irq_work's use of the DEC SPR is racy with guest<->host switch and guest
entry which flips the DEC interrupt to guest, which could lose a host
work interrupt.
This patch closes one race, and attempts to comment several others.
(XXX: should think a bit harder about this)
Signed-off-by: Nicholas Piggin <npiggin@xxxxxxxxx>
---
arch/powerpc/include/asm/paca.h | 1 +
arch/powerpc/kvm/book3s_hv.c | 15 ++++++++++++++-
2 files changed, 15 insertions(+), 1 deletion(-)
diff --git a/arch/powerpc/include/asm/paca.h b/arch/powerpc/include/asm/paca.h
index ec18ac818e3a..23c12048fbc9 100644
--- a/arch/powerpc/include/asm/paca.h
+++ b/arch/powerpc/include/asm/paca.h
@@ -174,6 +174,7 @@ struct paca_struct {
u8 irq_happened; /* irq happened while soft-disabled */
u8 irq_work_pending; /* IRQ_WORK interrupt while soft-disable */
#ifdef CONFIG_KVM_BOOK3S_HV_POSSIBLE
+ /* Could have irq_work_using_hdec here, but what about nested HV entry modifying DEC? Could have a pointer to the hv struct time limit */
u8 pmcregs_in_use; /* pseries puts this in lppaca */
#endif
u64 sprg_vdso; /* Saved user-visible sprg */
diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index d98958b78830..1997cf347d3e 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -3689,6 +3689,18 @@ static int kvmhv_p9_guest_entry(struct kvm_vcpu *vcpu, u64 time_limit,
if (!(vcpu->arch.ctrl & 1))
mtspr(SPRN_CTRLT, mfspr(SPRN_CTRLF) & ~1);
+ /*
+ * XXX: must always deal with irq_work_raise via NMI vs setting DEC.
+ * The problem occurs right as we switch into guest mode if a NMI
+ * hits and sets pending work and sets DEC, then that will apply to
+ * the guest and not bring us back to the host.
+ *
+ * irq_work_raise could check a flag (or possibly LPCR[HDICE] for
+ * example) and set HDEC to 1? That wouldn't solve the nested hv
+ * case which needs to abort the hcall or zero the time limit.
+ *
+ * Another day's problem.
+ */
mtspr(SPRN_DEC, vcpu->arch.dec_expires - tb);
if (kvmhv_on_pseries()) {
@@ -3822,7 +3834,8 @@ static int kvmhv_p9_guest_entry(struct kvm_vcpu *vcpu, u64 time_limit,
vc->entry_exit_map = 0x101;
vc->in_guest = 0;
- mtspr(SPRN_DEC, local_paca->kvm_hstate.dec_expires - tb);
+ set_dec_or_work(local_paca->kvm_hstate.dec_expires - tb);
+
mtspr(SPRN_SPRG_VDSO_WRITE, local_paca->sprg_vdso);
kvmhv_load_host_pmu();
--
2.23.0
