Commit 407a99c4 authored by Mark Rutland's avatar Mark Rutland Committed by Marc Zyngier
Browse files

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:

  861262ab ("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:

  375110ab ("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 861262ab.

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: default avatarMark Rutland <mark.rutland@arm.com>
Reviewed-by: default avatarMark Brown <broonie@kernel.org>
Tested-by: default avatarMark Brown <broonie@kernel.org>
Acked-by: default avatarWill 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: default avatarOliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20250210195226.1215254-5-mark.rutland@arm.com


Signed-off-by: default avatarMarc Zyngier <maz@kernel.org>
parent 459f059b

arch/arm64/include/asm/kvm_host.h

+0 −1
Original line number Diff line number Diff line
@@ -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;

arch/arm64/kvm/fpsimd.c

+0 −21
Original line number Diff line number Diff line
@@ -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);