KVM: PPC: Book3S HV: Close race with testing for signals on guest entry

At present, interrupts are hard-disabled fairly late in the guest
entry path, in the assembly code.  Since we check for pending signals
for the vCPU(s) task(s) earlier in the guest entry path, it is
possible for a signal to be delivered before we enter the guest but
not be noticed until after we exit the guest for some other reason.

Similarly, it is possible for the scheduler to request a reschedule
while we are in the guest entry path, and we won't notice until after
we have run the guest, potentially for a whole timeslice.

Furthermore, with a radix guest on POWER9, we can take the interrupt
with the MMU on.  In this case we end up leaving interrupts
hard-disabled after the guest exit, and they are likely to stay
hard-disabled until we exit to userspace or context-switch to
another process.  This was masking the fact that we were also not
setting the RI (recoverable interrupt) bit in the MSR, meaning
that if we had taken an interrupt, it would have crashed the host
kernel with an unrecoverable interrupt message.

To close these races, we need to check for signals and reschedule
requests after hard-disabling interrupts, and then keep interrupts
hard-disabled until we enter the guest.  If there is a signal or a
reschedule request from another CPU, it will send an IPI, which will
cause a guest exit.

This puts the interrupt disabling before we call kvmppc_start_thread()
for all the secondary threads of this core that are going to run vCPUs.
The reason for that is that once we have started the secondary threads
there is no easy way to back out without going through at least part
of the guest entry path.  However, kvmppc_start_thread() includes some
code for radix guests which needs to call smp_call_function(), which
must be called with interrupts enabled.  To solve this problem, this
patch moves that code into a separate function that is called earlier.

When the guest exit is caused by an external interrupt, a hypervisor
doorbell or a hypervisor maintenance interrupt, we now handle these
using the replay facility.  __kvmppc_vcore_entry() now returns the
trap number that caused the exit on this thread, and instead of the
assembly code jumping to the handler entry, we return to C code with
interrupts still hard-disabled and set the irq_happened flag in the
PACA, so that when we do local_irq_enable() the appropriate handler
gets called.

With all this, we now have the interrupt soft-enable flag clear while
we are in the guest.  This is useful because code in the real-mode
hypercall handlers that checks whether interrupts are enabled will
now see that they are disabled, which is correct, since interrupts
are hard-disabled in the real-mode code.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

arch/powerpc/kvm/book3s_hv.c

@@ -647,6 +647,7 @@ static void kvmppc_create_dtl_entry(struct kvm_vcpu *vcpu,
 	unsigned long stolen;
 	unsigned long core_stolen;
 	u64 now;
+	unsigned long flags;
 
 	dt = vcpu->arch.dtl_ptr;
 	vpa = vcpu->arch.vpa.pinned_addr;
@@ -654,10 +655,10 @@ static void kvmppc_create_dtl_entry(struct kvm_vcpu *vcpu,
 	core_stolen = vcore_stolen_time(vc, now);
 	stolen = core_stolen - vcpu->arch.stolen_logged;
 	vcpu->arch.stolen_logged = core_stolen;
-	spin_lock_irq(&vcpu->arch.tbacct_lock);
+	spin_lock_irqsave(&vcpu->arch.tbacct_lock, flags);
 	stolen += vcpu->arch.busy_stolen;
 	vcpu->arch.busy_stolen = 0;
-	spin_unlock_irq(&vcpu->arch.tbacct_lock);
+	spin_unlock_irqrestore(&vcpu->arch.tbacct_lock, flags);
 	if (!dt || !vpa)
 		return;
 	memset(dt, 0, sizeof(struct dtl_entry));
@@ -2085,7 +2086,7 @@ static void kvmppc_end_cede(struct kvm_vcpu *vcpu)
 	}
 }
 
-extern void __kvmppc_vcore_entry(void);
+extern int __kvmppc_vcore_entry(void);
 
 static void kvmppc_remove_runnable(struct kvmppc_vcore *vc,
 				   struct kvm_vcpu *vcpu)
@@ -2167,21 +2168,9 @@ static void radix_flush_cpu(struct kvm *kvm, int cpu, struct kvm_vcpu *vcpu)
 			smp_call_function_single(cpu + i, do_nothing, NULL, 1);
 }
 
