linux-kernel - [PATCH v9 05/30] arm64/fpsimd: Determine maximum virtualisable SME vector length

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Message-Id: <20251223-kvm-arm64-sme-v9-5-8be3867cb883@kernel.org>
Date: Tue, 23 Dec 2025 01:20:59 +0000
From: Mark Brown <broonie@...nel.org>
To: Marc Zyngier <maz@...nel.org>, Joey Gouly <joey.gouly@....com>, 
 Catalin Marinas <catalin.marinas@....com>, 
 Suzuki K Poulose <suzuki.poulose@....com>, Will Deacon <will@...nel.org>, 
 Paolo Bonzini <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>, 
 Shuah Khan <shuah@...nel.org>, Oliver Upton <oupton@...nel.org>
Cc: Dave Martin <Dave.Martin@....com>, Fuad Tabba <tabba@...gle.com>, 
 Mark Rutland <mark.rutland@....com>, Ben Horgan <ben.horgan@....com>, 
 linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev, 
 linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
 linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org, 
 Peter Maydell <peter.maydell@...aro.org>, 
 Eric Auger <eric.auger@...hat.com>, Mark Brown <broonie@...nel.org>
Subject: [PATCH v9 05/30] arm64/fpsimd: Determine maximum virtualisable SME
 vector length

As with SVE we can only virtualise SME vector lengths that are supported by
all CPUs in the system, implement similar checks to those for SVE. Since
unlike SVE there are no specific vector lengths that are architecturally
required the handling is subtly different, we report a system where this
happens with a maximum vector length of -1.

Signed-off-by: Mark Brown <broonie@...nel.org>
---
 arch/arm64/kernel/fpsimd.c | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index f4e8cee00198..22f8397c67f0 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -1257,7 +1257,8 @@ void cpu_enable_sme(const struct arm64_cpu_capabilities *__always_unused p)
 void __init sme_setup(void)
 {
 	struct vl_info *info = &vl_info[ARM64_VEC_SME];
-	int min_bit, max_bit;
+	DECLARE_BITMAP(tmp_map, SVE_VQ_MAX);
+	int min_bit, max_bit, b;
 
 	if (!system_supports_sme())
 		return;
@@ -1288,12 +1289,32 @@ void __init sme_setup(void)
 	 */
 	set_sme_default_vl(find_supported_vector_length(ARM64_VEC_SME, 32));
 
+	bitmap_andnot(tmp_map, info->vq_partial_map, info->vq_map,
+		      SVE_VQ_MAX);
+
+	b = find_last_bit(tmp_map, SVE_VQ_MAX);
+	if (b >= SVE_VQ_MAX)
+		/* All VLs virtualisable */
+		info->max_virtualisable_vl = SVE_VQ_MAX;
+	else if (b == SVE_VQ_MAX - 1)
+		/* No virtualisable VLs */
+		info->max_virtualisable_vl = -1;
+	else
+		info->max_virtualisable_vl = sve_vl_from_vq(__bit_to_vq(b +  1));
+
+	if (info->max_virtualisable_vl > info->max_vl)
+		info->max_virtualisable_vl = info->max_vl;
+
 	pr_info("SME: minimum available vector length %u bytes per vector\n",
 		info->min_vl);
 	pr_info("SME: maximum available vector length %u bytes per vector\n",
 		info->max_vl);
 	pr_info("SME: default vector length %u bytes per vector\n",
 		get_sme_default_vl());
+
+	/* KVM decides whether to support mismatched systems. Just warn here: */
+	if (info->max_virtualisable_vl < info->max_vl)
+		pr_warn("SME: unvirtualisable vector lengths present\n");
 }
 
 void sme_suspend_exit(void)

-- 
2.47.3