[Trusty] UBUNTU: SAUCE: Fix kvm hangs on i386

[Stefan Bader]

Unfortunately completely failed to proceed with this one (me and
upstream). This hack is curing at least the big issue with kvm.
As noted in the patch, there is a series of fixes after 3.13 which
I had been picking up while looking into this. These would be stable
material (minus maybe the idle changes though they might be useful for
other reasons and I did not want to make guesses about the things that
patch #3 changes there).

1. x86, acpi, idle: Restructure the mwait idle routines
2. x86, idle: Use static_cpu_has() for CLFLUSH workaround, add barriers
3. sched/preempt: Fix up missed PREEMPT_NEED_RESCHED folding
4. sched/preempt/x86: Fix voluntary preempt for x86

This just for documentation and in case anything else looks odd in
the scheduler and this might be a reason. Its now too late to pick those
and the hack works without.

-Stefan

---

>From 951bca51ab8456d2a2cbca33f440680ae675b5e4 Mon Sep 17 00:00:00 2001
From: Stefan Bader <stefan.bader at canonical.com>
Date: Tue, 1 Apr 2014 20:26:00 +0200
Subject: [PATCH] UBUNTU: SAUCE: kvm: Use schedule instead of cond_reschedule
 on i386

Since commit c2daa3bed53a81171cf8c1a36db798e82b91afe8
  Author: Peter Zijlstra <peterz at infradead.org>
  sched, x86: Provide a per-cpu preempt_count implementation

there is a problem on 32bit x86 which causes kvm to run wildly in a
tight loop. The cause is that since the preempt_count change it seems
possible on 32bit to have the thread info flag TIF_NEED_RESCHED set
but the IPI that in theory should update that flag into the per-cpu
counter does not see it. This leaves us in a bad place as this
counts as the process should reschule but is not allowed to. And
kvm unfortunately relies on those things to be in sync.

There is actually a list of fixes to this in general which was not yet
pushed to stable (since we never found a resolution to this issue). So
i386 may behave unfair in general. This here just allows to run kvm on
i386 at all.

Signed-off-by: Stefan Bader <stefan.bader at canonical.com>
---
 arch/x86/kvm/x86.c |   14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 6dc4904..512b1ce 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6092,7 +6092,21 @@ static int __vcpu_run(struct kvm_vcpu *vcpu)
 		}
 		if (need_resched()) {
 			srcu_read_unlock(&kvm->srcu, vcpu->srcu_idx);
+#ifdef CONFIG_X86_32
+			/*
+			 * This is a hack around a current bug on i386
+			 * that causes TIF_NEED_RESCHED not being propagated
+			 * into the per-cpu counter. This causes cond_resched()
+			 * which is what kvm_resched() calls to exit without
+			 * actually rescheduling. We continue immediately with
+			 * the loop but vcpu_enter_guest checks for the process
+			 * flag and immediately exits and this loops busily
+			 * runs forever.
+			 */
+			schedule();
+#else
 			kvm_resched(vcpu);
+#endif
 			vcpu->srcu_idx = srcu_read_lock(&kvm->srcu);
 		}
 	}
-- 
1.7.9.5