File 58cf9277-x86-time-dont-use-vTSC-if-host-guest-freqs-match.patch of Package xen.11319
References: bsc#1026236
# Commit 4fc380ac0077ecd6b0e0013ca7ca977cb7361662
# Date 2017-03-20 09:27:35 +0100
# Author Boris Ostrovsky <boris.ostrovsky@oracle.com>
# Committer Jan Beulich <jbeulich@suse.com>
x86/time: don't use virtual TSC if host and guest frequencies are equal
Commit 82713ec8d2 ("x86: use native RDTSC(P) execution when guest and
host frequencies are the same") left out optimization for PV guests
when host and guest run at the same frequency.
For such a case we should be able not to use virtual TSC regardless
of whether we are runing before or after a migration (i.e. regardless
of incarnation value).
Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
[jb: retain parts of the original comment]
Reviewed-by: Jan Beulich <jbeulich@suse.com>
# Commit 9614f0246688d1e4cee4d6bec47894b6e289c721
# Date 2017-03-30 15:12:25 +0200
# Author Boris Ostrovsky <boris.ostrovsky@oracle.com>
# Committer Jan Beulich <jbeulich@suse.com>
docs: update xen-tscmode.pod.7 to reflect default TSC mode changes
A number of changes have been made to how we determine whether TSC
is emulated (e.g. commit 4fc380ac0077 ("x86/time: don't use virtual TSC
if host and guest frequencies are equal")).
Update the man page to reflect those changes
Suggested-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
--- a/docs/misc/tscmode.txt
+++ b/docs/misc/tscmode.txt
@@ -168,12 +168,12 @@ The default mode (tsc_mode==0) checks TS
hardware on which the virtual machine is launched. If it is
TSC-safe, rdtsc will execute at hardware speed; if it is not, rdtsc
will be emulated. Once a virtual machine is save/restored or migrated,
-however, there are two possibilities: For a paravirtualized (PV) domain,
-TSC will always be emulated. For a fully-virtualized (HVM) domain,
-TSC remains native IF the source physical machine and target physical machine
-have the same TSC frequency; else TSC is emulated. Note that, though
-emulated, the "apparent" TSC frequency will be the TSC frequency
-of the initial physical machine, even after migration.
+however, there are two possibilities: TSC remains native IF the source
+physical machine and target physical machine have the same TSC frequency
+(or, for HVM/PVH guests, if TSC scaling support is available); else TSC
+is emulated. Note that, though emulated, the "apparent" TSC frequency
+will be the TSC frequency of the initial physical machine, even after
+migration.
For environments where both TSC-safeness AND highest performance
even across migration is a requirement, application code can be specially
--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -1999,16 +1999,17 @@ void tsc_set_info(struct domain *d,
d->arch.vtsc_offset = get_s_time() - elapsed_nsec;
d->arch.tsc_khz = gtsc_khz ? gtsc_khz : cpu_khz;
set_time_scale(&d->arch.vtsc_to_ns, d->arch.tsc_khz * 1000 );
+
/*
- * Use native TSC if the host has safe TSC and:
- * HVM/PVH: host and guest frequencies are the same (either
- * "naturally" or via TSC scaling)
- * PV: guest has not migrated yet (and thus arch.tsc_khz == cpu_khz)
+ * Use native TSC if the host has safe TSC and host and guest
+ * frequencies are the same (either "naturally" or - for HVM/PVH -
+ * via TSC scaling).
+ * When a guest is created, gtsc_khz is passed in as zero, making
+ * d->arch.tsc_khz == cpu_khz. Thus no need to check incarnation.
*/
if ( host_tsc_is_safe() &&
- ((has_hvm_container_domain(d) &&
- (d->arch.tsc_khz == cpu_khz || cpu_has_tsc_ratio)) ||
- incarnation == 0) )
+ (d->arch.tsc_khz == cpu_khz ||
+ (has_hvm_container_domain(d) && cpu_has_tsc_ratio)) )
d->arch.vtsc = 0;
else
d->arch.ns_to_vtsc = scale_reciprocal(d->arch.vtsc_to_ns);
