linux-kernel - [PATCH v4 15/16] arch/arm64: Introduce a capability to tell whether 16-bit VMID is available

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Date:   Wed, 14 Apr 2021 12:23:11 +0100
From:   Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>
To:     <linux-arm-kernel@...ts.infradead.org>,
        <kvmarm@...ts.cs.columbia.edu>, <linux-kernel@...r.kernel.org>
CC:     <maz@...nel.org>, <will@...nel.org>, <catalin.marinas@....com>,
        <james.morse@....com>, <julien.thierry.kdev@...il.com>,
        <suzuki.poulose@....com>, <jean-philippe@...aro.org>,
        <julien@....org>, <linuxarm@...wei.com>
Subject: [PATCH v4 15/16] arch/arm64: Introduce a capability to tell whether 16-bit VMID is available

From: Julien Grall <julien.grall@....com>

At the moment, the function kvm_get_vmid_bits() is looking up for the
sanitized value of ID_AA64MMFR1_EL1 and extract the information
regarding the number of VMID bits supported.

This is fine as the function is mainly used during VMID roll-over. New
use in a follow-up patch will require the function to be called a every
context switch so we want the function to be more efficient.

A new capability is introduced to tell whether 16-bit VMID is
available.

Signed-off-by: Julien Grall <julien.grall@....com>
---
 arch/arm64/include/asm/cpucaps.h | 3 ++-
 arch/arm64/include/asm/kvm_mmu.h | 4 +---
 arch/arm64/kernel/cpufeature.c   | 9 +++++++++
 3 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/asm/cpucaps.h b/arch/arm64/include/asm/cpucaps.h
index c40f2490cd7b..acb92da5c254 100644
--- a/arch/arm64/include/asm/cpucaps.h
+++ b/arch/arm64/include/asm/cpucaps.h
@@ -67,7 +67,8 @@
 #define ARM64_HAS_LDAPR				59
 #define ARM64_KVM_PROTECTED_MODE		60
 #define ARM64_WORKAROUND_NVIDIA_CARMEL_CNP	61
+#define ARM64_HAS_16BIT_VMID			62
 
-#define ARM64_NCAPS				62
+#define ARM64_NCAPS				63
 
 #endif /* __ASM_CPUCAPS_H */
diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
index 90873851f677..c3080966ef83 100644
--- a/arch/arm64/include/asm/kvm_mmu.h
+++ b/arch/arm64/include/asm/kvm_mmu.h
@@ -213,9 +213,7 @@ void kvm_toggle_cache(struct kvm_vcpu *vcpu, bool was_enabled);
 
 static inline unsigned int kvm_get_vmid_bits(void)
 {
-	int reg = read_sanitised_ftr_reg(SYS_ID_AA64MMFR1_EL1);
-
-	return get_vmid_bits(reg);
+	return cpus_have_const_cap(ARM64_HAS_16BIT_VMID) ? 16 : 8;
 }
 
 /*
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index e5281e1c8f1d..ff956fb2f712 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -2203,6 +2203,15 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
 		.matches = has_cpuid_feature,
 		.min_field_value = 1,
 	},
+	{
+		.capability = ARM64_HAS_16BIT_VMID,
+		.type = ARM64_CPUCAP_SYSTEM_FEATURE,
+		.sys_reg = SYS_ID_AA64MMFR1_EL1,
+		.field_pos = ID_AA64MMFR1_VMIDBITS_SHIFT,
+		.sign = FTR_UNSIGNED,
+		.min_field_value = ID_AA64MMFR1_VMIDBITS_16,
+		.matches = has_cpuid_feature,
+	},
 	{},
 };
 
-- 
2.17.1