[PATCH 0/9] RISC-V: Apply Zicboz to clear_page and memset

[Andrew Jones]

When the Zicboz extension is available we can more rapidly zero naturally
aligned Zicboz block sized chunks of memory. As pages are always page
aligned and are larger than any Zicboz block size will be, then
clear_page() appears to be a good candidate for the extension. While cycle
count and energy consumption should also be considered, we can be pretty
certain that implementing clear_page() with the Zicboz extension is a win
by comparing the new dynamic instruction count with its current count[1].
Doing so we see that the new count is less than half the old count (see
patch4's commit message for more details). Another candidate for the
extension is memset(), but, since memset() isn't just used for zeroing
memory and it accepts arbitrarily aligned addresses and arbitrary sizes,
it's not as obvious if adding support for Zicboz will be an overall win.
In order to make a determination, I've done some analysis and wrote my
conclusions in the bullets below.

* When compiling the kernel without CONFIG_RISCV_ISA_ZICBOZ, memset()
  doesn't change, so that's fine.

* The overhead added to memset() when the Zicboz extension isn't present,
  but CONFIG_RISCV_ISA_ZICBOZ is selected, is 3 jumps to known targets,
  which I believe is fine.

* The overhead added to a memset() invocation which is not zeroing memory
  is 7 instructions, where 3 are branches. This seems fine and,
  furthermore, memset() is almost always invoked to zero memory (99% [2]).

* When memset() is invoked to zero memory, the proposed Zicboz extended
  memset() always has a lower dynamic instruction count than the current
  memset() as long as the input address is Zicboz block aligned and the
  length is >= the block size.

* When memset() is invoked to zero memory, the proposed Zicboz extended
  memset() is always worse for unaligned or too small inputs than the
  current memset(), but it's only at most a few dozen instructions worse.
  I think this is probably fine, especially considering the large majority
  of zeroing invocations are 64 bytes or larger and are aligned to a
  power-of-2 boundary, 64-byte or larger (77% [2]).

[1] I ported the functions under test to userspace and linked them with
    a test program. Then, I ran them under gdb with a script[3] which
    counted instructions by single stepping.

[2] I wrote bpftrace scripts[4] to count memset() invocations to see the
    frequency of it being used to zero memory and have block size aligned
    input addresses with block size or larger lengths. The workload was
    just random desktop stuff including streaming video and compiling.
    While I did run this on my x86 notebook, I still expect the data to
    be representative on RISC-V. Note, x86 has clear_page() so the
    memset() data regarding alignment and size weren't over inflated by
    page zeroing invocations. Grepping also shows the large majority of
    memset() calls are to zero memory (93%).

[3] https://gist.github.com/jones-drew/487791c956ceca8c18adc2847eec9c60
[4] https://gist.github.com/jones-drew/1e860692cf6fc0fb2a82a04c9ce720fe

These patches are based on the following pending series

1. "[PATCH v2 0/3] RISC-V: Ensure Zicbom has a valid block size"
   20221024091309.406906-1-ajones at ventanamicro.com

2. "[PATCH 0/8] riscv: improve boot time isa extensions handling"
   20221006070818.3616-1-jszhang at kernel.org
   Also including the additional patch proposed here
   20221013162038.ehseju2neic2xu5z at kamzik

The patches are also available here
https://github.com/jones-drew/linux/commits/riscv/zicboz

To test over QEMU this branch may be used to enable Zicboz
https://gitlab.com/jones-drew/qemu/-/commits/riscv/zicboz

To test running a KVM guest with Zicboz this kvmtool branch may be used
https://github.com/jones-drew/kvmtool/commits/riscv/zicboz

Thanks,
drew

Andrew Jones (9):
  RISC-V: Factor out body of riscv_init_cbom_blocksize loop
  RISC-V: Add Zicboz detection and block size parsing
  RISC-V: insn-def: Define cbo.zero
  RISC-V: Use Zicboz in clear_page when available
  RISC-V: KVM: Provide UAPI for Zicboz block size
  RISC-V: KVM: Expose Zicboz to the guest
  RISC-V: lib: Improve memset assembler formatting
  RISC-V: lib: Use named labels in memset
  RISC-V: Use Zicboz in memset when available

 arch/riscv/Kconfig                  |  13 ++
 arch/riscv/include/asm/cacheflush.h |   3 +-
 arch/riscv/include/asm/hwcap.h      |   1 +
 arch/riscv/include/asm/insn-def.h   |  50 ++++++
 arch/riscv/include/asm/page.h       |   6 +-
 arch/riscv/include/uapi/asm/kvm.h   |   2 +
 arch/riscv/kernel/cpu.c             |   1 +
 arch/riscv/kernel/cpufeature.c      |  10 ++
 arch/riscv/kernel/setup.c           |   2 +-
 arch/riscv/kvm/vcpu.c               |  11 ++
 arch/riscv/lib/Makefile             |   1 +
 arch/riscv/lib/clear_page.S         |  28 ++++
 arch/riscv/lib/memset.S             | 241 +++++++++++++++++++---------
 arch/riscv/mm/cacheflush.c          |  64 +++++---
 14 files changed, 325 insertions(+), 108 deletions(-)
 create mode 100644 arch/riscv/lib/clear_page.S

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2.37.3