-static void kvmppc_start_thread(struct kvm_vcpu *vcpu, struct kvmppc_vcore *vc)
+static void kvmppc_prepare_radix_vcpu(struct kvm_vcpu *vcpu, int pcpu)
 {
-	int cpu;
-	struct paca_struct *tpaca;
-	struct kvm *kvm = vc->kvm;
-
-	cpu = vc->pcpu;
-	if (vcpu) {
-		if (vcpu->arch.timer_running) {
-			hrtimer_try_to_cancel(&vcpu->arch.dec_timer);
-			vcpu->arch.timer_running = 0;
-		}
-		cpu += vcpu->arch.ptid;
-		vcpu->cpu = vc->pcpu;
-		vcpu->arch.thread_cpu = cpu;
+	struct kvm *kvm = vcpu->kvm;
 
 	/*
 	 * With radix, the guest can do TLB invalidations itself,
@@ -2195,13 +2184,30 @@ static void kvmppc_start_thread(struct kvm_vcpu *vcpu, struct kvmppc_vcore *vc)
 	 * ran to flush the TLB.  The TLB is shared between threads,
 	 * so we use a single bit in .need_tlb_flush for all 4 threads.
 	 */
-		if (kvm_is_radix(kvm) && vcpu->arch.prev_cpu != cpu) {
+	if (vcpu->arch.prev_cpu != pcpu) {
 		if (vcpu->arch.prev_cpu >= 0 &&
 		    cpu_first_thread_sibling(vcpu->arch.prev_cpu) !=
-			    cpu_first_thread_sibling(cpu))
+		    cpu_first_thread_sibling(pcpu))
 			radix_flush_cpu(kvm, vcpu->arch.prev_cpu, vcpu);
-			vcpu->arch.prev_cpu = cpu;
+		vcpu->arch.prev_cpu = pcpu;
+	}
+}
+
+static void kvmppc_start_thread(struct kvm_vcpu *vcpu, struct kvmppc_vcore *vc)
+{
+	int cpu;
+	struct paca_struct *tpaca;
+	struct kvm *kvm = vc->kvm;
+
+	cpu = vc->pcpu;
+	if (vcpu) {
+		if (vcpu->arch.timer_running) {
+			hrtimer_try_to_cancel(&vcpu->arch.dec_timer);
+			vcpu->arch.timer_running = 0;
 		}
+		cpu += vcpu->arch.ptid;
+		vcpu->cpu = vc->pcpu;
+		vcpu->arch.thread_cpu = cpu;
 		cpumask_set_cpu(cpu, &kvm->arch.cpu_in_guest);
 	}
 	tpaca = &paca[cpu];
@@ -2470,6 +2476,18 @@ static void collect_piggybacks(struct core_info *cip, int target_threads)
 	spin_unlock(&lp->lock);
 }
 
+static bool recheck_signals(struct core_info *cip)
+{
+	int sub, i;
+	struct kvm_vcpu *vcpu;
+
+	for (sub = 0; sub < cip->n_subcores; ++sub)
+		for_each_runnable_thread(i, vcpu, cip->vc[sub])
+			if (signal_pending(vcpu->arch.run_task))
+				return true;
+	return false;
+}
+
 static void post_guest_process(struct kvmppc_vcore *vc, bool is_master)
 {
 	int still_running = 0, i;
@@ -2568,6 +2586,21 @@ static inline int kvmppc_set_host_core(unsigned int cpu)
 	return 0;
 }
 
+static void set_irq_happened(int trap)
+{
+	switch (trap) {
+	case BOOK3S_INTERRUPT_EXTERNAL:
+		local_paca->irq_happened |= PACA_IRQ_EE;
+		break;
+	case BOOK3S_INTERRUPT_H_DOORBELL:
+		local_paca->irq_happened |= PACA_IRQ_DBELL;
+		break;
+	case BOOK3S_INTERRUPT_HMI:
+		local_paca->irq_happened |= PACA_IRQ_HMI;
+		break;
+	}
+}
+
 /*
  * Run a set of guest threads on a physical core.
  * Called with vc->lock held.
@@ -2587,6 +2620,7 @@ static noinline void kvmppc_run_core(struct kvmppc_vcore *vc)
 	int pcpu, thr;
 	int target_threads;
 	int controlled_threads;
+	int trap;
 
 	/*
 	 * Remove from the list any threads that have a signal pending
@@ -2638,6 +2672,43 @@ static noinline void kvmppc_run_core(struct kvmppc_vcore *vc)
 	if (vc->num_threads < target_threads)
 		collect_piggybacks(&core_info, target_threads);
 
