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

[Andrew Jones]

On Thu, Oct 27, 2022 at 03:02:38PM +0200, Andrew Jones wrote:
> 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
> 
> -- 
> 2.37.3
>

FYI, I just tried this with clang. It compiles but doesn't boot when
Zicboz is present (it does boot when Zicboz is disabled in QEMU). I'm
suspicious of the ALTERNATIVE() stuff, but I'll debug more on Monday.
Also, I see building with LLVM=1 doesn't work, but that appears to be
an issue introduced with "[PATCH 0/8] riscv: improve boot time isa
extensions handling" which this series is based on.

drew