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by senderista 370 days ago
> Semi-related: one thing that most people never think about: it is exactly the same amount of work for the kernel to zero a page of memory (in preparation for a future mmap) as for a userland process to zero it out (for its own internal reuse)

Possibly more work since the kernel can't use SIMD
1 comments
LtdJorge 370 days ago
Why is that? Doesn't Linux use SIMD for the crypto operations?
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dwattttt 370 days ago
Allowing SIMD instructions to be used arbitrarily in kernel actually has a fair penalty to it. I'm not sure what Linux does specifically, but:

When a syscall is made, the kernel has to backup the user mode state of the thread, so it can restore it later.

If any kernel code could use SIMD registers, you'll have to backup and restore that too, and those registers get big. You could easily be looking at adding a 1kb copy to every syscall, and most of the time it wouldn't be needed.

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kstrauser 370 days ago
Why is that? Couldn’t there be push_simd()/pop_simd() that the syscall itself uses around its SIMD calls?

If no syscalls use SIMD today, I’d think we’re starting from a safe position.

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durrrrrrrrrrrrr 369 days ago
push_simd/pop_simd exist and are called kernel_fpu_begin/kernel_fpu_end. Their use is practically prohibited in most areas and iiuc not available on all archs, but it's available if needed.
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kstrauser 369 days ago
Today I learned. Thanks!
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durrrrrrrrrrrrr 370 days ago
It's not so much that you can't ever use it, it's more a you really shouldn't. It's more expensive, harder to use and rarely worth it. Main users currently are crypto and raid checksumming.

https://www.kernel.org/doc/html/next/core-api/floating-point...

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