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Currently, the SVE code is compiled only when -march supports SVE
(e.g. '- march=armv8.2a+sve'), there maybe some problem[1] with this

approach.

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The solution:
a. If the minimum instruction set support SVE then compiles it.
b. Else if the compiler support SVE then compiles it.
c. Otherwise don't compile it.

[1] https://mails.dpdk.org/archives/dev/2021-April/208189.html

Fixes: 8c25b02b082a ("net/hns3: fix enabling SVE Rx/Tx")
Fixes: 952ebacce4f2 ("net/hns3: support SVE Rx")
Cc: stable@dpdk.org

Signed-off-by: Chengwen Feng <redacted>
---
 drivers/net/hns3/hns3_rxtx.c |  2 +-  drivers/net/hns3/meson.build
| 13 +++++++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/drivers/net/hns3/hns3_rxtx.c

b/drivers/net/hns3/hns3_rxtx.c index
1d7a769..4ef20c6 100644

--- a/drivers/net/hns3/hns3_rxtx.c
+++ b/drivers/net/hns3/hns3_rxtx.c

@@ -2808,7 +2808,7 @@ hns3_get_default_vec_support(void)
 static bool
 hns3_get_sve_support(void)
 {
-#if defined(RTE_ARCH_ARM64) && defined(__ARM_FEATURE_SVE)
+#if defined(CC_SVE_SUPPORT)
 	if (rte_vect_get_max_simd_bitwidth() < RTE_VECT_SIMD_256)
 		return false;
 	if (rte_cpu_get_flag_enabled(RTE_CPUFLAG_SVE))

diff --git a/drivers/net/hns3/meson.build

b/drivers/net/hns3/meson.build index
53c7df7..8563d70 100644

--- a/drivers/net/hns3/meson.build
+++ b/drivers/net/hns3/meson.build

@@ -35,7 +35,20 @@ deps += ['hash']

 if arch_subdir == 'arm' and dpdk_conf.get('RTE_ARCH_64')
     sources += files('hns3_rxtx_vec.c')
+
+    # compile SVE when:
+    # a. support SVE in minimum instruction set baseline
+    # b. it's not minimum instruction set, but compiler support
     if cc.get_define('__ARM_FEATURE_SVE', args: machine_args) != ''
+        cflags += ['-DCC_SVE_SUPPORT']

Why is the CC_SVE_SUPPORT flag needed? The compiler has
__ARM_FEATURE_SVE flag already which gets defined when '+sve" is
added to '-march'.

The CC_SVE_SUPPORT is used to implement the hns3_get_sve_support API
(below), this API located in another file which is hns3_rxtx.c, and this file was
compiled with default machine_args.

	static bool
	hns3_get_sve_support(void)
	{
	#if defined(CC_SVE_SUPPORT)
		if (rte_vect_get_max_simd_bitwidth() < RTE_VECT_SIMD_256)
			return false;
		if (rte_cpu_get_flag_enabled(RTE_CPUFLAG_SVE))
			return true;
	#endif
		return false;
	}

If the machine_args doesn't support SVE but compiler support, we will
compile SVE code too, so in this case we need define CC_SVE_SUPPORT, so
that in runtime we could try judge whether support SVE. In this case,
__ARM_FEATURE_SVE was not defined because it is in hns3_rxtx.c which use
default machine_args.

If the machine_args supports SVE we sure compile SVE code, in this case, to
maintain consistency, we also define this macro.

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         sources += files('hns3_rxtx_vec_sve.c')
+    elif cc.has_argument('-march=armv8.2-a+sve')

I think this check and the above check do the same thing. i.e. both
of them check if +sve flag is passed to the compiler.

Yes it is.

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+        cflags += ['-DCC_SVE_SUPPORT']
+        hns3_sve_lib = static_library('hns3_sve_lib',
+                        'hns3_rxtx_vec_sve.c',
+                        dependencies: [static_rte_ethdev],
+                        include_directories: includes,
+                        c_args: [cflags, '-march=armv8.2-a+sve'])
+        objs += hns3_sve_lib.extract_objects('hns3_rxtx_vec_sve.c')

I do not understand the need of this block of code, appreciate if you could

explain why this is required.

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This is typical usage for compiling some soure file with custom flags (here is '-
march=armv8.2-a+sve') Please ref [1] with keyword 'extract_objects'

[1] https://mesonbuild.com/Reference-manual.html

Ack, understand this now, thanks for your patience

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     endif
 endif
--
2.8.1

.