Re: [PATCH 24/30] KVM: PPC: booke: call resched after every exit

On 18.02.2012, at 00:00, Scott Wood wrote:

On 02/17/2012 11:13 AM, Alexander Graf wrote:

quoted

Instead of checking whether we should reschedule only when we exited
due to an interrupt, let's always check before entering the guest =

back

quoted

again. This gets the target more in line with the other archs.
=20
Signed-off-by: Alexander Graf <redacted>
---
arch/powerpc/kvm/booke.c |   15 ++++++++++-----
1 files changed, 10 insertions(+), 5 deletions(-)
=20

diff --git a/arch/powerpc/kvm/booke.c b/arch/powerpc/kvm/booke.c
index bfb2092..de30b6d 100644
--- a/arch/powerpc/kvm/booke.c
+++ b/arch/powerpc/kvm/booke.c

@@ -572,6 +572,7 @@ int kvmppc_handle_exit(struct kvm_run *run, =

struct kvm_vcpu *vcpu,

quoted

                       unsigned int exit_nr)
{
	int r =3D RESUME_HOST;
+	int resched_needed =3D 1;
=20
	/* update before a new last_exit_type is rewritten */
	kvmppc_update_timing_stats(vcpu);

@@ -602,25 +603,21 @@ int kvmppc_handle_exit(struct kvm_run *run, =

struct kvm_vcpu *vcpu,

quoted

=20
	switch (exit_nr) {
	case BOOKE_INTERRUPT_MACHINE_CHECK:
-		kvm_resched(vcpu);
		r =3D RESUME_GUEST;
		break;
=20
	case BOOKE_INTERRUPT_EXTERNAL:
		kvmppc_account_exit(vcpu, EXT_INTR_EXITS);
-		kvm_resched(vcpu);
		r =3D RESUME_GUEST;
		break;
=20
	case BOOKE_INTERRUPT_DECREMENTER:
		kvmppc_account_exit(vcpu, DEC_EXITS);
-		kvm_resched(vcpu);
		r =3D RESUME_GUEST;
		break;
=20
	case BOOKE_INTERRUPT_DOORBELL:
		kvmppc_account_exit(vcpu, DBELL_EXITS);
-		kvm_resched(vcpu);
		r =3D RESUME_GUEST;
		break;
=20

@@ -869,8 +866,16 @@ int kvmppc_handle_exit(struct kvm_run *run, =

struct kvm_vcpu *vcpu,

quoted

		BUG();
	}
=20
-	local_irq_disable();
+	/* make sure we reschedule if we need to */
+	while (resched_needed) {
+		local_irq_disable();
=20
+		resched_needed =3D need_resched();
+		if (resched_needed) {
+			local_irq_enable();
+			cond_resched();
+		}
+	}
	kvmppc_core_prepare_to_enter(vcpu);
=20
	if (!(r & RESUME_HOST)) {

=20
kvmppc_core_prepare_to_enter can enable interrupts (and block) if =

guest

MSR_WE is set.  We may take an interrupt that wants a resched after
waking but before interrupts are disabled again.
=20
We also want to check for a resched in kvmppc_vcpu_run.  So, the =

resched

check belongs in kvmppc_core_prepare_to_enter, something like:
=20
/* Check pending exceptions and deliver one, if possible. */
void kvmppc_core_prepare_to_enter(struct kvm_vcpu *vcpu)
{
	WARN_ON_ONCE(!irqs_disabled());
=20
	while (true) {
		if (signal_pending(current))
			break;
=20
		if (need_resched()) {
			local_irq_enable();
			cond_resched();
			local_irq_disable();
			continue;
		}
=20
		kvmppc_core_check_exceptions(vcpu);
=20
		if (vcpu->arch.shared->msr & MSR_WE) {
			local_irq_enable();
			kvm_vcpu_block(vcpu);
			local_irq_disable();
=09
			kvmppc_set_exit_type(vcpu,
				EMULATED_MTMSRWE_EXITS);
			continue;
		}
=20
		break;
	}
}
=20
It would be simpler (both here and in the idle hcall) if we could just
drop support for CONFIG_PREEMPT=3Dn. :-P

When running with CONFIG_PREEMPT=3Dn we don't have to worry about =
interrupts being enabled though, since we only preempt on known good =
checkpoints, right? While for CONFIG_PREEMPT=3Dy we can always get =
preempted, rendering most of these checks void.

So essentially we could just be lazy and do a "best effort" resched =
check, but not worry about races wrt guest entry/exit, right? And for =
real preemption modes we don't have to worry about any of the resched =
stuff IIUC, correct?


Alex