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docs: kvm: Convert timekeeping.txt to ReST format 

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Signed-off-by: default avatarMauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
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Documentation/virt/kvm/index.rst
View file @ 6012d9a9
......@@ -18,6 +18,7 @@ KVM
nested-vmx
ppc-pv
s390-diag
timekeeping
vcpu-requests
arm/index
......
Documentation/virt/kvm/timekeeping.txt → Documentation/virt/kvm/timekeeping.rst
View file @ 6012d9a9
.. SPDX-License-Identifier: GPL-2.0
Timekeeping Virtualization for X86-Based Architectures
======================================================
Timekeeping Virtualization for X86-Based Architectures
======================================================
Zachary Amsden <zamsden@redhat.com>
Copyright (c) 2010, Red Hat. All rights reserved.
:Author: Zachary Amsden <zamsden@redhat.com>
:Copyright: (c) 2010, Red Hat. All rights reserved.
1) Overview
2) Timing Devices
3) TSC Hardware
4) Virtualization Problems
.. Contents
=========================================================================
1) Overview
2) Timing Devices
3) TSC Hardware
4) Virtualization Problems
1) Overview
1. Overview
===========
One of the most complicated parts of the X86 platform, and specifically,
the virtualization of this platform is the plethora of timing devices available
......@@ -27,15 +31,15 @@ The purpose of this document is to collect data and information relevant to
timekeeping which may be difficult to find elsewhere, specifically,
information relevant to KVM and hardware-based virtualization.
=========================================================================
2) Timing Devices
2. Timing Devices
=================
First we discuss the basic hardware devices available. TSC and the related
KVM clock are special enough to warrant a full exposition and are described in
the following section.
2.1) i8254 - PIT
2.1. i8254 - PIT
----------------
One of the first timer devices available is the programmable interrupt timer,
or PIT. The PIT has a fixed frequency 1.193182 MHz base clock and three
......@@ -50,13 +54,13 @@ The PIT uses I/O ports 0x40 - 0x43. Access to the 16-bit counters is done
using single or multiple byte access to the I/O ports. There are 6 modes
available, but not all modes are available to all timers, as only timer 2
has a connected gate input, required for modes 1 and 5. The gate line is
controlled by port 61h, bit 0, as illustrated in the following diagram.
controlled by port 61h, bit 0, as illustrated in the following diagram::
-------------- ----------------
| | | |
| 1.1932 MHz |---------->| CLOCK OUT | ---------> IRQ 0
| Clock | | | |
-------------- | +->| GATE TIMER 0 |
-------------- ----------------
| | | |
| 1.1932 MHz|---------->| CLOCK OUT | ---------> IRQ 0
| Clock | | | |
-------------- | +->| GATE TIMER 0 |
| ----------------
|
| ----------------
......@@ -70,29 +74,33 @@ controlled by port 61h, bit 0, as illustrated in the following diagram.
| | |
|------>| CLOCK OUT | ---------> Port 61h, bit 5
| | |
Port 61h, bit 0 ---------->| GATE TIMER 2 | \_.---- ____
Port 61h, bit 0 -------->| GATE TIMER 2 | \_.---- ____
---------------- _| )--|LPF|---Speaker
/ *---- \___/
Port 61h, bit 1 -----------------------------------/
Port 61h, bit 1 ---------------------------------/
The timer modes are now described.
Mode 0: Single Timeout. This is a one-shot software timeout that counts down
Mode 0: Single Timeout.
This is a one-shot software timeout that counts down
when the gate is high (always true for timers 0 and 1). When the count
reaches zero, the output goes high.
Mode 1: Triggered One-shot. The output is initially set high. When the gate
Mode 1: Triggered One-shot.
The output is initially set high. When the gate
line is set high, a countdown is initiated (which does not stop if the gate is
lowered), during which the output is set low. When the count reaches zero,
the output goes high.
Mode 2: Rate Generator. The output is initially set high. When the countdown
Mode 2: Rate Generator.
