linux/arch
Zachary Amsden 4dd7980b21 KVM: Leave TSC synchronization window open with each new sync
Currently, when the TSC is written by the guest, the variable
ns is updated to force the current write to appear to have taken
place at the time of the first write in this sync phase.  This
leaves a cliff at the end of the match window where updates will
fall of the end.  There are two scenarios where this can be a
problem in practe - first, on a system with a large number of
VCPUs, the sync period may last for an extended period of time.

The second way this can happen is if the VM reboots very rapidly
and we catch a VCPU TSC synchronization just around the edge.
We may be unaware of the reboot, and thus the first VCPU might
synchronize with an old set of the timer (at, say 0.97 seconds
ago, when first powered on).  The second VCPU can come in 0.04
seconds later to try to synchronize, but it misses the window
because it is just over the threshold.

Instead, stop doing this artificial setback of the ns variable
and just update it with every write of the TSC.

It may be observed that doing so causes values computed by
compute_guest_tsc to diverge slightly across CPUs - note that
the last_tsc_ns and last_tsc_write variable are used here, and
now they last_tsc_ns will be different for each VCPU, reflecting
the actual time of the update.

However, compute_guest_tsc is used only for guests which already
have TSC stability issues, and further, note that the previous
patch has caused last_tsc_write to be incremented by the difference
in nanoseconds, converted back into guest cycles.  As such, only
boundary rounding errors should be visible, which given the
resolution in nanoseconds, is going to only be a few cycles and
only visible in cross-CPU consistency tests.  The problem can be
fixed by adding a new set of variables to track the start offset
and start write value for the current sync cycle.

Signed-off-by: Zachary Amsden <zamsden@gmail.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
2012-03-08 14:10:04 +02:00
..
alpha Merge branch 'linux-next' of git://git.kernel.org/pub/scm/linux/kernel/git/jbarnes/pci 2012-01-11 18:50:26 -08:00
arm IOMMU fixes for Linux 3.3-rc5 2012-02-28 09:15:31 -08:00
avr32 avr32: select generic atomic64_t support 2012-02-03 16:16:40 -08:00
blackfin Merge branch 'linux-next' of git://git.kernel.org/pub/scm/linux/kernel/git/jbarnes/pci 2012-01-11 18:50:26 -08:00
c6x Kbuild: Use dtc's -d (dependency) option 2012-02-14 21:14:44 -05:00
cris lib: use generic pci_iomap on all architectures 2012-01-10 18:04:27 -08:00
frv Merge branch 'for-next' of git://git.infradead.org/users/dhowells/linux-headers 2012-01-14 18:03:30 -08:00
h8300 Merge branch 'linux-next' of git://git.kernel.org/pub/scm/linux/kernel/git/jbarnes/pci 2012-01-11 18:50:26 -08:00
hexagon
ia64 KVM: provide synchronous registers in kvm_run 2012-03-05 14:52:22 +02:00
m32r Merge branch 'for-next' of git://git.infradead.org/users/dhowells/linux-headers 2012-01-14 18:03:30 -08:00
m68k m68k: Do not set global share for non-kernel shared pages 2012-02-07 11:25:57 +10:00
microblaze Revert "microblaze: Add topology init" 2012-01-31 09:06:52 +01:00
mips mips: use the the PCI controller's io_map_base 2012-01-31 23:20:30 +02:00
mn10300 Merge branch 'for-next' of git://git.infradead.org/users/dhowells/linux-headers 2012-01-14 18:03:30 -08:00
openrisc openrisc: Fix up audit_syscall_[entry|exit]() usage 2012-02-17 09:53:12 +01:00
parisc parisc, exec: remove redundant set_fs(USER_DS) 2012-01-12 20:13:04 -08:00
powerpc KVM: PPC: Add HPT preallocator 2012-03-05 14:57:28 +02:00
s390 KVM: s390: provide access guest registers via kvm_run 2012-03-05 14:52:22 +02:00
score score: fix off-by-one index into syscall table 2012-01-23 08:38:49 -08:00
sh SuperH fixes for 3.3-rc5 2012-02-24 08:56:51 -08:00
sparc lib: Fix multiple definitions of clz_tab 2012-02-02 10:34:23 +11:00
tile Merge branch 'for-next' of git://git.infradead.org/users/dhowells/linux-headers 2012-01-14 18:03:30 -08:00
um Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/audit 2012-01-17 16:41:31 -08:00
unicore32 Merge branch 'linux-next' of git://git.kernel.org/pub/scm/linux/kernel/git/jbarnes/pci 2012-01-11 18:50:26 -08:00
x86 KVM: Leave TSC synchronization window open with each new sync 2012-03-08 14:10:04 +02:00
xtensa xtensa: fix memscan() 2012-02-03 16:16:40 -08:00
.gitignore
Kconfig mm,x86,um: move CMPXCHG_DOUBLE config option 2012-01-12 20:13:03 -08:00