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

[ 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@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>
[Update for rework of flags storage -- broonie]
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 181e49120e0c4027aa52dd389de13f9ce5cd7b57..757f4dea1e563657eb5c79e624e4b91f514a113a 100644 (file)
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -556,8 +556,6 @@ struct kvm_vcpu_arch {
 /* Save TRBE context if active  */
 #define DEBUG_STATE_SAVE_TRBE  __vcpu_single_flag(iflags, BIT(6))
 
-/* 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 8d073a37c266db3dc2726c6a0bb39c7e2586f53f..df050e4d3562d2ef36b7c27602a6feaa431dfa93 100644 (file)
--- a/arch/arm64/kvm/fpsimd.c
+++ b/arch/arm64/kvm/fpsimd.c
@@ -87,21 +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;
-
-       /*
-        * We don't currently support SME guests but if we leave
-        * things in streaming mode then when the guest starts running
-        * FPSIMD or SVE code it may generate SME traps so as a
-        * special case if we are in streaming mode we force the host
-        * state to be saved now and exit streaming mode so that we
-        * don't have to handle any SME traps for valid guest
-        * operations. Do this for ZA as well for now for simplicity.
-        */
-       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);
-       }
 }
 
 /*
@@ -162,22 +147,6 @@ 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
-        * CPACR_EL1_DEFAULT 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);
-       }
-
        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);