The output is initially set high. When the countdown
reaches 1, the output goes low for one count and then returns high. The value
is reloaded and the countdown automatically resumes. If the gate line goes
low, the count is halted. If the output is low when the gate is lowered, the
output automatically goes high (this only affects timer 2).
Mode 3: Square Wave. This generates a high / low square wave. The count
Mode 3: Square Wave.
This generates a high / low square wave. The count
determines the length of the pulse, which alternates between high and low
when zero is reached. The count only proceeds when gate is high and is
automatically reloaded on reaching zero. The count is decremented twice at
......@@ -103,12 +111,14 @@ Mode 3: Square Wave. This generates a high / low square wave. The count
values are not observed when reading. This is the intended mode for timer 2,
which generates sine-like tones by low-pass filtering the square wave output.
Mode 4: Software Strobe. After programming this mode and loading the counter,
Mode 4: Software Strobe.
After programming this mode and loading the counter,
the output remains high until the counter reaches zero. Then the output
goes low for 1 clock cycle and returns high. The counter is not reloaded.
Counting only occurs when gate is high.
Mode 5: Hardware Strobe. After programming and loading the counter, the
Mode 5: Hardware Strobe.
After programming and loading the counter, the
output remains high. When the gate is raised, a countdown is initiated
(which does not stop if the gate is lowered). When the counter reaches zero,
the output goes low for 1 clock cycle and then returns high. The counter is
......@@ -118,49 +128,49 @@ In addition to normal binary counting, the PIT supports BCD counting. The
command port, 0x43 is used to set the counter and mode for each of the three
timers.
PIT commands, issued to port 0x43, using the following bit encoding:
PIT commands, issued to port 0x43, using the following bit encoding::
Bit 7-4: Command (See table below)
Bit 3-1: Mode (000 = Mode 0, 101 = Mode 5, 11X = undefined)
Bit 0 : Binary (0) / BCD (1)
Bit 7-4: Command (See table below)
Bit 3-1: Mode (000 = Mode 0, 101 = Mode 5, 11X = undefined)
Bit 0 : Binary (0) / BCD (1)
Command table:
Command table::
0000 - Latch Timer 0 count for port 0x40
0000 - Latch Timer 0 count for port 0x40
sample and hold the count to be read in port 0x40;
additional commands ignored until counter is read;
mode bits ignored.
0001 - Set Timer 0 LSB mode for port 0x40
0001 - Set Timer 0 LSB mode for port 0x40
set timer to read LSB only and force MSB to zero;
mode bits set timer mode
0010 - Set Timer 0 MSB mode for port 0x40
0010 - Set Timer 0 MSB mode for port 0x40
set timer to read MSB only and force LSB to zero;
mode bits set timer mode
0011 - Set Timer 0 16-bit mode for port 0x40
0011 - Set Timer 0 16-bit mode for port 0x40
set timer to read / write LSB first, then MSB;
mode bits set timer mode
0100 - Latch Timer 1 count for port 0x41 - as described above
0101 - Set Timer 1 LSB mode for port 0x41 - as described above
0110 - Set Timer 1 MSB mode for port 0x41 - as described above
0111 - Set Timer 1 16-bit mode for port 0x41 - as described above
0100 - Latch Timer 1 count for port 0x41 - as described above
0101 - Set Timer 1 LSB mode for port 0x41 - as described above
0110 - Set Timer 1 MSB mode for port 0x41 - as described above
0111 - Set Timer 1 16-bit mode for port 0x41 - as described above
1000 - Latch Timer 2 count for port 0x42 - as described above
1001 - Set Timer 2 LSB mode for port 0x42 - as described above
1010 - Set Timer 2 MSB mode for port 0x42 - as described above
1011 - Set Timer 2 16-bit mode for port 0x42 as described above
1000 - Latch Timer 2 count for port 0x42 - as described above
1001 - Set Timer 2 LSB mode for port 0x42 - as described above
1010 - Set Timer 2 MSB mode for port 0x42 - as described above
1011 - Set Timer 2 16-bit mode for port 0x42 as described above
1101 - General counter latch
1101 - General counter latch
Latch combination of counters into corresponding ports
Bit 3 = Counter 2
Bit 2 = Counter 1
Bit 1 = Counter 0
Bit 0 = Unused
1110 - Latch timer status
1110 - Latch timer status
Latch combination of counter mode into corresponding ports
Bit 3 = Counter 2
Bit 2 = Counter 1
......@@ -177,7 +187,8 @@ Command table:
Bit 3-1 = Mode
Bit 0 = Binary (0) / BCD mode (1)
2.2) RTC
2.2. RTC
--------
The second device which was available in the original PC was the MC146818 real
time clock. The original device is now obsolete, and usually emulated by the
......@@ -201,21 +212,21 @@ in progress, as indicated in the status register.
