One of the things we've not dealt with in libvirt yet is how to model the
various evil hacks most virt products have for dealing with timers in
guests. This email tries to outlines the problems & way each virt system
has dealt with them. Finally it suggests how to manage this in libvirt
domain XML. Comments please :-)
Virtual machine timer management in libvirt
===========================================
On PC hardware there are a number of terrible timers / clock sources available
to operating systems
* PIT
Timer with periodic interrupts
* RTC
Time of Day clock, continuous running
Timer with periodic interrupts
* Local APIC Timer
Timer with periodic interrupts
* ACPI Timer
Timer with periodic interrupts
* TSC
Read via rdtsc instruction. No interrupts
Unreliable on some hardware. eg changes frequency.
Not synced between cores
Different HZ across hosts
* HPET
Multiple timers with periodic interrupts
Can replace PIT/RTC timers
They all generally suck in real hardware, and this gets worse in virtual machines.
Many different approaches to making them suck less in VMWare, Xen & KVM, but there
are some reasonably common concepts....
Virtual timekeeping problems
----------------------------
Three primary problems / areas to deal with:
* Time of day clock (RTC)
- Initialized to UTC/Localtime/Timezone/UTC+offset
- Two modes of operation:
1. Guest wallclock: only runs when guest is executing. ie stopped across save/restore, etc
2. Host wallclock: runs continuously with host wall time.
* Interrupt timers
- Ticks can not always be delivered on time
Policies to deal with "missed" ticks:
1. Deliver at normal rate without catchup
2. Deliver at higher rate to catch up
3. Merge into 1 tick & deliver asap
4. Discard all missed ticks
* TSC
- rdtsc instruction can be exposed to guests in two ways
1. Trap + emulate (slow, but more reliable)
2. Native (fast, but possibly unreliable)
Optionally also expose a 'rdtscp' instruction
Possiblly set a fixed HZ independant of host.
VMWare timekeeping
------------------
* All timers run in "apparant time" ie track guest wallclock
* Missed tick policy is to deliver at higher rate to catchup
* TSC can be switched between native/emulate (virtual_rdtsc=TRUE|FALSE)
* TSC can have hardcoded HZ in emulate mode (apparantHZ=VALUE)
* RTC time of day is synced to host at startup (rtc.diffFromUTC or rtc.startTime)
* VMWare tools reset guest TOD if it gets out of sync
Xen timekeeping
---------------
* Virtual platform timer (VPT) used as source for other timers
* VPT has 4 modes
0: delay_for_missed_ticks
Missed ticks are delivered when next scheduled, at the normal
rate. RTC runs in guest wallclock, so is delayed. No catchup is
attempted
1: no_delay_for_missed_ticks
Missed ticks are delivered when next scheduled, at the normal
rate. RTC runs in host wallclock, so is not delayed.
2: no_missed_ticks_pending
Missed ticks are discarded & next tick is delivered normally. RTC
runs in host wallclock.
3: one_missed_tick_pending
Missed interrupts are collapsed into a single late tick. RTC
run in host wallclock.
* HPET
Optionally enabled
* TSC. Can run in 4 modes
- auto: emulate if host TSC is unstable. native with invariant TSC
- native: always native regardless of host TSC stability
- emulate: trap + emulate regardless of host TSC invariant
- pvrdtsc: native, requiring invariant TSC. Also exposes rdtscp instruction
KVM timekeeping
---------------
* PIT: can be in kernel, or userspace (userspace deprecated for KVM)
Default tick policies differ for both impls
- Userspace: Default: missed ticks are delivered when next scheduled at normal rate
-tdf flag enable tick reinjection to catchup
- Kernel: Default: Missed ticks are delivered at higher rate to catch up
-no-kvm-pit-reinjection to disable tick reinjection catchup
* RTC
TOD clock can run in host or guest wallclock (clock=host|guest)
Default: missed ticks are delivered when next scheduled at normal rate
-rtc-td-hack or -clock driftfix=slew: missed ticks are delivered at a
higher rate to catchup
* TSC
- Always runs native.
* HPET
- Optionally enabled/disabled
Mapping in libvirt XML
----------------------
Currently supports setting Time of Day clock via
<clock offset="utc"/>
Always sync to UTC
<clock offset="localtime"/>
Always sync to host timezone
<clock offset="timezone" timezone='Europe/Paris'/>
Sync to arbitrary timezone
<clock offset="variable" adjustment='123456'/>
Sync to UTC + arbitrary offset
Proposal to model all timers policies as sub-elements of this <clock/>
In general we wil allow zero or more <timer/> elements following the
syntax:
<timer name='platform|pit|rtc|hpet|tsc'
wallclock='host|guest'
tickpolicy='none|catchup|merge|discard'
frequency='123'
mode='auto|native|emulate|paravirt'
present='yes|no' />
Meaning of 'name':
Names map to regular PC timers / clocks. 'Platform' refers to the
(optional) master virtual clock source that may be used to drive
policy of "other" clocks (eg used in Xen, which clocks are controlled
by the platform clock is to be undefined because it has varied in
Xen over time).
Meaning of 'tickpolicy':
none: continue to deliver at normal rate (ie ticks are delayed)
catchup: deliver at higher rate to catchup
merge: ticks merged into 1 single tick
discard: all missed ticks are discarded
Meaning of 'wallclock':
Only valid for name='rtc' or 'platform'
host: RTC wallclock always tracks host time
guest: RTC wallclock always tracks host time
Meaning of 'frequency':
Set a fixed frequency in HZ.
NB: Only relevant for TSC. All other timers are fixed (PIT, RTC), or
fully guest controlled frequency (HPET)
Meaning of 'mode':
Control how the clock is exposed to guest.
auto: native if safe, otherwise emulate
native: always native
emulate: always emulate
paravirt: native + paravirtualize
NB: Only relevant for TSC. All other timers are always emulated.
Meaing of 'present':
Used to override default set of timers visible to the guest. eg to
enable or disable the HPET
Mapping to VMWare
-----------------
eg with guest config showing
diffFromUTC='123456'
apparentHZ='123456'
virtual_rdtsc=False
libvirt XML gets:
<clock mode='variable' adjustment='123456'>
<timer name='tsc' frequency='123456' mode='native'/>
</clock>
Mapping to Xen
--------------
eg with guest config showing
timer_mode=3
hpet=1
tsc_mode=2
localtime=1
<clock mode='localtime'>
<timer name='platform' tickpolicy='merge' wallclock='host'/>
<timer name='hpet'/>
<timer name='tsc' mode='native'/>
</clock>
Mapping to KVM
--------------
eg with guest ARGV showing
-no-kvm-pit-reinjection
-clock base=localtime,clock=guest,driftfix=slew
-no-hpet
<clock mode='localtime'>
<timer name='rtc' tickpolicy='catchup' wallclock='guest'/>
<timer name='pit' tickpolicy='none'/>
<timer name='hpet' present='no'/>
</clock>
Further reading
---------------
VMWare has the best doc:
http://www.vmware.com/pdf/vmware_timekeeping.pdf
Xen:
Docs on 'tsc_mode' at
$SOURCETREE/docs/misc/tscmode.txt
Docs for 'timer_mode' in the source code only:
xen/include/public/hvm/params.h
KVM:
No docs at all. Guess from -help descriptions, reading source code & asking
clever people about it :-)
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