KVM: PPC: Book3S HV: Make secondary threads more robust against stray IPIs

Currently on POWER7, if we are running the guest on a core and we don't
need all the hardware threads, we do nothing to ensure that the unused
threads aren't executing in the kernel (other than checking that they
are offline).  We just assume they're napping and we don't do anything
to stop them trying to enter the kernel while the guest is running.
This means that a stray IPI can wake up the hardware thread and it will
then try to enter the kernel, but since the core is in guest context,
it will execute code from the guest in hypervisor mode once it turns the
MMU on, which tends to lead to crashes or hangs in the host.

This fixes the problem by adding two new one-byte flags in the
kvmppc_host_state structure in the PACA which are used to interlock
between the primary thread and the unused secondary threads when entering
the guest.  With these flags, the primary thread can ensure that the
unused secondaries are not already in kernel mode (i.e. handling a stray
IPI) and then indicate that they should not try to enter the kernel
if they do get woken for any reason.  Instead they will go into KVM code,
find that there is no vcpu to run, acknowledge and clear the IPI and go
back to nap mode.
Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/powerpc/include/asm/kvm_book3s_asm.h

@@ -79,6 +79,9 @@ struct kvmppc_host_state {
 	u8 napping;
 
 #ifdef CONFIG_KVM_BOOK3S_64_HV
+	u8 hwthread_req;
+	u8 hwthread_state;
+
 	struct kvm_vcpu *kvm_vcpu;
 	struct kvmppc_vcore *kvm_vcore;
 	unsigned long xics_phys;
@@ -122,4 +125,9 @@ struct kvmppc_book3s_shadow_vcpu {
 
 #endif /*__ASSEMBLY__ */
 
+/* Values for kvm_state */
+#define KVM_HWTHREAD_IN_KERNEL	0
+#define KVM_HWTHREAD_IN_NAP	1
+#define KVM_HWTHREAD_IN_KVM	2
+
 #endif /* __ASM_KVM_BOOK3S_ASM_H__ */

arch/powerpc/kernel/asm-offsets.c

@@ -540,6 +540,8 @@ int main(void)
 	HSTATE_FIELD(HSTATE_IN_GUEST, in_guest);
 	HSTATE_FIELD(HSTATE_RESTORE_HID5, restore_hid5);
 	HSTATE_FIELD(HSTATE_NAPPING, napping);
+	HSTATE_FIELD(HSTATE_HWTHREAD_REQ, hwthread_req);
+	HSTATE_FIELD(HSTATE_HWTHREAD_STATE, hwthread_state);
 
 #ifdef CONFIG_KVM_BOOK3S_64_HV
 	HSTATE_FIELD(HSTATE_KVM_VCPU, kvm_vcpu);

arch/powerpc/kernel/exceptions-64s.S

@@ -63,11 +63,13 @@ BEGIN_FTR_SECTION
 	GET_PACA(r13)
 
 #ifdef CONFIG_KVM_BOOK3S_64_HV
-	lbz	r0,PACAPROCSTART(r13)
-	cmpwi	r0,0x80
-	bne	1f
-	li	r0,1
-	stb	r0,PACAPROCSTART(r13)
+	li	r0,KVM_HWTHREAD_IN_KERNEL
+	stb	r0,HSTATE_HWTHREAD_STATE(r13)
+	/* Order setting hwthread_state vs. testing hwthread_req */
+	sync
+	lbz	r0,HSTATE_HWTHREAD_REQ(r13)
+	cmpwi	r0,0
+	beq	1f
 	b	kvm_start_guest
 1:
 #endif

arch/powerpc/kernel/idle_power7.S

@@ -16,6 +16,7 @@
 #include <asm/asm-offsets.h>
 #include <asm/ppc-opcode.h>
 #include <asm/hw_irq.h>
+#include <asm/kvm_book3s_asm.h>
 
 #undef DEBUG
 
@@ -81,6 +82,12 @@ _GLOBAL(power7_idle)
 	std	r9,_MSR(r1)
 	std	r1,PACAR1(r13)
 
+#ifdef CONFIG_KVM_BOOK3S_64_HV
+	/* Tell KVM we're napping */
+	li	r4,KVM_HWTHREAD_IN_NAP
+	stb	r4,HSTATE_HWTHREAD_STATE(r13)
+#endif
+
 	/* Magic NAP mode enter sequence */
 	std	r0,0(r1)
 	ptesync

arch/powerpc/kvm/book3s_hv.c

@@ -569,6 +569,45 @@ static void kvmppc_remove_runnable(struct kvmppc_vcore *vc,
 	list_del(&vcpu->arch.run_list);
 }
 
