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target/arm: Add sve-default-vector-length cpu property

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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>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20210723203344.968563-4-richard.henderson@linaro.org
[PMM: tweaked docs formatting, document -1 special-case,
 added fixup patch from RTH mentioning QEMU's maximum veclen.]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
This commit is contained in:
Richard Henderson 2021-07-23 10:33:44 -10:00 • committed by Peter Maydell
parent ce440581c1
commit b3d52804c5
4 changed files with 92 additions and 2 deletions
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15
docs/system/arm/cpu-features.rst
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@ -376,3 +376,18 @@ verbose command lines. However, the recommended way to select vector
lengths is to explicitly enable each desired length. Therefore only
example's (1), (4), and (6) exhibit recommended uses of the properties.
SVE User-mode Default Vector Length Property
--------------------------------------------
For qemu-aarch64, the cpu property ``sve-default-vector-length=N`` is
defined to mirror the Linux kernel parameter file
``/proc/sys/abi/sve_default_vector_length``. The default length, ``N``,
is in units of bytes and must be between 16 and 8192.
If not specified, the default vector length is 64.
If the default length is larger than the maximum vector length enabled,
the actual vector length will be reduced. Note that the maximum vector
length supported by QEMU is 256.
If this property is set to ``-1`` then the default vector length
is set to the maximum possible length.