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Re: [PATCH 2/2] target/arm: Add sve-default-vector-length cpu property

From: Peter Maydell
Subject: Re: [PATCH 2/2] target/arm: Add sve-default-vector-length cpu property
Date: Fri, 16 Jul 2021 10:14:16 +0100

On Wed, 14 Jul 2021 at 19:06, Richard Henderson
<richard.henderson@linaro.org> wrote:
> Mirror the behavour of /proc/sys/abi/sve_default_vector_length
> under the real linux kernel.  We have no way of passing along
> a real default across exec like the kernel can, but this is a
> decent way of adjusting the startup vector length of a process.
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/482
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

> +/* Mirror linux /proc/sys/abi/sve_default_vector_length. */

/proc/sys/abi/sve_default_vector_length wants a vector length in
bytes, and it looks like we take a length in bits. I assume that's
to match other places where the user can specify vector lengths,
but we should mention the units we expect and that it's not what
the kernel uses.

We also don't support the kernel's (undocumented) "-1 means set
to the maximum" behaviour -- do we need it, or is that more reasonably
achievable by the user via other properties ?

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>

Regarding documentation, yeah, we don't have a great place
to put usermode specific CPU properties. I would just put
the documentation with the docs of the other properties.
Maybe we'll sort out the document structure later (or at least
put a link from the usermode doc section into the CPU part of
the system-emulation doc section), but the usermode emulation
documentation overall is in need of a massive overhaul some day...

-- PMM

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