+static int kvmppc_grab_hwthread(int cpu)
+{
+	struct paca_struct *tpaca;
+	long timeout = 1000;
+
+	tpaca = &paca[cpu];
+
+	/* Ensure the thread won't go into the kernel if it wakes */
+	tpaca->kvm_hstate.hwthread_req = 1;
+
+	/*
+	 * If the thread is already executing in the kernel (e.g. handling
+	 * a stray interrupt), wait for it to get back to nap mode.
+	 * The smp_mb() is to ensure that our setting of hwthread_req
+	 * is visible before we look at hwthread_state, so if this
+	 * races with the code at system_reset_pSeries and the thread
+	 * misses our setting of hwthread_req, we are sure to see its
+	 * setting of hwthread_state, and vice versa.
+	 */
+	smp_mb();
+	while (tpaca->kvm_hstate.hwthread_state == KVM_HWTHREAD_IN_KERNEL) {
+		if (--timeout <= 0) {
+			pr_err("KVM: couldn't grab cpu %d\n", cpu);
+			return -EBUSY;
+		}
+		udelay(1);
+	}
+	return 0;
+}
+
+static void kvmppc_release_hwthread(int cpu)
+{
+	struct paca_struct *tpaca;
+
+	tpaca = &paca[cpu];
+	tpaca->kvm_hstate.hwthread_req = 0;
+	tpaca->kvm_hstate.kvm_vcpu = NULL;
+}
+
 static void kvmppc_start_thread(struct kvm_vcpu *vcpu)
 {
 	int cpu;
@@ -588,8 +627,7 @@ static void kvmppc_start_thread(struct kvm_vcpu *vcpu)
 	smp_wmb();
 #if defined(CONFIG_PPC_ICP_NATIVE) && defined(CONFIG_SMP)
 	if (vcpu->arch.ptid) {
-		tpaca->cpu_start = 0x80;
-		wmb();
+		kvmppc_grab_hwthread(cpu);
 		xics_wake_cpu(cpu);
 		++vc->n_woken;
 	}
@@ -639,7 +677,7 @@ static int kvmppc_run_core(struct kvmppc_vcore *vc)
 	struct kvm_vcpu *vcpu, *vcpu0, *vnext;
 	long ret;
 	u64 now;
-	int ptid;
+	int ptid, i;
 
 	/* don't start if any threads have a signal pending */
 	list_for_each_entry(vcpu, &vc->runnable_threads, arch.run_list)
@@ -686,12 +724,17 @@ static int kvmppc_run_core(struct kvmppc_vcore *vc)
 	vc->napping_threads = 0;
 	list_for_each_entry(vcpu, &vc->runnable_threads, arch.run_list)
 		kvmppc_start_thread(vcpu);
+	/* Grab any remaining hw threads so they can't go into the kernel */
+	for (i = ptid; i < threads_per_core; ++i)
+		kvmppc_grab_hwthread(vc->pcpu + i);
 
 	preempt_disable();
 	spin_unlock(&vc->lock);
 
 	kvm_guest_enter();
 	__kvmppc_vcore_entry(NULL, vcpu0);
+	for (i = 0; i < threads_per_core; ++i)
+		kvmppc_release_hwthread(vc->pcpu + i);
 
 	spin_lock(&vc->lock);
 	/* disable sending of IPIs on virtual external irqs */

arch/powerpc/kvm/book3s_hv_rmhandlers.S

@@ -26,6 +26,7 @@
 #include <asm/hvcall.h>
 #include <asm/asm-offsets.h>
 #include <asm/exception-64s.h>
+#include <asm/kvm_book3s_asm.h>
 
 /*****************************************************************************
  *                                                                           *
@@ -82,6 +83,7 @@ _GLOBAL(kvmppc_hv_entry_trampoline)
 
 #define XICS_XIRR		4
 #define XICS_QIRR		0xc
+#define XICS_IPI		2	/* interrupt source # for IPIs */
 
 /*
  * We come in here when wakened from nap mode on a secondary hw thread.
@@ -94,26 +96,54 @@ kvm_start_guest:
 	subi	r1,r1,STACK_FRAME_OVERHEAD
 	ld	r2,PACATOC(r13)
 
-	/* were we napping due to cede? */
-	lbz	r0,HSTATE_NAPPING(r13)
-	cmpwi	r0,0
-	bne	kvm_end_cede
+	li	r0,KVM_HWTHREAD_IN_KVM
+	stb	r0,HSTATE_HWTHREAD_STATE(r13)
 
-	/* get vcpu pointer */
-	ld	r4, HSTATE_KVM_VCPU(r13)
+	/* NV GPR values from power7_idle() will no longer be valid */
+	li	r0,1
+	stb	r0,PACA_NAPSTATELOST(r13)
 
