[PATCH 1/1] KVM: powerpc/book3s_hv: Use generic xfer to guest work function

[Vishal Chourasia]

Use the generic infrastructure to check for and handle pending work
before transitioning into guest mode, replacing the open-coded
need_resched() and cond_resched() checks.

This picks up handling for TIF_NOTIFY_RESUME, which was previously
ignored, meaning task work will now be correctly handled on every
guest re-entry.

Signed-off-by: Vishal Chourasia <vishalc@xxxxxxxxxxxxx>
---
 arch/powerpc/kvm/Kconfig     |  1 +
 arch/powerpc/kvm/book3s_hv.c | 58 +++++++++++++++++++++++++++++++-----
 2 files changed, 52 insertions(+), 7 deletions(-)

diff --git a/arch/powerpc/kvm/Kconfig b/arch/powerpc/kvm/Kconfig
index 9a0d1c1aca6c..36aec58c5f22 100644
--- a/arch/powerpc/kvm/Kconfig
+++ b/arch/powerpc/kvm/Kconfig
@@ -81,6 +81,7 @@ config KVM_BOOK3S_64_HV
 	depends on KVM_BOOK3S_64 && PPC_POWERNV
 	select KVM_BOOK3S_HV_POSSIBLE
 	select KVM_BOOK3S_HV_PMU
+	select VIRT_XFER_TO_GUEST_WORK
 	select CMA
 	help
 	  Support running unmodified book3s_64 guest kernels in
diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 61dbeea317f3..b012512342e6 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -3850,10 +3850,20 @@ static noinline void kvmppc_run_core(struct kvmppc_vcore *vc)
 	 * and return without going into the guest(s).
 	 * If the mmu_ready flag has been cleared, don't go into the
 	 * guest because that means a HPT resize operation is in progress.
+	 *
+	 * xfer_to_guest_mode_work_pending() is the IRQs-disabled recheck for
+	 * pending guest-mode work (reschedule, signals, and TIF_NOTIFY_RESUME
+	 * task_work such as the deferred CFS throttle). It is the pre-POWER9
+	 * analog of the final gate in kvmhv_run_single_vcpu(), and a superset
+	 * of the old need_resched() check: it catches work that raced in after
+	 * the drain in kvmppc_run_vcpu(), so a CPU-bound vCPU is throttled here
+	 * instead of running one more guest dispatch past its quota. IRQs are
+	 * hard-disabled just above, so the non-__ variant (which asserts that)
+	 * is the correct one.
 	 */
 	local_irq_disable();
 	hard_irq_disable();
-	if (lazy_irq_pending() || need_resched() ||
+	if (lazy_irq_pending() || xfer_to_guest_mode_work_pending() ||
 	    recheck_signals_and_mmu(&core_info)) {
 		local_irq_enable();
 		vc->vcore_state = VCORE_INACTIVE;
@@ -4824,10 +4834,24 @@ static int kvmppc_run_vcpu(struct kvm_vcpu *vcpu)
 		vc->runner = vcpu;
 		if (n_ceded == vc->n_runnable) {
 			kvmppc_vcore_blocked(vc);
-		} else if (need_resched()) {
+		} else if (__xfer_to_guest_mode_work_pending()) {
 			kvmppc_vcore_preempt(vc);
-			/* Let something else run */
-			cond_resched_lock(&vc->lock);
+			/*
+			 * Let something else run, and run pending guest-mode
+			 * work (reschedule, and TIF_NOTIFY_RESUME task_work such
+			 * as the deferred CFS throttle) before we would re-enter
+			 * the guest, so a CPU-bound vCPU is actually throttled
+			 * here instead of running past its quota. This is a
+			 * superset of the old need_resched() check. Use the raw
+			 * helper, not the kvm_ wrapper: signals (KVM_EXIT_INTR
+			 * and the signal_exits stat) are accounted by this path's
+			 * existing handling below, so going through the wrapper
+			 * here would double-count them. The helper may schedule(),
+			 * so the vcore lock is dropped around it.
+			 */
+			spin_unlock(&vc->lock);
+			xfer_to_guest_mode_handle_work();
+			spin_lock(&vc->lock);
 			if (vc->vcore_state == VCORE_PREEMPT)
 				kvmppc_vcore_end_preempt(vc);
 		} else {
@@ -4899,8 +4923,21 @@ int kvmhv_run_single_vcpu(struct kvm_vcpu *vcpu, u64 time_limit,
 		}
 	}
 
-	if (need_resched())
-		cond_resched();
+	/*
+	 * Run pending work before (re-)entering the guest, most importantly
+	 * task_work queued via TWA_RESUME (e.g. the deferred CFS bandwidth
+	 * throttle, which only sets TIF_NOTIFY_RESUME). Without this a CPU-bound
+	 * vCPU that keeps returning RESUME_GUEST never reaches an exit-to-user
+	 * point, so the throttle is never enforced and the task runs far beyond
+	 * its quota. The helper also handles reschedule and signals, replacing
+	 * the cond_resched() that was here. It may schedule(), so it runs before
+	 * preemption and IRQs are disabled, with no vcore/KVM locks held. This
+	 * is the per-reentry site shared by the bare-metal and pseries (nested)
+	 * paths, so both are covered.
+	 */
+	r = kvm_xfer_to_guest_mode_handle_work(vcpu);
+	if (r)	/* -EINTR: signal pending, exit to userspace (KVM_EXIT_INTR) */
+		return r;
 
 	kvmppc_update_vpas(vcpu);
 
@@ -4916,7 +4953,14 @@ int kvmhv_run_single_vcpu(struct kvm_vcpu *vcpu, u64 time_limit,
 
 	if (signal_pending(current))
 		goto sigpend;
-	if (need_resched() || !kvm->arch.mmu_ready)
+	/*
+	 * Re-check for pending guest-mode work with IRQs disabled, to catch
+	 * anything (e.g. a TIF_NOTIFY_RESUME task_work such as the deferred CFS
+	 * throttle) that raced in after the check above. Bail back to the outer
+	 * loop, which re-enters here and runs the work. This is a superset of
+	 * the previous need_resched() check.
+	 */
+	if (xfer_to_guest_mode_work_pending() || !kvm->arch.mmu_ready)
 		goto out;
 
 	vcpu->cpu = pcpu;
-- 
2.54.0