[Xen-changelog] [xen master] x86/time: implement PVCLOCK_TSC_STABLE_BIT

[patchbot@xxxxxxx]

commit fe0364d29323cf2acccdf37d000f6c2f2f7267df
Author:     Joao Martins <joao.m.martins@xxxxxxxxxx>
AuthorDate: Fri Sep 23 18:25:49 2016 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Sep 23 18:25:49 2016 +0200

    x86/time: implement PVCLOCK_TSC_STABLE_BIT
    
    This patch proposes relying on host TSC synchronization and
    passthrough to the guest, when running on a TSC-safe platform. On
    time_calibration we retrieve the platform time in ns and the counter
    read by the clocksource that was used to compute system time. We can
    guarantee that on a platform with a constant and reliable TSC, that the
    time read on vcpu B right after A is bigger independently of the VCPU
    calibration error. Since pvclock time infos are monotonic as seen by any
    vCPU set PVCLOCK_TSC_STABLE_BIT, which then enables usage of VDSO on
    Linux.  IIUC, this is similar to how it's implemented on KVM. Add also a
    comment regarding this bit changing and that guests are expected to
    check this bit on every read.
    
    Should note that I've yet to see time going backwards in a long running
    test I ran for 2 weeks (in a dual socket machine), plus few other
    tests I did on older platforms.
    
    Signed-off-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/time.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
index 128e653..d307d93 100644
--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -955,6 +955,14 @@ static void __update_vcpu_system_time(struct vcpu *v, int 
force)
     _u.tsc_timestamp = tsc_stamp;
     _u.system_time   = t->stamp.local_stime;
 
+    /*
+     * It's expected that domains cope with this bit changing on every
+     * pvclock read to check whether they can resort solely on this tuple
+     * or if it further requires monotonicity checks with other vcpus.
+     */
+    if ( clocksource_is_tsc() )
+        _u.flags |= XEN_PVCLOCK_TSC_STABLE_BIT;
+
     if ( is_hvm_domain(d) )
         _u.tsc_timestamp += v->arch.hvm_vcpu.cache_tsc_offset;
 
--
generated by git-patchbot for /home/xen/git/xen.git#master

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