arm64: KVM: Add ARCH_WORKAROUND_2 support for guests

In order to offer ARCH_WORKAROUND_2 support to guests, we need
a bit of infrastructure.

Let's add a flag indicating whether or not the guest uses
SSBD mitigation. Depending on the state of this flag, allow
KVM to disable ARCH_WORKAROUND_2 before entering the guest,
and enable it when exiting it.
Reviewed-by: Christoffer Dall <christoffer.dall@arm.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm/include/asm/kvm_mmu.h

@@ -319,6 +319,11 @@ static inline int kvm_map_vectors(void)
 	return 0;
 }
 
+static inline int hyp_map_aux_data(void)
+{
+	return 0;
+}
+
 #define kvm_phys_to_vttbr(addr)		(addr)
 
 #endif	/* !__ASSEMBLY__ */

arch/arm64/include/asm/kvm_asm.h

@@ -33,6 +33,9 @@
 #define KVM_ARM64_DEBUG_DIRTY_SHIFT	0
 #define KVM_ARM64_DEBUG_DIRTY		(1 << KVM_ARM64_DEBUG_DIRTY_SHIFT)
 
+#define	VCPU_WORKAROUND_2_FLAG_SHIFT	0
+#define	VCPU_WORKAROUND_2_FLAG		(_AC(1, UL) << VCPU_WORKAROUND_2_FLAG_SHIFT)
+
 /* Translate a kernel address of @sym into its equivalent linear mapping */
 #define kvm_ksym_ref(sym)						\
 	({								\

arch/arm64/include/asm/kvm_host.h

@@ -216,6 +216,9 @@ struct kvm_vcpu_arch {
 	/* Exception Information */
 	struct kvm_vcpu_fault_info fault;
 
+	/* State of various workarounds, see kvm_asm.h for bit assignment */
+	u64 workaround_flags;
+
 	/* Guest debug state */
 	u64 debug_flags;
 

arch/arm64/include/asm/kvm_mmu.h

@@ -456,6 +456,30 @@ static inline int kvm_map_vectors(void)
 }
 #endif
 
+#ifdef CONFIG_ARM64_SSBD
+DECLARE_PER_CPU_READ_MOSTLY(u64, arm64_ssbd_callback_required);
+
+static inline int hyp_map_aux_data(void)
+{
+	int cpu, err;
+
+	for_each_possible_cpu(cpu) {
+		u64 *ptr;
+
+		ptr = per_cpu_ptr(&arm64_ssbd_callback_required, cpu);
+		err = create_hyp_mappings(ptr, ptr + 1, PAGE_HYP);
+		if (err)
+			return err;
+	}
+	return 0;
+}
+#else
+static inline int hyp_map_aux_data(void)
+{
+	return 0;
+}
+#endif
+
 #define kvm_phys_to_vttbr(addr)		phys_to_ttbr(addr)
 
 #endif /* __ASSEMBLY__ */

arch/arm64/kvm/hyp/switch.c

@@ -15,6 +15,7 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+#include <linux/arm-smccc.h>
 #include <linux/types.h>
 #include <linux/jump_label.h>
 #include <uapi/linux/psci.h>
@@ -389,6 +390,39 @@ static bool __hyp_text fixup_guest_exit(struct kvm_vcpu *vcpu, u64 *exit_code)
 	return false;
 }
 
+static inline bool __hyp_text __needs_ssbd_off(struct kvm_vcpu *vcpu)
+{
+	if (!cpus_have_const_cap(ARM64_SSBD))
+		return false;
+
+	return !(vcpu->arch.workaround_flags & VCPU_WORKAROUND_2_FLAG);
+}
+
+static void __hyp_text __set_guest_arch_workaround_state(struct kvm_vcpu *vcpu)
+{
+#ifdef CONFIG_ARM64_SSBD
+	/*
+	 * The host runs with the workaround always present. If the
+	 * guest wants it disabled, so be it...
+	 */
+	if (__needs_ssbd_off(vcpu) &&
+	    __hyp_this_cpu_read(arm64_ssbd_callback_required))
+		arm_smccc_1_1_smc(ARM_SMCCC_ARCH_WORKAROUND_2, 0, NULL);
+#endif
+}
+
+static void __hyp_text __set_host_arch_workaround_state(struct kvm_vcpu *vcpu)
+{
+#ifdef CONFIG_ARM64_SSBD
+	/*
+	 * If the guest has disabled the workaround, bring it back on.
+	 */
+	if (__needs_ssbd_off(vcpu) &&
+	    __hyp_this_cpu_read(arm64_ssbd_callback_required))
+		arm_smccc_1_1_smc(ARM_SMCCC_ARCH_WORKAROUND_2, 1, NULL);
+#endif
+}
+
 /* Switch to the guest for VHE systems running in EL2 */
 int kvm_vcpu_run_vhe(struct kvm_vcpu *vcpu)
 {
@@ -409,6 +443,8 @@ int kvm_vcpu_run_vhe(struct kvm_vcpu *vcpu)
 	sysreg_restore_guest_state_vhe(guest_ctxt);
 	__debug_switch_to_guest(vcpu);
 
+	__set_guest_arch_workaround_state(vcpu);
+
 	do {
 		/* Jump in the fire! */
 		exit_code = __guest_enter(vcpu, host_ctxt);
@@ -416,6 +452,8 @@ int kvm_vcpu_run_vhe(struct kvm_vcpu *vcpu)
 		/* And we're baaack! */
 	} while (fixup_guest_exit(vcpu, &exit_code));
 
+	__set_host_arch_workaround_state(vcpu);
+
 	fp_enabled = fpsimd_enabled_vhe();
 
 	sysreg_save_guest_state_vhe(guest_ctxt);
@@ -465,6 +503,8 @@ int __hyp_text __kvm_vcpu_run_nvhe(struct kvm_vcpu *vcpu)
 	__sysreg_restore_state_nvhe(guest_ctxt);
 	__debug_switch_to_guest(vcpu);
 
+	__set_guest_arch_workaround_state(vcpu);
+
 	do {
 		/* Jump in the fire! */
 		exit_code = __guest_enter(vcpu, host_ctxt);
@@ -472,6 +512,8 @@ int __hyp_text __kvm_vcpu_run_nvhe(struct kvm_vcpu *vcpu)
 		/* And we're baaack! */
 	} while (fixup_guest_exit(vcpu, &exit_code));
 
+	__set_host_arch_workaround_state(vcpu);
+
 	fp_enabled = __fpsimd_enabled_nvhe();
 
 	__sysreg_save_state_nvhe(guest_ctxt);

virt/kvm/arm/arm.c

@@ -1490,6 +1490,10 @@ static int init_hyp_mode(void)
 		}
 	}
 
+	err = hyp_map_aux_data();
+	if (err)
+		kvm_err("Cannot map host auxilary data: %d\n", err);
+
 	return 0;
 
 out_err: