KVM: PPC: Book3S HV: Set LPCR[HDICE] before writing HDEC

POWER8 and POWER9 machines have a hardware deviation where generation
of a hypervisor decrementer exception is suppressed if the HDICE bit
in the LPCR register is 0 at the time when the HDEC register
decrements from 0 to -1.  When entering a guest, KVM first writes the
HDEC register with the time until it wants the CPU to exit the guest,
and then writes the LPCR with the guest value, which includes
HDICE = 1.  If HDEC decrements from 0 to -1 during the interval
between those two events, it is possible that we can enter the guest
with HDEC already negative but no HDEC exception pending, meaning that
no HDEC interrupt will occur while the CPU is in the guest, or at
least not until HDEC wraps around.  Thus it is possible for the CPU to
keep executing in the guest for a long time; up to about 4 seconds on
POWER8, or about 4.46 years on POWER9 (except that the host kernel
hard lockup detector will fire first).

To fix this, we set the LPCR[HDICE] bit before writing HDEC on guest
entry.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

arch/powerpc/kvm/book3s_hv.c

@@ -3442,9 +3442,19 @@ static int kvmhv_load_hv_regs_and_go(struct kvm_vcpu *vcpu, u64 time_limit,
 	unsigned long host_psscr = mfspr(SPRN_PSSCR);
 	unsigned long host_pidr = mfspr(SPRN_PID);
 
+	/*
+	 * P8 and P9 suppress the HDEC exception when LPCR[HDICE] = 0,
+	 * so set HDICE before writing HDEC.
+	 */
+	mtspr(SPRN_LPCR, vcpu->kvm->arch.host_lpcr | LPCR_HDICE);
+	isync();
+
 	hdec = time_limit - mftb();
-	if (hdec < 0)
+	if (hdec < 0) {
+		mtspr(SPRN_LPCR, vcpu->kvm->arch.host_lpcr);
+		isync();
 		return BOOK3S_INTERRUPT_HV_DECREMENTER;
+	}
 	mtspr(SPRN_HDEC, hdec);
 
 	if (vc->tb_offset) {
@@ -3572,7 +3582,7 @@ int kvmhv_p9_guest_entry(struct kvm_vcpu *vcpu, u64 time_limit,
 
 	dec = mfspr(SPRN_DEC);
 	tb = mftb();
-	if (dec < 512)
+	if (dec < 0)
 		return BOOK3S_INTERRUPT_HV_DECREMENTER;
 	local_paca->kvm_hstate.dec_expires = dec + tb;
 	if (local_paca->kvm_hstate.dec_expires < time_limit)

arch/powerpc/kvm/book3s_hv_interrupts.S

@@ -58,13 +58,16 @@ END_FTR_SECTION_IFCLR(CPU_FTR_ARCH_207S)
 	/*
 	 * Put whatever is in the decrementer into the
 	 * hypervisor decrementer.
+	 * Because of a hardware deviation in P8 and P9,
+	 * we need to set LPCR[HDICE] before writing HDEC.
 	 */
-BEGIN_FTR_SECTION
 	ld	r5, HSTATE_KVM_VCORE(r13)
 	ld	r6, VCORE_KVM(r5)
 	ld	r9, KVM_HOST_LPCR(r6)
-	andis.	r9, r9, LPCR_LD@h
-END_FTR_SECTION_IFSET(CPU_FTR_ARCH_300)
+	ori	r8, r9, LPCR_HDICE
+	mtspr	SPRN_LPCR, r8
+	isync
+	andis.	r0, r9, LPCR_LD@h
 	mfspr	r8,SPRN_DEC
 	mftb	r7
 BEGIN_FTR_SECTION