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>

1 files changed, 23 insertions, 4 deletions

diff --git a/arch/mips/kvm/kvm_trap_emul.c b/arch/mips/kvm/kvm_trap_emul.c
index f1e8389f8d33..9908f2b0ff46 100644
--- a/arch/mips/kvm/kvm_trap_emul.c
+++ b/arch/mips/kvm/kvm_trap_emul.c
@@ -407,8 +407,7 @@ static int kvm_trap_emul_get_one_reg(struct kvm_vcpu *vcpu,
 {
 	switch (reg->id) {
 	case KVM_REG_MIPS_CP0_COUNT:
-		/* XXXKYMA: Run the Guest count register @ 1/4 the rate of the host */
-		*v = (read_c0_count() >> 2);
+		*v = kvm_mips_read_count(vcpu);
 		break;
 	default:
 		return -EINVAL;
@@ -424,10 +423,30 @@ static int kvm_trap_emul_set_one_reg(struct kvm_vcpu *vcpu,
 
 	switch (reg->id) {
 	case KVM_REG_MIPS_CP0_COUNT:
-		/* Not supported yet */
+		kvm_mips_write_count(vcpu, v);
 		break;
 	case KVM_REG_MIPS_CP0_COMPARE:
-		kvm_write_c0_guest_compare(cop0, v);
+		kvm_mips_write_compare(vcpu, v);
+		break;
+	case KVM_REG_MIPS_CP0_CAUSE:
+		/*
+		 * If the timer is stopped or started (DC bit) it must look
+		 * atomic with changes to the interrupt pending bits (TI, IRQ5).
+		 * A timer interrupt should not happen in between.
+		 */
+		if ((kvm_read_c0_guest_cause(cop0) ^ v) & CAUSEF_DC) {
+			if (v & CAUSEF_DC) {
+				/* disable timer first */
+				kvm_mips_count_disable_cause(vcpu);
+				kvm_change_c0_guest_cause(cop0, ~CAUSEF_DC, v);
+			} else {
+				/* enable timer last */
+				kvm_change_c0_guest_cause(cop0, ~CAUSEF_DC, v);
+				kvm_mips_count_enable_cause(vcpu);
+			}
+		} else {
+			kvm_write_c0_guest_cause(cop0, v);
+		}
 		break;
 	default:
 		return -EINVAL;