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Comment by vladimirralev | Hacker News Reader

The VP9 codec I imagine would heavily benefit from SIMD? Just SIMD should massively outperform any advantage gained by putting this into the kernel. Why does the kernel need an unoptimized codec?

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I don't think that the driver implements VP9 in software. It uses hardware acceleration to perform the actual encode/decode, but is managing preparing of coefficients and DMA of data buffers in the driver.

SIMD can be pretty architecture specific. For example, does the CPU support AVX-512 or SSE3? So, you have to have a few code paths if you're going to support a wide variety of hardware.

I don't have an answer to your question it just occurs to me that maybe the Linux kernel doesn't allow a lot of SIMD for this reason or maybe they require a fall back/slow code path to be available if you're submitting a patch that includes SIMD operations?

Curious if anyone knows the answer to this.

Kernel avoids SIMD because it doesn't have a kernel keeping its registers coherent https://stackoverflow.com/a/46677815

Thank you for the answer! Very interesting.

Fwiw, Rust's standard library has a cross-platform Simd abstraction: https://doc.rust-lang.org/std/simd/index.html

It's nightly-only for now, but I've used it and it's lovely. I've added Simd to projects that I never otherwise would have, just because this made it so easy and accessible

That is metal af ;)

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