KVM: arm64: Remove VHE host restore of CPACR_EL1.SMEN

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@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Tested-by: Mark Brown <broonie@kernel.org>
Acked-by: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Fuad Tabba <tabba@google.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Oliver Upton <oliver.upton@linux.dev>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20250210195226.1215254-5-mark.rutland@arm.com
Signed-off-by: Marc Zyngier <maz@kernel.org>

2 files changed, 0 insertions, 22 deletions

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index ed6841bf21b2..c77acc990457 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -615,7 +615,6 @@ struct cpu_sve_state {
 struct kvm_host_data {
 #define KVM_HOST_DATA_FLAG_HAS_SPE			0
 #define KVM_HOST_DATA_FLAG_HAS_TRBE			1
-#define KVM_HOST_DATA_FLAG_HOST_SME_ENABLED		3
 #define KVM_HOST_DATA_FLAG_TRBE_ENABLED			4
 #define KVM_HOST_DATA_FLAG_EL1_TRACING_CONFIGURED	5
 	unsigned long flags;
diff --git a/arch/arm64/kvm/fpsimd.c b/arch/arm64/kvm/fpsimd.c
index 4ff0dee1a403..f64724197958 100644
--- a/arch/arm64/kvm/fpsimd.c
+++ b/arch/arm64/kvm/fpsimd.c
@@ -65,12 +65,6 @@ void kvm_arch_vcpu_load_fp(struct kvm_vcpu *vcpu)
 	fpsimd_save_and_flush_cpu_state();
 	*host_data_ptr(fp_owner) = FP_STATE_FREE;
 
-	if (system_supports_sme()) {
-		host_data_clear_flag(HOST_SME_ENABLED);
-		if (read_sysreg(cpacr_el1) & CPACR_EL1_SMEN_EL0EN)
-			host_data_set_flag(HOST_SME_ENABLED);
-	}
-
 	/*
 	 * If normal guests gain SME support, maintain this behavior for pKVM
 	 * guests, which don't support SME.
@@ -141,21 +135,6 @@ 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 (host_data_test_flag(HOST_SME_ENABLED))
-			sysreg_clear_set(CPACR_EL1, 0, CPACR_EL1_SMEN);
-		else
-			sysreg_clear_set(CPACR_EL1,
-					 CPACR_EL1_SMEN_EL0EN,
-					 CPACR_EL1_SMEN_EL1EN);
-		isb();
-	}
-
 	if (guest_owns_fp_regs()) {
 		if (vcpu_has_sve(vcpu)) {
 			u64 zcr = read_sysreg_el1(SYS_ZCR);