+	/*
+	 * On radix, arrange for TLB flushing if necessary.
+	 * This has to be done before disabling interrupts since
+	 * it uses smp_call_function().
+	 */
+	pcpu = smp_processor_id();
+	if (kvm_is_radix(vc->kvm)) {
+		for (sub = 0; sub < core_info.n_subcores; ++sub)
+			for_each_runnable_thread(i, vcpu, core_info.vc[sub])
+				kvmppc_prepare_radix_vcpu(vcpu, pcpu);
+	}
+
+	/*
+	 * Hard-disable interrupts, and check resched flag and signals.
+	 * If we need to reschedule or deliver a signal, clean up
+	 * and return without going into the guest(s).
+	 */
+	local_irq_disable();
+	hard_irq_disable();
+	if (lazy_irq_pending() || need_resched() ||
+	    recheck_signals(&core_info)) {
+		local_irq_enable();
+		vc->vcore_state = VCORE_INACTIVE;
+		/* Unlock all except the primary vcore */
+		for (sub = 1; sub < core_info.n_subcores; ++sub) {
+			pvc = core_info.vc[sub];
+			/* Put back on to the preempted vcores list */
+			kvmppc_vcore_preempt(pvc);
+			spin_unlock(&pvc->lock);
+		}
+		for (i = 0; i < controlled_threads; ++i)
+			kvmppc_release_hwthread(pcpu + i);
+		return;
+	}
+
+	kvmppc_clear_host_core(pcpu);
+
 	/* Decide on micro-threading (split-core) mode */
 	subcore_size = threads_per_subcore;
 	cmd_bit = stat_bit = 0;
@@ -2665,7 +2736,6 @@ static noinline void kvmppc_run_core(struct kvmppc_vcore *vc)
 		/* order writes to split_info before kvm_split_mode pointer */
 		smp_wmb();
 	}
-	pcpu = smp_processor_id();
 	for (thr = 0; thr < controlled_threads; ++thr)
 		paca[pcpu + thr].kvm_hstate.kvm_split_mode = sip;
 
@@ -2685,8 +2755,6 @@ static noinline void kvmppc_run_core(struct kvmppc_vcore *vc)
 		}
 	}
 
-	kvmppc_clear_host_core(pcpu);
-
 	/* Start all the threads */
 	active = 0;
 	for (sub = 0; sub < core_info.n_subcores; ++sub) {
@@ -2738,14 +2806,25 @@ static noinline void kvmppc_run_core(struct kvmppc_vcore *vc)
 	for (sub = 0; sub < core_info.n_subcores; ++sub)
 		spin_unlock(&core_info.vc[sub]->lock);
 
+	/*
+	 * Interrupts will be enabled once we get into the guest,
+	 * so tell lockdep that we're about to enable interrupts.
+	 */
+	trace_hardirqs_on();
+
 	guest_enter();
 
 	srcu_idx = srcu_read_lock(&vc->kvm->srcu);
 
-	__kvmppc_vcore_entry();
+	trap = __kvmppc_vcore_entry();
 
 	srcu_read_unlock(&vc->kvm->srcu, srcu_idx);
 
+	guest_exit();
+
+	trace_hardirqs_off();
+	set_irq_happened(trap);
+
 	spin_lock(&vc->lock);
 	/* prevent other vcpu threads from doing kvmppc_start_thread() now */
 	vc->vcore_state = VCORE_EXITING;
@@ -2773,6 +2852,10 @@ static noinline void kvmppc_run_core(struct kvmppc_vcore *vc)
 		split_info.do_nap = 0;
 	}
 
+	kvmppc_set_host_core(pcpu);
+
+	local_irq_enable();
+
 	/* Let secondaries go back to the offline loop */
 	for (i = 0; i < controlled_threads; ++i) {
 		kvmppc_release_hwthread(pcpu + i);
@@ -2781,13 +2864,10 @@ static noinline void kvmppc_run_core(struct kvmppc_vcore *vc)
 		cpumask_clear_cpu(pcpu + i, &vc->kvm->arch.cpu_in_guest);
 	}
 
-	kvmppc_set_host_core(pcpu);
-
 	spin_unlock(&vc->lock);
 
 	/* make sure updates to secondary vcpu structs are visible now */
 	smp_mb();
-	guest_exit();
 
 	for (sub = 0; sub < core_info.n_subcores; ++sub) {
 		pvc = core_info.vc[sub];

arch/powerpc/kvm/book3s_hv_interrupts.S

@@ -61,13 +61,6 @@ BEGIN_FTR_SECTION
 	std	r3, HSTATE_DABR(r13)
 END_FTR_SECTION_IFCLR(CPU_FTR_ARCH_207S)
 
