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>

2 files changed, 100 insertions, 41 deletions

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 01294a5099dd..e87f6196d222 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/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 */
diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
index b70bf22a3ff3..d595033bd449 100644
--- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S
+++ b/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)