KVM: PPC: Book3S HV: Enable migration of decrementer register

This adds a register identifier for use with the one_reg interface
to allow the decrementer expiry time to be read and written by
userspace.  The decrementer expiry time is in guest timebase units
and is equal to the sum of the decrementer and the guest timebase.
(The expiry time is used rather than the decrementer value itself
because the expiry time is not constantly changing, though the
decrementer value is, while the guest vcpu is not running.)

Without this, a guest vcpu migrated to a new host will see its
decrementer set to some random value.  On POWER8 and earlier, the
decrementer is 32 bits wide and counts down at 512MHz, so the
guest vcpu will potentially see no decrementer interrupts for up
to about 4 seconds, which will lead to a stall.  With POWER9, the
decrementer is now 56 bits side, so the stall can be much longer
(up to 2.23 years) and more noticeable.

To help work around the problem in cases where userspace has not been
updated to migrate the decrementer expiry time, we now set the
default decrementer expiry at vcpu creation time to the current time
rather than the maximum possible value.  This should mean an
immediate decrementer interrupt when a migrated vcpu starts
running.  In cases where the decrementer is 32 bits wide and more
than 4 seconds elapse between the creation of the vcpu and when it
first runs, the decrementer would have wrapped around to positive
values and there may still be a stall - but this is no worse than
the current situation.  In the large-decrementer case, we are sure
to get an immediate decrementer interrupt (assuming the time from
vcpu creation to first run is less than 2.23 years) and we thus
avoid a very long stall.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

4 files changed, 12 insertions, 1 deletions

diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
index f670e4b9e7f3..c6f9eebb79f2 100644
--- a/Documentation/virtual/kvm/api.txt
+++ b/Documentation/virtual/kvm/api.txt
@@ -1841,6 +1841,7 @@ registers, find a list below:
   PPC   | KVM_REG_PPC_DBSR              | 32
   PPC   | KVM_REG_PPC_TIDR              | 64
   PPC   | KVM_REG_PPC_PSSCR             | 64
+  PPC   | KVM_REG_PPC_DEC_EXPIRY        | 64
   PPC   | KVM_REG_PPC_TM_GPR0           | 64
           ...
   PPC   | KVM_REG_PPC_TM_GPR31          | 64
diff --git a/arch/powerpc/include/uapi/asm/kvm.h b/arch/powerpc/include/uapi/asm/kvm.h
index 61d6049f4c1e..8aaec831053a 100644
--- a/arch/powerpc/include/uapi/asm/kvm.h
+++ b/arch/powerpc/include/uapi/asm/kvm.h
@@ -607,6 +607,8 @@ struct kvm_ppc_rmmu_info {
 #define KVM_REG_PPC_TIDR        (KVM_REG_PPC | KVM_REG_SIZE_U64 | 0xbc)
 #define KVM_REG_PPC_PSSCR       (KVM_REG_PPC | KVM_REG_SIZE_U64 | 0xbd)
 
+#define KVM_REG_PPC_DEC_EXPIRY  (KVM_REG_PPC | KVM_REG_SIZE_U64 | 0xbe)
+
 /* Transactional Memory checkpointed state:
  * This is all GPRs, all VSX regs and a subset of SPRs
  */
diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index c4f0bebfc5ba..b2d448c75008 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -1497,6 +1497,10 @@ static int kvmppc_get_one_reg_hv(struct kvm_vcpu *vcpu, u64 id,
         case KVM_REG_PPC_ARCH_COMPAT:
                 *val = get_reg_val(id, vcpu->arch.vcore->arch_compat);
                 break;
+        case KVM_REG_PPC_DEC_EXPIRY:
+                *val = get_reg_val(id, vcpu->arch.dec_expires +
+                                   vcpu->arch.vcore->tb_offset);
+                break;
         default:
                 r = -EINVAL;
                 break;
@@ -1724,6 +1728,10 @@ static int kvmppc_set_one_reg_hv(struct kvm_vcpu *vcpu, u64 id,
         case KVM_REG_PPC_ARCH_COMPAT:
                 r = kvmppc_set_arch_compat(vcpu, set_reg_val(id, *val));
                 break;
+        case KVM_REG_PPC_DEC_EXPIRY:
+                vcpu->arch.dec_expires = set_reg_val(id, *val) -
+                        vcpu->arch.vcore->tb_offset;
+                break;
         default:
                 r = -EINVAL;
                 break;
diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
index c2c7ef330553..7c9e45f54186 100644
--- a/arch/powerpc/kvm/powerpc.c
+++ b/arch/powerpc/kvm/powerpc.c
@@ -758,7 +758,7 @@ int kvm_arch_vcpu_init(struct kvm_vcpu *vcpu)
 
         hrtimer_init(&vcpu->arch.dec_timer, CLOCK_REALTIME, HRTIMER_MODE_ABS);
         vcpu->arch.dec_timer.function = kvmppc_decrementer_wakeup;
-        vcpu->arch.dec_expires = ~(u64)0;
+        vcpu->arch.dec_expires = get_tb();
 
 #ifdef CONFIG_KVM_EXIT_TIMING
         mutex_init(&vcpu->arch.exit_timing_lock);