linux-kernel - [PATCH RFC v3 02/27] arm64/fpsimd: Decide to save ZT0 and streaming mode FFR at bind time

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Message-Id: <20241220-kvm-arm64-sme-v3-2-05b018c1ffeb@kernel.org>
Date: Fri, 20 Dec 2024 16:46:27 +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>, 
 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 RFC v3 02/27] arm64/fpsimd: Decide to save ZT0 and
 streaming mode FFR at bind time

Some parts of the SME state are optional, enabled by additional features
on top of the base FEAT_SME and controlled with enable bits in SMCR_ELx. We
unconditionally enable these for the host but for KVM we will allow the
feature set exposed to guests to be restricted by the VMM. These are the
FFR register (FEAT_SME_FA64) and ZT0 (FEAT_SME2).

We defer saving of guest floating point state for non-protected guests to
the host kernel. We also want to avoid having to reconfigure the guest
floating point state if nothing used the floating point state while running
the host. If the guest was running with the optional features disabled then
traps will be enabled for them so the host kernel will need to skip
accessing that state when saving state for the guest.

Support this by moving the decision about saving this state to the point
where we bind floating point state to the CPU, adding a new variable to
the cpu_fp_state which uses the enable bits in SMCR_ELx to flag which
features are enabled.

Signed-off-by: Mark Brown <broonie@...nel.org>
---
 arch/arm64/include/asm/fpsimd.h |  1 +
 arch/arm64/kernel/fpsimd.c      | 10 ++++++++--
 arch/arm64/kvm/fpsimd.c         |  1 +
 3 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/fpsimd.h b/arch/arm64/include/asm/fpsimd.h
index 95355892d47b3ec1c77a3ab19ccad0d7f9a8d621..144cc805bfea112341b89c9c6028cf4b2a201c6c 100644
--- a/arch/arm64/include/asm/fpsimd.h
+++ b/arch/arm64/include/asm/fpsimd.h
@@ -88,6 +88,7 @@ struct cpu_fp_state {
 	void *sme_state;
 	u64 *svcr;
 	u64 *fpmr;
+	u64 sme_features;
 	unsigned int sve_vl;
 	unsigned int sme_vl;
 	enum fp_type *fp_type;
diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index 92c085288ed2cbc4f51f49546c6abbde6ba891a3..7c66ed6e43c34d1b5e1cc00595c12244d13d3d0d 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -478,12 +478,12 @@ static void fpsimd_save_user_state(void)
 
 		if (*svcr & SVCR_ZA_MASK)
 			sme_save_state(last->sme_state,
-				       system_supports_sme2());
+				       last->sme_features & SMCR_ELx_EZT0);
 
 		/* If we are in streaming mode override regular SVE. */
 		if (*svcr & SVCR_SM_MASK) {
 			save_sve_regs = true;
-			save_ffr = system_supports_fa64();
+			save_ffr = last->sme_features & SMCR_ELx_FA64;
 			vl = last->sme_vl;
 		}
 	}
@@ -1722,6 +1722,12 @@ static void fpsimd_bind_task_to_cpu(void)
 	last->to_save = FP_STATE_CURRENT;
 	current->thread.fpsimd_cpu = smp_processor_id();
 
+	last->sme_features = 0;
+	if (system_supports_fa64())
+		last->sme_features |= SMCR_ELx_FA64;
+	if (system_supports_sme2())
+		last->sme_features |= SMCR_ELx_EZT0;
+
 	/*
 	 * Toggle SVE and SME trapping for userspace if needed, these
 	 * are serialsied by ret_to_user().
diff --git a/arch/arm64/kvm/fpsimd.c b/arch/arm64/kvm/fpsimd.c
index ea5484ce1f3ba3121b6938bda15f7a8057d49051..09b65abaf9db60cc57dbc554ad2108a80c2dc46b 100644
--- a/arch/arm64/kvm/fpsimd.c
+++ b/arch/arm64/kvm/fpsimd.c
@@ -138,6 +138,7 @@ void kvm_arch_vcpu_ctxsync_fp(struct kvm_vcpu *vcpu)
 		fp_state.svcr = &__vcpu_sys_reg(vcpu, SVCR);
 		fp_state.fpmr = &__vcpu_sys_reg(vcpu, FPMR);
 		fp_state.fp_type = &vcpu->arch.fp_type;
+		fp_state.sme_features = 0;
 
 		if (vcpu_has_sve(vcpu))
 			fp_state.to_save = FP_STATE_SVE;

-- 
2.39.5