MIPS: KVM: Rewrite count/compare timer emulation

Previously the emulation of the CPU timer was just enough to get a Linux
guest running but some shortcuts were taken:
 - The guest timer interrupt was hard coded to always happen every 10 ms
   rather than being timed to when CP0_Count would match CP0_Compare.
 - The guest's CP0_Count register was based on the host's CP0_Count
   register. This isn't very portable and fails on cores without a
   CP_Count register implemented such as Ingenic XBurst. It also meant
   that the guest's CP0_Cause.DC bit to disable the CP0_Count register
   took no effect.
 - The guest's CP0_Count register was emulated by just dividing the
   host's CP0_Count register by 4. This resulted in continuity problems
   when used as a clock source, since when the host CP0_Count overflows
   from 0x7fffffff to 0x80000000, the guest CP0_Count transitions
   discontinuously from 0x1fffffff to 0xe0000000.

Therefore rewrite & fix emulation of the guest timer based on the
monotonic kernel time (i.e. ktime_get()). Internally a 32-bit count_bias
value is added to the frequency scaled nanosecond monotonic time to get
the guest's CP0_Count. The frequency of the timer is initialised to
100MHz and cannot yet be changed, but a later patch will allow the
frequency to be configured via the KVM_{GET,SET}_ONE_REG ioctl
interface.

The timer can now be stopped via the CP0_Cause.DC bit (by the guest or
via the KVM_SET_ONE_REG ioctl interface), at which point the current
CP0_Count is stored and can be read directly. When it is restarted the
bias is recalculated such that the CP0_Count value is continuous.

Due to the nature of hrtimer interrupts any read of the guest's
CP0_Count register while it is running triggers a check for whether the
hrtimer has expired, so that the guest/userland cannot observe the
CP0_Count passing CP0_Compare without queuing a timer interrupt. This is
also taken advantage of when stopping the timer to ensure that a pending
timer interrupt is queued.

This replaces the implementation of:
 - Guest read of CP0_Count
 - Guest write of CP0_Count
 - Guest write of CP0_Compare
 - Guest write of CP0_Cause
 - Guest read of HWR 2 (CC) with RDHWR
 - Host read of CP0_Count via KVM_GET_ONE_REG ioctl interface
 - Host write of CP0_Count via KVM_SET_ONE_REG ioctl interface
 - Host write of CP0_Compare via KVM_SET_ONE_REG ioctl interface
 - Host write of CP0_Cause via KVM_SET_ONE_REG ioctl interface

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Gleb Natapov <gleb@kernel.org>
Cc: kvm@vger.kernel.org
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: Sanjay Lal <sanjayl@kymasys.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/mips/include/asm/kvm_host.h

@@ -404,8 +404,15 @@ struct kvm_vcpu_arch {
 
 	u32 io_gpr;		/* GPR used as IO source/target */
 
-	/* Used to calibrate the virutal count register for the guest */
-	int32_t host_cp0_count;
+	struct hrtimer comparecount_timer;
+	/* Count bias from the raw time */
+	uint32_t count_bias;
+	/* Frequency of timer in Hz */
+	uint32_t count_hz;
+	/* Dynamic nanosecond bias (multiple of count_period) to avoid overflow */
+	s64 count_dyn_bias;
+	/* Period of timer tick in ns */
+	u64 count_period;
 
 	/* Bitmask of exceptions that are pending */
 	unsigned long pending_exceptions;
@@ -426,8 +433,6 @@ struct kvm_vcpu_arch {
 	uint32_t guest_kernel_asid[NR_CPUS];
 	struct mm_struct guest_kernel_mm, guest_user_mm;
 
-	struct hrtimer comparecount_timer;
-
 	int last_sched_cpu;
 
 	/* WAIT executed */
@@ -705,7 +710,13 @@ extern enum emulation_result kvm_mips_emulate_bp_exc(unsigned long cause,
 extern enum emulation_result kvm_mips_complete_mmio_load(struct kvm_vcpu *vcpu,
 							 struct kvm_run *run);
 
-enum emulation_result kvm_mips_emulate_count(struct kvm_vcpu *vcpu);
+uint32_t kvm_mips_read_count(struct kvm_vcpu *vcpu);
+void kvm_mips_write_count(struct kvm_vcpu *vcpu, uint32_t count);
+void kvm_mips_write_compare(struct kvm_vcpu *vcpu, uint32_t compare);
+void kvm_mips_init_count(struct kvm_vcpu *vcpu);
+void kvm_mips_count_enable_cause(struct kvm_vcpu *vcpu);
+void kvm_mips_count_disable_cause(struct kvm_vcpu *vcpu);
+enum hrtimer_restart kvm_mips_count_timeout(struct kvm_vcpu *vcpu);
 
 enum emulation_result kvm_mips_check_privilege(unsigned long cause,
 					       uint32_t *opc,

arch/mips/kvm/kvm_mips.c

@@ -363,7 +363,7 @@ struct kvm_vcpu *kvm_arch_vcpu_create(struct kvm *kvm, unsigned int id)
 	vcpu->arch.last_sched_cpu = -1;
 
 	/* Start off the timer */
-	kvm_mips_emulate_count(vcpu);
+	kvm_mips_init_count(vcpu);
 
 	return vcpu;
 
@@ -707,9 +707,6 @@ static int kvm_mips_set_reg(struct kvm_vcpu *vcpu,
 	case KVM_REG_MIPS_CP0_STATUS:
 		kvm_write_c0_guest_status(cop0, v);
 		break;
-	case KVM_REG_MIPS_CP0_CAUSE:
-		kvm_write_c0_guest_cause(cop0, v);
-		break;
 	case KVM_REG_MIPS_CP0_EPC:
 		kvm_write_c0_guest_epc(cop0, v);
 		break;
@@ -719,6 +716,7 @@ static int kvm_mips_set_reg(struct kvm_vcpu *vcpu,
 	/* registers to be handled specially */
 	case KVM_REG_MIPS_CP0_COUNT:
 	case KVM_REG_MIPS_CP0_COMPARE:
+	case KVM_REG_MIPS_CP0_CAUSE:
 		return kvm_mips_callbacks->set_one_reg(vcpu, reg, v);
 	default:
 		return -EINVAL;
@@ -992,9 +990,7 @@ enum hrtimer_restart kvm_mips_comparecount_wakeup(struct hrtimer *timer)
 
 	vcpu = container_of(timer, struct kvm_vcpu, arch.comparecount_timer);
 	kvm_mips_comparecount_func((unsigned long) vcpu);
-	hrtimer_forward_now(&vcpu->arch.comparecount_timer,
-			    ktime_set(0, MS_TO_NS(10)));
-	return HRTIMER_RESTART;
+	return kvm_mips_count_timeout(vcpu);
 }
 
 int kvm_arch_vcpu_init(struct kvm_vcpu *vcpu)