-	/* We got here with an IPI; clear it */
-	ld	r5, HSTATE_XICS_PHYS(r13)
-	li	r0, 0xff
-	li	r6, XICS_QIRR
-	li	r7, XICS_XIRR
-	lwzcix	r8, r5, r7		/* ack the interrupt */
+	/* get vcpu pointer, NULL if we have no vcpu to run */
+	ld	r4,HSTATE_KVM_VCPU(r13)
+	cmpdi	cr1,r4,0
+
+	/* Check the wake reason in SRR1 to see why we got here */
+	mfspr	r3,SPRN_SRR1
+	rlwinm	r3,r3,44-31,0x7		/* extract wake reason field */
+	cmpwi	r3,4			/* was it an external interrupt? */
+	bne	27f
+
+	/*
+	 * External interrupt - for now assume it is an IPI, since we
+	 * should never get any other interrupts sent to offline threads.
+	 * Only do this for secondary threads.
+	 */
+	beq	cr1,25f
+	lwz	r3,VCPU_PTID(r4)
+	cmpwi	r3,0
+	beq	27f
+25:	ld	r5,HSTATE_XICS_PHYS(r13)
+	li	r0,0xff
+	li	r6,XICS_QIRR
+	li	r7,XICS_XIRR
+	lwzcix	r8,r5,r7		/* get and ack the interrupt */
 	sync
-	stbcix	r0, r5, r6		/* clear it */
-	stwcix	r8, r5, r7		/* EOI it */
+	clrldi.	r9,r8,40		/* get interrupt source ID. */
+	beq	27f			/* none there? */
+	cmpwi	r9,XICS_IPI
+	bne	26f
+	stbcix	r0,r5,r6		/* clear IPI */
+26:	stwcix	r8,r5,r7		/* EOI the interrupt */
 
-	/* NV GPR values from power7_idle() will no longer be valid */
-	stb	r0, PACA_NAPSTATELOST(r13)
+27:	/* XXX should handle hypervisor maintenance interrupts etc. here */
+
+	/* if we have no vcpu to run, go back to sleep */
+	beq	cr1,kvm_no_guest
+
+	/* were we napping due to cede? */
+	lbz	r0,HSTATE_NAPPING(r13)
+	cmpwi	r0,0
+	bne	kvm_end_cede
 
 .global kvmppc_hv_entry
 kvmppc_hv_entry:
@@ -1445,8 +1475,8 @@ END_FTR_SECTION_IFCLR(CPU_FTR_ARCH_206)
 	 * Take a nap until a decrementer or external interrupt occurs,
 	 * with PECE1 (wake on decr) and PECE0 (wake on external) set in LPCR
 	 */
-	li	r0,0x80
-	stb	r0,PACAPROCSTART(r13)
+	li	r0,1
+	stb	r0,HSTATE_HWTHREAD_REQ(r13)
 	mfspr	r5,SPRN_LPCR
 	ori	r5,r5,LPCR_PECE0 | LPCR_PECE1
 	mtspr	SPRN_LPCR,r5
@@ -1463,26 +1493,7 @@ END_FTR_SECTION_IFCLR(CPU_FTR_ARCH_206)
 kvm_end_cede:
 	/* Woken by external or decrementer interrupt */
 	ld	r1, HSTATE_HOST_R1(r13)
-	ld	r2, PACATOC(r13)
 
-	/* If we're a secondary thread and we got here by an IPI, ack it */
-	ld	r4,HSTATE_KVM_VCPU(r13)
-	lwz	r3,VCPU_PTID(r4)
-	cmpwi	r3,0
-	beq	27f
-	mfspr	r3,SPRN_SRR1
-	rlwinm	r3,r3,44-31,0x7		/* extract wake reason field */
-	cmpwi	r3,4			/* was it an external interrupt? */
-	bne	27f
-	ld	r5, HSTATE_XICS_PHYS(r13)
-	li	r0,0xff
-	li	r6,XICS_QIRR
-	li	r7,XICS_XIRR
-	lwzcix	r8,r5,r7		/* ack the interrupt */
-	sync
-	stbcix	r0,r5,r6		/* clear it */
-	stwcix	r8,r5,r7		/* EOI it */
-27:
 	/* load up FP state */
 	bl	kvmppc_load_fp
 
@@ -1580,12 +1591,17 @@ secondary_nap:
 	stwcx.	r3, 0, r4
 	bne	51b
 
+kvm_no_guest:
+	li	r0, KVM_HWTHREAD_IN_NAP
+	stb	r0, HSTATE_HWTHREAD_STATE(r13)
+	li	r0, 0
+	std	r0, HSTATE_KVM_VCPU(r13)
+
 	li	r3, LPCR_PECE0
 	mfspr	r4, SPRN_LPCR
 	rlwimi	r4, r3, 0, LPCR_PECE0 | LPCR_PECE1
 	mtspr	SPRN_LPCR, r4
 	isync
-	li	r0, 0
 	std	r0, HSTATE_SCRATCH0(r13)
 	ptesync
 	ld	r0, HSTATE_SCRATCH0(r13)