KVM: PPC: Book3S HV: Use decrementer to wake napping threads

This arranges for threads that are napping due to their vcpu having
ceded or due to not having a vcpu to wake up at the end of the guest's
timeslice without having to be poked with an IPI.  We do that by
arranging for the decrementer to contain a value no greater than the
number of timebase ticks remaining until the end of the timeslice.
In the case of a thread with no vcpu, this number is in the hypervisor
decrementer already.  In the case of a ceded vcpu, we use the smaller
of the HDEC value and the DEC value.

Using the DEC like this when ceded means we need to save and restore
the guest decrementer value around the nap.
Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>

arch/powerpc/kvm/book3s_hv_rmhandlers.S

@@ -172,6 +172,9 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)
 
 kvmppc_primary_no_guest:
 	/* We handle this much like a ceded vcpu */
+	/* put the HDEC into the DEC, since HDEC interrupts don't wake us */
+	mfspr	r3, SPRN_HDEC
+	mtspr	SPRN_DEC, r3
 	/* set our bit in napping_threads */
 	ld	r5, HSTATE_KVM_VCORE(r13)
 	lbz	r7, HSTATE_PTID(r13)
@@ -223,6 +226,12 @@ kvm_novcpu_wakeup:
 	cmpdi	r3, 0
 	bge	kvm_novcpu_exit
 
+	/* See if our timeslice has expired (HDEC is negative) */
+	mfspr	r0, SPRN_HDEC
+	li	r12, BOOK3S_INTERRUPT_HV_DECREMENTER
+	cmpwi	r0, 0
+	blt	kvm_novcpu_exit
+
 	/* Got an IPI but other vcpus aren't yet exiting, must be a latecomer */
 	ld	r4, HSTATE_KVM_VCPU(r13)
 	cmpdi	r4, 0
@@ -1493,10 +1502,10 @@ kvmhv_do_exit:			/* r12 = trap, r13 = paca */
 	cmpwi	r3,0x100	/* Are we the first here? */
 	bge	43f
 	cmpwi	r12,BOOK3S_INTERRUPT_HV_DECREMENTER
-	beq	40f
+	beq	43f
 	li	r0,0
 	mtspr	SPRN_HDEC,r0
-40:
+
 	/*
 	 * Send an IPI to any napping threads, since an HDEC interrupt
 	 * doesn't wake CPUs up from nap.
@@ -2124,6 +2133,27 @@ _GLOBAL(kvmppc_h_cede)		/* r3 = vcpu pointer, r11 = msr, r13 = paca */
 	/* save FP state */
 	bl	kvmppc_save_fp
 
+	/*
+	 * Set DEC to the smaller of DEC and HDEC, so that we wake
+	 * no later than the end of our timeslice (HDEC interrupts
+	 * don't wake us from nap).
+	 */
+	mfspr	r3, SPRN_DEC
+	mfspr	r4, SPRN_HDEC
+	mftb	r5
+	cmpw	r3, r4
+	ble	67f
+	mtspr	SPRN_DEC, r4
+67:
+	/* save expiry time of guest decrementer */
+	extsw	r3, r3
+	add	r3, r3, r5
+	ld	r4, HSTATE_KVM_VCPU(r13)
+	ld	r5, HSTATE_KVM_VCORE(r13)
+	ld	r6, VCORE_TB_OFFSET(r5)
+	subf	r3, r6, r3	/* convert to host TB value */
+	std	r3, VCPU_DEC_EXPIRES(r4)
+
 #ifdef CONFIG_KVM_BOOK3S_HV_EXIT_TIMING
 	ld	r4, HSTATE_KVM_VCPU(r13)
 	addi	r3, r4, VCPU_TB_CEDE
@@ -2181,6 +2211,15 @@ kvm_end_cede:
 	/* load up FP state */
 	bl	kvmppc_load_fp
 
+	/* Restore guest decrementer */
+	ld	r3, VCPU_DEC_EXPIRES(r4)
+	ld	r5, HSTATE_KVM_VCORE(r13)
+	ld	r6, VCORE_TB_OFFSET(r5)
+	add	r3, r3, r6	/* convert host TB to guest TB value */
+	mftb	r7
+	subf	r3, r7, r3
+	mtspr	SPRN_DEC, r3
+
 	/* Load NV GPRS */
 	ld	r14, VCPU_GPR(R14)(r4)
 	ld	r15, VCPU_GPR(R15)(r4)