The clock uses a 32.768kHz crystal, so bits 6-4 of register A should be
programmed to a 32kHz divider if the RTC is to count seconds.
This is the RAM map originally used for the RTC/CMOS:
Location Size Description
------------------------------------------
00h byte Current second (BCD)
01h byte Seconds alarm (BCD)
02h byte Current minute (BCD)
03h byte Minutes alarm (BCD)
04h byte Current hour (BCD)
05h byte Hours alarm (BCD)
06h byte Current day of week (BCD)
07h byte Current day of month (BCD)
08h byte Current month (BCD)
09h byte Current year (BCD)
0Ah byte Register A
This is the RAM map originally used for the RTC/CMOS::
Location Size Description
------------------------------------------
00h byte Current second (BCD)
01h byte Seconds alarm (BCD)
02h byte Current minute (BCD)
03h byte Minutes alarm (BCD)
04h byte Current hour (BCD)
05h byte Hours alarm (BCD)
06h byte Current day of week (BCD)
07h byte Current day of month (BCD)
08h byte Current month (BCD)
09h byte Current year (BCD)
0Ah byte Register A
bit 7 = Update in progress
bit 6-4 = Divider for clock
000 = 4.194 MHz
......@@ -234,7 +245,7 @@ Location Size Description
1101 = 125 mS
1110 = 250 mS
1111 = 500 mS
0Bh byte Register B
0Bh byte Register B
bit 7 = Run (0) / Halt (1)
bit 6 = Periodic interrupt enable
bit 5 = Alarm interrupt enable
......@@ -243,19 +254,20 @@ Location Size Description
bit 2 = BCD calendar (0) / Binary (1)
bit 1 = 12-hour mode (0) / 24-hour mode (1)
bit 0 = 0 (DST off) / 1 (DST enabled)
OCh byte Register C (read only)
OCh byte Register C (read only)
bit 7 = interrupt request flag (IRQF)
bit 6 = periodic interrupt flag (PF)
bit 5 = alarm interrupt flag (AF)
bit 4 = update interrupt flag (UF)
bit 3-0 = reserved
ODh byte Register D (read only)
ODh byte Register D (read only)
bit 7 = RTC has power
bit 6-0 = reserved
32h byte Current century BCD (*)
32h byte Current century BCD (*)
(*) location vendor specific and now determined from ACPI global tables
2.3) APIC
2.3. APIC
---------
On Pentium and later processors, an on-board timer is available to each CPU
as part of the Advanced Programmable Interrupt Controller. The APIC is
......@@ -276,7 +288,8 @@ timer is programmed through the LVT (local vector timer) register, is capable
of one-shot or periodic operation, and is based on the bus clock divided down
by the programmable divider register.
2.4) HPET
2.4. HPET
---------
HPET is quite complex, and was originally intended to replace the PIT / RTC
support of the X86 PC. It remains to be seen whether that will be the case, as
......@@ -297,7 +310,8 @@ indicated through ACPI tables by the BIOS.
Detailed specification of the HPET is beyond the current scope of this
document, as it is also very well documented elsewhere.
2.5) Offboard Timers
2.5. Offboard Timers
--------------------
Several cards, both proprietary (watchdog boards) and commonplace (e1000) have
timing chips built into the cards which may have registers which are accessible
......@@ -307,9 +321,8 @@ general frowned upon as not playing by the agreed rules of the game. Such a
timer device would require additional support to be virtualized properly and is
not considered important at this time as no known operating system does this.
