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Using memset() to zero a 4K page takes 563 total instructions, where 20 are branches. clear_page(), with Zicboz and a 64 byte block size, takes 169 total instructions, where 4 are branches and 33 are nops. Even though the block size is a variable, thanks to alternatives, we can still implement a Duff device without having to do any preliminary calculations. This is achieved by using the alternatives' cpufeature value (the upper 16 bits of patch_id). The value used is the maximum zicboz block size order accepted at the patch site. This enables us to stop patching / unrolling when 4K bytes have been zeroed (we would loop and continue after 4K if the page size would be larger) For 4K pages, unrolling 16 times allows block sizes of 64 and 128 to only loop a few times and larger block sizes to not loop at all. Since cbo.zero doesn't take an offset, we also need an 'add' after each instruction, making the loop body 112 to 160 bytes. Hopefully this is small enough to not cause icache misses. Signed-off-by: Andrew Jones <ajones@ventanamicro.com> Acked-by: Conor Dooley <conor.dooley@microchip.com> Link: https://lore.kernel.org/r/20230224162631.405473-7-ajones@ventanamicro.com Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.