linux-kernel - [PATCH v4 20/27] KVM: arm64: Support userspace access to streaming mode Z and P registers

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Message-Id: <20250214-kvm-arm64-sme-v4-20-d64a681adcc2@kernel.org>
Date: Fri, 14 Feb 2025 01:58:03 +0000
From: Mark Brown <broonie@...nel.org>
To: Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>, 
 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>
Cc: Dave Martin <Dave.Martin@....com>, Fuad Tabba <tabba@...gle.com>, 
 Mark Rutland <mark.rutland@....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, 
 Mark Brown <broonie@...nel.org>
Subject: [PATCH v4 20/27] KVM: arm64: Support userspace access to streaming
 mode Z and P registers

SME introduces a mode called streaming mode where the Z, P and optionally
FFR registers can be accessed using the SVE instructions but with the SME
vector length. Reflect this in the ABI for accessing the guest registers by
making the vector length for the vcpu reflect the vector length that would
be seen by the guest were it running, using the SME vector length when the
guest is configured for streaming mode.

Since SME may be present without SVE we also update the existing checks for
access to the Z, P and V registers to check for either SVE or streaming
mode. When not in streaming mode the guest floating point state may be
accessed via the V registers.

Any VMM that supports SME must be aware of the need to configure streaming
mode prior to writing the floating point registers that this creates.

Signed-off-by: Mark Brown <broonie@...nel.org>
---
 arch/arm64/include/asm/kvm_host.h |  3 +++
 arch/arm64/kvm/guest.c            | 38 ++++++++++++++++++++++++++++++++++----
 2 files changed, 37 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index acd243a31ccb77cce3d95829e22456a71e87725c..67ac3e37fde31901513c2e692693d3ba1967b0bd 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -1600,4 +1600,7 @@ void kvm_set_vm_id_reg(struct kvm *kvm, u32 reg, u64 val);
 #define vcpu_has_fa64(vcpu)	kvm_has_fa64((vcpu)->kvm)
 #endif
 
+#define vcpu_in_streaming_mode(vcpu) \
+	(__vcpu_sys_reg(vcpu, SVCR) & SVCR_SM_MASK)
+
 #endif /* __ARM64_KVM_HOST_H__ */
diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
index 62df7830d11e6e7d01d175566e4b6a771ac2ec6d..87a0a027e1e7e2487eff703dee2958137c9640ef 100644
--- a/arch/arm64/kvm/guest.c
+++ b/arch/arm64/kvm/guest.c
@@ -73,6 +73,11 @@ static u64 core_reg_offset_from_id(u64 id)
 	return id & ~(KVM_REG_ARCH_MASK | KVM_REG_SIZE_MASK | KVM_REG_ARM_CORE);
 }
 
+static bool vcpu_has_sve_regs(const struct kvm_vcpu *vcpu)
+{
+	return vcpu_has_sve(vcpu) || vcpu_in_streaming_mode(vcpu);
+}
+
 static int core_reg_size_from_offset(const struct kvm_vcpu *vcpu, u64 off)
 {
 	int size;
@@ -110,9 +115,10 @@ static int core_reg_size_from_offset(const struct kvm_vcpu *vcpu, u64 off)
 	/*
 	 * The KVM_REG_ARM64_SVE regs must be used instead of
 	 * KVM_REG_ARM_CORE for accessing the FPSIMD V-registers on
-	 * SVE-enabled vcpus:
+	 * SVE-enabled vcpus or when a SME enabled vcpu is in
+	 * streaming mode:
 	 */
-	if (vcpu_has_sve(vcpu) && core_reg_offset_is_vreg(off))
+	if (vcpu_has_sve_regs(vcpu) && core_reg_offset_is_vreg(off))
 		return -EINVAL;
 
 	return size;
@@ -426,6 +432,24 @@ struct vec_state_reg_region {
 	unsigned int upad;	/* extra trailing padding in user memory */
 };
 
+/*
+ * We represent the Z and P registers to userspace using either the
+ * SVE or SME vector length, depending on which features the guest has
+ * and if the guest is in streaming mode.
+ */
+static unsigned int vcpu_sve_cur_vq(struct kvm_vcpu *vcpu)
+{
+	unsigned int vq = 0;
+
+	if (vcpu_has_sve(vcpu))
+		vq = vcpu_sve_max_vq(vcpu);
+
+	if (vcpu_in_streaming_mode(vcpu))
+		vq = vcpu_sme_max_vq(vcpu);
+
+	return vq;
+}
+
 /*
  * Validate SVE register ID and get sanitised bounds for user/kernel SVE
  * register copy
@@ -466,7 +490,7 @@ static int sve_reg_to_region(struct vec_state_reg_region *region,
 		if (!vcpu_has_sve(vcpu) || (reg->id & SVE_REG_SLICE_MASK) > 0)
 			return -ENOENT;
 
-		vq = vcpu_sve_max_vq(vcpu);
+		vq = vcpu_sve_cur_vq(vcpu);
 
 		reqoffset = SVE_SIG_ZREG_OFFSET(vq, reg_num) -
 				SVE_SIG_REGS_OFFSET;
@@ -476,7 +500,7 @@ static int sve_reg_to_region(struct vec_state_reg_region *region,
 		if (!vcpu_has_sve(vcpu) || (reg->id & SVE_REG_SLICE_MASK) > 0)
 			return -ENOENT;
 
-		vq = vcpu_sve_max_vq(vcpu);
+		vq = vcpu_sve_cur_vq(vcpu);
 
 		reqoffset = SVE_SIG_PREG_OFFSET(vq, reg_num) -
 				SVE_SIG_REGS_OFFSET;
@@ -515,6 +539,9 @@ static int get_sve_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
 	if (!kvm_arm_vcpu_vec_finalized(vcpu))
 		return -EPERM;
 
+	if (!vcpu_has_sve_regs(vcpu))
+		return -EBUSY;
+
 	if (copy_to_user(uptr, vcpu->arch.sve_state + region.koffset,
 			 region.klen) ||
 	    clear_user(uptr + region.klen, region.upad))
@@ -541,6 +568,9 @@ static int set_sve_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
 	if (!kvm_arm_vcpu_vec_finalized(vcpu))
 		return -EPERM;
 
+	if (!vcpu_has_sve_regs(vcpu))
+		return -EBUSY;
+
 	if (copy_from_user(vcpu->arch.sve_state + region.koffset, uptr,
 			   region.klen))
 		return -EFAULT;

-- 
2.39.5