=========================================================================
3) TSC Hardware
3. TSC Hardware
===============
The TSC or time stamp counter is relatively simple in theory; it counts
instruction cycles issued by the processor, which can be used as a measure of
......@@ -340,7 +353,8 @@ allows the guest visible TSC to be offset by a constant. Newer implementations
promise to allow the TSC to additionally be scaled, but this hardware is not
yet widely available.
3.1) TSC synchronization
3.1. TSC synchronization
------------------------
The TSC is a CPU-local clock in most implementations. This means, on SMP
platforms, the TSCs of different CPUs may start at different times depending
......@@ -357,7 +371,8 @@ practice, getting a perfectly synchronized TSC will not be possible unless all
values are read from the same clock, which generally only is possible on single
socket systems or those with special hardware support.
3.2) TSC and CPU hotplug
3.2. TSC and CPU hotplug
------------------------
As touched on already, CPUs which arrive later than the boot time of the system
may not have a TSC value that is synchronized with the rest of the system.
......@@ -367,7 +382,8 @@ a guarantee. This can have the effect of bringing a system from a state where
TSC is synchronized back to a state where TSC synchronization flaws, however
small, may be exposed to the OS and any virtualization environment.
3.3) TSC and multi-socket / NUMA
3.3. TSC and multi-socket / NUMA
--------------------------------
Multi-socket systems, especially large multi-socket systems are likely to have
individual clocksources rather than a single, universally distributed clock.
......@@ -385,7 +401,8 @@ standards for telecommunications and computer equipment.
It is recommended not to trust the TSCs to remain synchronized on NUMA or
multiple socket systems for these reasons.
3.4) TSC and C-states
3.4. TSC and C-states
---------------------
C-states, or idling states of the processor, especially C1E and deeper sleep
states may be problematic for TSC as well. The TSC may stop advancing in such
......@@ -396,7 +413,8 @@ based on CPU and chipset identifications.
The TSC in such a case may be corrected by catching it up to a known external
clocksource.
3.5) TSC frequency change / P-states
3.5. TSC frequency change / P-states
------------------------------------
To make things slightly more interesting, some CPUs may change frequency. They
may or may not run the TSC at the same rate, and because the frequency change
......@@ -416,14 +434,16 @@ other processors. In such cases, the TSC on halted CPUs could advance faster
than that of non-halted processors. AMD Turion processors are known to have
this problem.
3.6) TSC and STPCLK / T-states
3.6. TSC and STPCLK / T-states
------------------------------
External signals given to the processor may also have the effect of stopping
the TSC. This is typically done for thermal emergency power control to prevent
an overheating condition, and typically, there is no way to detect that this
condition has happened.
3.7) TSC virtualization - VMX
3.7. TSC virtualization - VMX
-----------------------------
VMX provides conditional trapping of RDTSC, RDMSR, WRMSR and RDTSCP
instructions, which is enough for full virtualization of TSC in any manner. In
......@@ -431,14 +451,16 @@ addition, VMX allows passing through the host TSC plus an additional TSC_OFFSET
field specified in the VMCS. Special instructions must be used to read and
write the VMCS field.
3.8) TSC virtualization - SVM
3.8. TSC virtualization - SVM
-----------------------------
SVM provides conditional trapping of RDTSC, RDMSR, WRMSR and RDTSCP
instructions, which is enough for full virtualization of TSC in any manner. In
addition, SVM allows passing through the host TSC plus an additional offset
field specified in the SVM control block.
3.9) TSC feature bits in Linux
3.9. TSC feature bits in Linux
------------------------------
In summary, there is no way to guarantee the TSC remains in perfect
synchronization unless it is explicitly guaranteed by the architecture. Even
......@@ -448,13 +470,16 @@ despite being locally consistent.
The following feature bits are used by Linux to signal various TSC attributes,
but they can only be taken to be meaningful for UP or single node systems.
