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Re: [PATCH 15/27] arm64/sve: Probe SVE capabilities and usable vector lengths

On 09/08/17 13:05, Dave Martin wrote:
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This patch uses the cpufeatures framework to determine common SVE
capabilities and vector lengths, and configures the runtime SVE
support code appropriately.

ZCR_ELx is not really a feature register, but it is convenient to
use it as a template for recording the maximum vector length
supported by a CPU, using the LEN field.  This field is similar to
a feature field in that it is a contiguous bitfield for which we
want to determine the minimum system-wide value.  This patch adds
ZCR as a pseudo-register in cpuinfo/cpufeatures, with appropriate
custom code to populate it.  Finding the minimum supported value of
the LEN field is left to the cpufeatures framework in the usual

The meaning of ID_AA64ZFR0_EL1 is not architecturally defined yet,
so for now we just require it to be zero.

Note that much of this code is dormant and SVE still won't be used
yet, since system_supports_sve() remains hardwired to false.

Signed-off-by: Dave Martin <>


The cpufeature bits look good to me, with one minor comment.

diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index bce95de..955c873 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c


+void __init sve_setup(void)
+       u64 zcr;
+       unsigned int max_vl;
+       if (!system_supports_sve())
+               return;
+       /*
+        * The architecture mandates 128-bit vectors be supported, and
+        * the code assumes elsewhere that sve_vq_map is non-empty:
+        */
+       BUG_ON(!test_bit(vq_to_bit(1), sve_vq_map));
+       sve_vq_map_finalised = true;

We have something local in cpufeature.c, sys_caps_initialised. May be we could
reuse it here ? With or without that change, FWIW.

Acked-by: Suzuki K Poulose <>

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