-	/* Hard-disable interrupts */
-	mfmsr   r10
-	std	r10, HSTATE_HOST_MSR(r13)
-	rldicl  r10,r10,48,1
-	rotldi  r10,r10,16
-	mtmsrd  r10,1
-
 	/* Save host PMU registers */
 BEGIN_FTR_SECTION
 	/* Work around P8 PMAE bug */
@@ -153,6 +146,7 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_300)
 	 *
 	 * R1       = host R1
 	 * R2       = host R2
+	 * R3       = trap number on this thread
 	 * R12      = exit handler id
 	 * R13      = PACA
 	 */

arch/powerpc/kvm/book3s_hv_rmhandlers.S

@@ -69,6 +69,7 @@ _GLOBAL_TOC(kvmppc_hv_entry_trampoline)
 	std	r0, PPC_LR_STKOFF(r1)
 	stdu	r1, -112(r1)
 	mfmsr	r10
+	std	r10, HSTATE_HOST_MSR(r13)
 	LOAD_REG_ADDR(r5, kvmppc_call_hv_entry)
 	li	r0,MSR_RI
 	andc	r0,r10,r0
@@ -165,6 +166,9 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)
 	addi	r1, r1, 112
 	ld	r7, HSTATE_HOST_MSR(r13)
 
+	/* Return the trap number on this thread as the return value */
+	mr	r3, r12
+
 	/*
 	 * If we came back from the guest via a relocation-on interrupt,
 	 * we will be in virtual mode at this point, which makes it a
@@ -174,59 +178,20 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)
 	andi.	r0, r0, MSR_IR		/* in real mode? */
 	bne	.Lvirt_return
 
-	cmpwi	r12, BOOK3S_INTERRUPT_EXTERNAL
-	beq	11f
-	cmpwi	r12, BOOK3S_INTERRUPT_H_DOORBELL
-	beq 	15f	/* Invoke the H_DOORBELL handler */
-	cmpwi	cr2, r12, BOOK3S_INTERRUPT_HMI
-	beq	cr2, 14f			/* HMI check */
-
-	/* RFI into the highmem handler, or branch to interrupt handler */
+	/* RFI into the highmem handler */
 	mfmsr	r6
 	li	r0, MSR_RI
 	andc	r6, r6, r0
 	mtmsrd	r6, 1			/* Clear RI in MSR */
 	mtsrr0	r8
 	mtsrr1	r7
-	/*
-	 * BOOK3S_INTERRUPT_MACHINE_CHECK is handled at the
-	 * time of guest exit
-	 */
 	RFI
 
-	/* On POWER7, we have external interrupts set to use HSRR0/1 */
-11:	mtspr	SPRN_HSRR0, r8
-	mtspr	SPRN_HSRR1, r7
-	ba	0x500
-
-14:	mtspr	SPRN_HSRR0, r8
-	mtspr	SPRN_HSRR1, r7
-	b	hmi_exception_after_realmode
-
-15:	mtspr SPRN_HSRR0, r8
-	mtspr SPRN_HSRR1, r7
-	ba    0xe80
-
-	/* Virtual-mode return - can't get here for HMI or machine check */
+	/* Virtual-mode return */
 .Lvirt_return:
-	cmpwi	r12, BOOK3S_INTERRUPT_EXTERNAL
-	beq	16f
-	cmpwi	r12, BOOK3S_INTERRUPT_H_DOORBELL
-	beq	17f
-	andi.	r0, r7, MSR_EE		/* were interrupts hard-enabled? */
-	beq	18f
-	mtmsrd	r7, 1			/* if so then re-enable them */
-18:	mtlr	r8
+	mtlr	r8
 	blr
 
-16:	mtspr	SPRN_HSRR0, r8		/* jump to reloc-on external vector */
-	mtspr	SPRN_HSRR1, r7
-	b	exc_virt_0x4500_hardware_interrupt
-
-17:	mtspr	SPRN_HSRR0, r8
-	mtspr	SPRN_HSRR1, r7
-	b	exc_virt_0x4e80_h_doorbell
-
 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 */
@@ -1258,6 +1223,15 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
 
 	stw	r12,VCPU_TRAP(r9)
 
+	/*
+	 * Now that we have saved away SRR0/1 and HSRR0/1,
+	 * interrupts are recoverable in principle, so set MSR_RI.
+	 * This becomes important for relocation-on interrupts from
+	 * the guest, which we can get in radix mode on POWER9.
+	 */
+	li	r0, MSR_RI
+	mtmsrd	r0, 1
+
 #ifdef CONFIG_KVM_BOOK3S_HV_EXIT_TIMING
 	addi	r3, r9, VCPU_TB_RMINTR
 	mr	r4, r9