[3.16.y-ckt stable] Patch "KVM: x86: update masterclock values on TSC writes" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    KVM: x86: update masterclock values on TSC writes

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt7.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From f474bf036e540daceea09cb04480932bef966fb5 Mon Sep 17 00:00:00 2001
From: Marcelo Tosatti <mtosatti at redhat.com>
Date: Tue, 4 Nov 2014 21:30:44 -0200
Subject: KVM: x86: update masterclock values on TSC writes

commit 7f187922ddf6b67f2999a76dcb71663097b75497 upstream.

When the guest writes to the TSC, the masterclock TSC copy must be
updated as well along with the TSC_OFFSET update, otherwise a negative
tsc_timestamp is calculated at kvm_guest_time_update.

Once "if (!vcpus_matched && ka->use_master_clock)" is simplified to
"if (ka->use_master_clock)", the corresponding "if (!ka->use_master_clock)"
becomes redundant, so remove the do_request boolean and collapse
everything into a single condition.

Signed-off-by: Marcelo Tosatti <mtosatti at redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 arch/x86/kvm/x86.c | 19 ++++++++++---------
 1 file changed, 10 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index eee371c996c2..a711a0d1ebc3 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1209,21 +1209,22 @@ void kvm_track_tsc_matching(struct kvm_vcpu *vcpu)
 {
 #ifdef CONFIG_X86_64
 	bool vcpus_matched;
-	bool do_request = false;
 	struct kvm_arch *ka = &vcpu->kvm->arch;
 	struct pvclock_gtod_data *gtod = &pvclock_gtod_data;

 	vcpus_matched = (ka->nr_vcpus_matched_tsc + 1 ==
 			 atomic_read(&vcpu->kvm->online_vcpus));

-	if (vcpus_matched && gtod->clock.vclock_mode == VCLOCK_TSC)
-		if (!ka->use_master_clock)
-			do_request = 1;
-
-	if (!vcpus_matched && ka->use_master_clock)
-			do_request = 1;
-
-	if (do_request)
+	/*
+	 * Once the masterclock is enabled, always perform request in
+	 * order to update it.
+	 *
+	 * In order to enable masterclock, the host clocksource must be TSC
+	 * and the vcpus need to have matched TSCs.  When that happens,
+	 * perform request to enable masterclock.
+	 */
+	if (ka->use_master_clock ||
+	    (gtod->clock.vclock_mode == VCLOCK_TSC && vcpus_matched))
 		kvm_make_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu);

 	trace_kvm_track_tsc(vcpu->vcpu_id, ka->nr_vcpus_matched_tsc,
--
2.1.4