X86_FEATURE_TSC : The TSC is available in hardware
X86_FEATURE_RDTSCP : The RDTSCP instruction is available
X86_FEATURE_CONSTANT_TSC : The TSC rate is unchanged with P-states
X86_FEATURE_NONSTOP_TSC : The TSC does not stop in C-states
X86_FEATURE_TSC_RELIABLE : TSC sync checks are skipped (VMware)
========================= =======================================
X86_FEATURE_TSC The TSC is available in hardware
X86_FEATURE_RDTSCP The RDTSCP instruction is available
X86_FEATURE_CONSTANT_TSC The TSC rate is unchanged with P-states
X86_FEATURE_NONSTOP_TSC The TSC does not stop in C-states
X86_FEATURE_TSC_RELIABLE TSC sync checks are skipped (VMware)
========================= =======================================
4) Virtualization Problems
4. Virtualization Problems
==========================
Timekeeping is especially problematic for virtualization because a number of
challenges arise. The most obvious problem is that time is now shared between
......@@ -473,7 +498,8 @@ BIOS, but not in such an extreme fashion. However, the fact that SMM mode may
cause similar problems to virtualization makes it a good justification for
solving many of these problems on bare metal.
4.1) Interrupt clocking
4.1. Interrupt clocking
-----------------------
One of the most immediate problems that occurs with legacy operating systems
is that the system timekeeping routines are often designed to keep track of
......@@ -502,7 +528,8 @@ thus requires interrupt slewing to keep proper time. It does use a low enough
rate (ed: is it 18.2 Hz?) however that it has not yet been a problem in
practice.
4.2) TSC sampling and serialization
4.2. TSC sampling and serialization
-----------------------------------
As the highest precision time source available, the cycle counter of the CPU
has aroused much interest from developers. As explained above, this timer has
......@@ -524,7 +551,8 @@ it may be necessary for an implementation to guard against "backwards" reads of
the TSC as seen from other CPUs, even in an otherwise perfectly synchronized
system.
4.3) Timespec aliasing
4.3. Timespec aliasing
----------------------
Additionally, this lack of serialization from the TSC poses another challenge
when using results of the TSC when measured against another time source. As
......@@ -548,7 +576,8 @@ This aliasing requires care in the computation and recalibration of kvmclock
and any other values derived from TSC computation (such as TSC virtualization
itself).
4.4) Migration
4.4. Migration
--------------
Migration of a virtual machine raises problems for timekeeping in two ways.
First, the migration itself may take time, during which interrupts cannot be
......@@ -566,7 +595,8 @@ always be caught up to the original rate. KVM clock avoids these problems by
simply storing multipliers and offsets against the TSC for the guest to convert
back into nanosecond resolution values.
4.5) Scheduling
4.5. Scheduling
---------------
Since scheduling may be based on precise timing and firing of interrupts, the
scheduling algorithms of an operating system may be adversely affected by
......@@ -579,7 +609,8 @@ In an attempt to work around this, several implementations have provided a
paravirtualized scheduler clock, which reveals the true amount of CPU time for
which a virtual machine has been running.
4.6) Watchdogs
4.6. Watchdogs
--------------
Watchdog timers, such as the lock detector in Linux may fire accidentally when
running under hardware virtualization due to timer interrupts being delayed or
......@@ -587,7 +618,8 @@ misinterpretation of the passage of real time. Usually, these warnings are
spurious and can be ignored, but in some circumstances it may be necessary to
disable such detection.
4.7) Delays and precision timing
4.7. Delays and precision timing
--------------------------------
Precise timing and delays may not be possible in a virtualized system. This
can happen if the system is controlling physical hardware, or issues delays to
......@@ -600,7 +632,8 @@ The second issue may cause performance problems, but this is unlikely to be a
significant issue. In many cases these delays may be eliminated through
configuration or paravirtualization.
4.8) Covert channels and leaks
4.8. Covert channels and leaks
------------------------------
In addition to the above problems, time information will inevitably leak to the
guest about the host in anything but a perfect implementation of virtualized
......
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