linux-kernel - [PATCH 6.6 5/8] KVM: arm64: Remove VHE host restore of CPACR

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Message-Id: <20250321-stable-sve-6-6-v1-5-0b3a6a14ea53@kernel.org>
Date: Fri, 21 Mar 2025 00:16:05 +0000
From: Mark Brown <broonie@...nel.org>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
 Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>, 
 James Morse <james.morse@....com>, 
 Suzuki K Poulose <suzuki.poulose@....com>, 
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>
Cc: linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev, 
 linux-kernel@...r.kernel.org, stable@...r.kernel.org, 
 Mark Brown <broonie@...nel.org>, Mark Rutland <mark.rutland@....com>, 
 Fuad Tabba <tabba@...gle.com>
Subject: [PATCH 6.6 5/8] KVM: arm64: Remove VHE host restore of
 CPACR_EL1.SMEN

From: Mark Rutland <mark.rutland@....com>

[ Upstream commit 407a99c4654e8ea65393f412c421a55cac539f5b ]

When KVM is in VHE mode, the host kernel tries to save and restore the
configuration of CPACR_EL1.SMEN (i.e. CPTR_EL2.SMEN when HCR_EL2.E2H=1)
across kvm_arch_vcpu_load_fp() and kvm_arch_vcpu_put_fp(), since the
configuration may be clobbered by hyp when running a vCPU. This logic
has historically been broken, and is currently redundant.

This logic was originally introduced in commit:

  861262ab86270206 ("KVM: arm64: Handle SME host state when running guests")

At the time, the VHE hyp code would reset CPTR_EL2.SMEN to 0b00 when
returning to the host, trapping host access to SME state. Unfortunately,
this was unsafe as the host could take a softirq before calling
kvm_arch_vcpu_put_fp(), and if a softirq handler were to use kernel mode
NEON the resulting attempt to save the live FPSIMD/SVE/SME state would
result in a fatal trap.

That issue was limited to VHE mode. For nVHE/hVHE modes, KVM always
saved/restored the host kernel's CPACR_EL1 value, and configured
CPTR_EL2.TSM to 0b0, ensuring that host usage of SME would not be
trapped.

The issue above was incidentally fixed by commit:

  375110ab51dec5dc ("KVM: arm64: Fix resetting SME trap values on reset for (h)VHE")

That commit changed the VHE hyp code to configure CPTR_EL2.SMEN to 0b01
when returning to the host, permitting host kernel usage of SME,
avoiding the issue described above. At the time, this was not identified
as a fix for commit 861262ab86270206.

Now that the host eagerly saves and unbinds its own FPSIMD/SVE/SME
state, there's no need to save/restore the state of the EL0 SME trap.
The kernel can safely save/restore state without trapping, as described
above, and will restore userspace state (including trap controls) before
returning to userspace.

Remove the redundant logic.

Signed-off-by: Mark Rutland <mark.rutland@....com>
Reviewed-by: Mark Brown <broonie@...nel.org>
Tested-by: Mark Brown <broonie@...nel.org>
Acked-by: Will Deacon <will@...nel.org>
Cc: Catalin Marinas <catalin.marinas@....com>
Cc: Fuad Tabba <tabba@...gle.com>
Cc: Marc Zyngier <maz@...nel.org>
Cc: Oliver Upton <oliver.upton@...ux.dev>
Reviewed-by: Oliver Upton <oliver.upton@...ux.dev>
Link: https://lore.kernel.org/r/20250210195226.1215254-5-mark.rutland@arm.com
Signed-off-by: Marc Zyngier <maz@...nel.org>
[Update for rework of flags storage -- broonie]
Signed-off-by: Mark Brown <broonie@...nel.org>
---
 arch/arm64/include/asm/kvm_host.h |  2 --
 arch/arm64/kvm/fpsimd.c           | 23 -----------------------
 2 files changed, 25 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 9c1056d6f6859522dcee4b8c4e4804a7abac93d5..27c53b603d693d1cfe8d81f94a59d3afb665be8e 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -717,8 +717,6 @@ struct kvm_vcpu_arch {
 /* vcpu running in HYP context */
 #define VCPU_HYP_CONTEXT	__vcpu_single_flag(iflags, BIT(7))
 
-/* SME enabled for EL0 */
-#define HOST_SME_ENABLED	__vcpu_single_flag(sflags, BIT(1))
 /* Physical CPU not in supported_cpus */
 #define ON_UNSUPPORTED_CPU	__vcpu_single_flag(sflags, BIT(2))
 /* WFIT instruction trapped */
diff --git a/arch/arm64/kvm/fpsimd.c b/arch/arm64/kvm/fpsimd.c
index f1fe7abbcb83393e7c3402405bb7b1fdbf6024ff..0492a1fc718130934f8bada836fe42bea6db7abb 100644
--- a/arch/arm64/kvm/fpsimd.c
+++ b/arch/arm64/kvm/fpsimd.c
@@ -87,12 +87,6 @@ void kvm_arch_vcpu_load_fp(struct kvm_vcpu *vcpu)
 	 */
 	fpsimd_save_and_flush_cpu_state();
 	vcpu->arch.fp_state = FP_STATE_FREE;
-
-	if (system_supports_sme()) {
-		vcpu_clear_flag(vcpu, HOST_SME_ENABLED);
-		if (read_sysreg(cpacr_el1) & CPACR_EL1_SMEN_EL0EN)
-			vcpu_set_flag(vcpu, HOST_SME_ENABLED);
-	}
 }
 
 /*
@@ -157,23 +151,6 @@ void kvm_arch_vcpu_put_fp(struct kvm_vcpu *vcpu)
 
 	local_irq_save(flags);
 
-	/*
-	 * If we have VHE then the Hyp code will reset CPACR_EL1 to
-	 * the default value and we need to reenable SME.
-	 */
-	if (has_vhe() && system_supports_sme()) {
-		/* Also restore EL0 state seen on entry */
-		if (vcpu_get_flag(vcpu, HOST_SME_ENABLED))
-			sysreg_clear_set(CPACR_EL1, 0,
-					 CPACR_EL1_SMEN_EL0EN |
-					 CPACR_EL1_SMEN_EL1EN);
-		else
-			sysreg_clear_set(CPACR_EL1,
-					 CPACR_EL1_SMEN_EL0EN,
-					 CPACR_EL1_SMEN_EL1EN);
-		isb();
-	}
-
 	if (vcpu->arch.fp_state == FP_STATE_GUEST_OWNED) {
 		if (vcpu_has_sve(vcpu)) {
 			__vcpu_sys_reg(vcpu, ZCR_EL1) = read_sysreg_el1(SYS_ZCR);

-- 
2.39.5