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Jisheng Zhang <jszhang@kernel.org> says: Previously, we use alternative mechanism to dynamically patch the CMO operations for THEAD C906/C910 during boot for performance reason. But as pointed out by Arnd, "there is already a significant cost in accessing the invalidated cache lines afterwards, which is likely going to be much higher than the cost of an indirect branch". And indeed, there's no performance difference with GMAC and EMMC per my test on Sipeed Lichee Pi 4A board. Use riscv_nonstd_cache_ops for THEAD C906/C910 CMO to simplify the alternative code, and to acchieve Arnd's goal -- "I think moving the THEAD ops at the same level as all nonstandard operations makes sense, but I'd still leave CMO as an explicit fast path that avoids the indirect branch. This seems like the right thing to do both for readability and for platforms on which the indirect branch has a noticeable overhead." To make bisect easy, I use two patches here: patch1 does the conversion which just mimics current CMO behavior via. riscv_nonstd_cache_ops, I assume no functionalities changes. patch2 uses T-HEAD PA based CMO instructions so that we don't need to covert PA to VA. * b4-shazam-merge: riscv: errata: thead: use pa based instructions for CMO riscv: errata: thead: use riscv_nonstd_cache_ops for CMO Link: https://lore.kernel.org/r/20231114143338.2406-1-jszhang@kernel.org Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com> |
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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.