KVM: PPC: Book3S HV: Signed extend decrementer value if not using large decrementer

On POWER9 the decrementer can operate in large decrementer mode where
the decrementer is 56 bits and signed extended to 64 bits. When not
operating in this mode the decrementer behaves as a 32 bit decrementer
which is NOT signed extended (as on POWER8).

Currently when reading a guest decrementer value we don't take into
account whether the large decrementer is enabled or not, and this
means the value will be incorrect when the guest is not using the
large decrementer. Fix this by sign extending the value read when the
guest isn't using the large decrementer.

Fixes: 95a6432c ("KVM: PPC: Book3S HV: Streamlined guest entry/exit path on P9 for radix guests")
Cc: stable@vger.kernel.org # v4.20+
Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kvm/book3s_hv.c

@@ -3607,6 +3607,8 @@ int kvmhv_p9_guest_entry(struct kvm_vcpu *vcpu, u64 time_limit,
 
 	vcpu->arch.slb_max = 0;
 	dec = mfspr(SPRN_DEC);
+	if (!(lpcr & LPCR_LD)) /* Sign extend if not using large decrementer */
+		dec = (s32) dec;
 	tb = mftb();
 	vcpu->arch.dec_expires = dec + tb;
 	vcpu->cpu = -1;