Re: [PATCH v4 28/46] KVM: PPC: Book3S HV P9: Reduce irq_work vs guest decrementer races

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 another class of
races.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
  arch/powerpc/kvm/book3s_hv.c | 15 ++++++++++++++-
  1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 1f38a0abc611..989a1ff5ad11 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c

@@ -3745,6 +3745,18 @@ 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);
  
+	/*
+	 * When setting DEC, we 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.
+	 *
+	 * XXX: Another day's problem.
+	 */
  	mtspr(SPRN_DEC, vcpu->arch.dec_expires - tb);
  
  	if (kvmhv_on_pseries()) {

@@ -3879,7 +3891,8 @@ 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);