isolation
> measures for the interrupt file are needed. (S)PMP and IOPMP may be good
> choices.
>
> 3. More consideration is needed to make this patch series collaborate
> with folios better.
>
> 4. Support 64K base page on IOMMU.
>
> 5. The performance test is on schedule to verify the actual performance
> improvement and the decrease in TLB miss rate.
>
> Thanks in advance for comments.
>
> Xu Lu (11):
>   mm: Fix misused APIs on huge pte
>   riscv: Introduce concept of hardware base page
>   riscv: Adapt pte struct to gap between hw page and sw page
>   riscv: Adapt pte operations to gap between hw page and sw page
>   riscv: Decouple pmd operations and pte operations
>   riscv: Distinguish pmd huge pte and napot huge pte
>   riscv: Adapt satp operations to gap between hw page and sw page
>   riscv: Apply Svnapot for base page mapping
>   riscv: Adjust fix_btmap slots number to match variable page size
>   riscv: kvm: Adapt kvm to gap between hw page and sw page
>   riscv: Introduce 64K page size
>
>  arch/Kconfig                        |   1 +
>  arch/riscv/Kconfig                  |  28 +++
>  arch/riscv/include/asm/fixmap.h     |   3 +-
>  arch/riscv/include/asm/hugetlb.h    |  71 ++++++-
>  arch/riscv/include/asm/page.h       |  16 +-
>  arch/riscv/include/asm/pgalloc.h    |  21 ++-
>  arch/riscv/include/asm/pgtable-32.h |   2 +-
>  arch/riscv/include/asm/pgtable-64.h |  45 +++--
>  arch/riscv/include/asm/pgtable.h    | 282 +++++++++++++++++++++++-----
>  arch/riscv/kernel/efi.c             |   2 +-
>  arch/riscv/kernel/head.S            |   4 +-
>  arch/riscv/kernel/hibernate.c       |   3 +-
>  arch/riscv/kvm/mmu.c                | 198 +++++++++++++------
>  arch/riscv/mm/context.c             |   7 +-
>  arch/riscv/mm/fault.c               |   1 +
>  arch/riscv/mm/hugetlbpage.c         |  42 +++--
>  arch/riscv/mm/init.c                |  25 +--
>  arch/riscv/mm/kasan_init.c          |   7 +-
>  arch/riscv/mm/pageattr.c            |   2 +-
>  fs/proc/task_mmu.c                  |   2 +-
>  include/asm-generic/hugetlb.h       |   7 +
>  include/asm-generic/pgtable-nopmd.h |   1 +
>  include/linux/pgtable.h             |   6 +
>  mm/hugetlb.c                        |   2 +-
>  mm/migrate.c                        |   5 +-
>  mm/mprotect.c                       |   2 +-
>  mm/rmap.c                           |  10 +-
>  mm/vmalloc.c                        |   3 +-
>  28 files changed, 616 insertions(+), 182 deletions(-)
>
> --
> 2.20.1
>