Re: [Qemu-devel] Question on kvm_clock working ...
On 09/09/2011 06:28 PM, al pat wrote: We are doing an experiment with kvm-clock to validate its effectiveness, particularly when running NTP on the host to make sure the host’s clock stays properly sync. Our observations leads us to a few unanswered questions, including the possibility of a bug (our our misunderstanding of how kvm_clock should work). Our understanding is that kvm_clock will help sync the clock between the host and the guest. We do not observe this to happen in reality and thus this question. We are using Ubuntu 11.04 on the host and the guest. The command we issue to launch the VM is the following: $ sudo kvm -m 500 -rtc clock=host guestos.img We also arranged for Ubuntu to show the seconds on the clock displayed in the menu. Observation 1: Upon launching the VM, we see a time difference between the 2 clock ranging from 1 to 2 seconds. Observation 2: If we change the date on the host (with a command such as “date --set 10:00:00 AM Sep 9, 2011”), the time on the guest remains the same, unaffected. Observation 3: After running for a while without NTP on the host, we run “ntpdate” to sync up the host, but the guest stick with whatever previous time. You probably meant ntpd -q Another test we will run is to have ntpd on the host and wait for an extended time to see if the guest drifts away from that original 1 or 2 second lag. In the meantime, we are asking you for some input in this regards: Questions -What does the “–rtc clock” option is supposed to mean exactly? According to the man page, the guest should get its time from the host, but neither date nor an “ntpdate” affected the clock on the guest. -What are the other options that we should use? -rtc [base=utc|localtime|date][,clock=host|vm][,driftfix=none|slew] Specify base as utc or localtime to let the RTC start at the current UTC or local time, respectively. localtime is required for correct date in MS-DOS or Windows. To start at a specific point in time, provide date in the format 2006-06-17T16:01:21 or 2006-06-17. The default base is UTC. By default the RTC is driven by the host system time. This allows to use the RTC as accurate reference clock inside the guest, specifically if the host time is smoothly following an accurate external reference clock, e.g. via NTP. If you want to isolate the guest time from the host, even prevent it from progressing during suspension, you can set clock to vm instead. Enable driftfix (i386 targets only) if you experience time drift problems, specifically with Windows' ACPI HAL. This option will try to figure out how many timer interrupts were not processed by the Windows guest and will re-inject them. Can someone shed light on what we are missing? Any pointers will be helpful. Thanks -a
Re: [Qemu-devel] Question on kvm_clock working ...
Hi Phil, Thanks for an excellent explanation. Follow up question - ...additional clock source named kvmclock, which uses the host clock and the TSC: The host exports its current system time (plus some configurable offset) and a snapshot value of TSC register when doing so. Than the guest can interpolate the current time by using the exported_system_time + scale * (current_TSC_value-snapshot_TSC_value). This kvmclock doesn't have anything to do with the RTC clock as far as I know. Question: Since I am changing time on host using date --set ..., it should change the value of the host clock, no? My question was since the host clock is changed, by the above logic, why is the guest clock also not changing? My current source is kvmclock. Thanks -a On Tue, Sep 13, 2011 at 2:49 AM, Philipp Hahn h...@univention.de wrote: Hello Al, I just debugged a kvmclock bug, so I claim to have some knowledge in this area now, but please take my answer with a grain of doubt. On Monday 12 September 2011 15:21:25 al pat wrote: Still seeking your guidance on this. Appreciate any pointers you may have. You have to distiguish between the real-time-clock (RTC), which in hardware is a battery powered clock running even when your PC is powered off. Since it's slow to access, most Linux distributions read out its value once during boot using hwclock --hctosys --utc and than don't care about that clock any more until shutdown, when they write back the system time to the RTC using ... --systohc During runtime, other methods are used for time keeping: Either by counting regular interrupts, using the ACPI-PM clock, or the High Performance Event Timer (HPET), or the Time Stamp Counter (TSC) register, or ...; see /sys/devices/system/clocksource/clocksource0/available_clocksource for a list of available clock sources. For virtual machines there is an additional clock source named kvmclock, which uses the host clock and the TSC: The host exports its current system time (plus some configurable offset) and a snapshot value of TSC register when doing so. Than the guest can interpolate the current time by using the exported_system_time + scale * (current_TSC_value-snapshot_TSC_value). This kvmclock doesn't have anything to do with the RTC clock as far as I know. Now to your problem: You should check the value of /sys/devices/system/clocksource/clocksource0/current_clocksource in your guest. If it is somethong other than kvmclock, you should if using hwclock --hctosys --utc re-synchronizes your guest clock to the host. Sincerely Philipp -- Philipp Hahn Open Source Software Engineer h...@univention.de Univention GmbHLinux for Your Businessfon: +49 421 22 232- 0 Mary-Somerville-Str.1 D-28359 Bremen fax: +49 421 22 232-99 http://www.univention.de/ Treffen Sie Univention auf der ITBusiness vom 20. bis 22. September 2011 auf dem Gemeinschaftsstand der Open Source Business Alliance in Stuttgart in Halle 3 Stand 3D27-7.
Re: [Qemu-devel] Question on kvm_clock working ...
Hello Al, I just debugged a kvmclock bug, so I claim to have some knowledge in this area now, but please take my answer with a grain of doubt. On Monday 12 September 2011 15:21:25 al pat wrote: Still seeking your guidance on this. Appreciate any pointers you may have. You have to distiguish between the real-time-clock (RTC), which in hardware is a battery powered clock running even when your PC is powered off. Since it's slow to access, most Linux distributions read out its value once during boot using hwclock --hctosys --utc and than don't care about that clock any more until shutdown, when they write back the system time to the RTC using ... --systohc During runtime, other methods are used for time keeping: Either by counting regular interrupts, using the ACPI-PM clock, or the High Performance Event Timer (HPET), or the Time Stamp Counter (TSC) register, or ...; see /sys/devices/system/clocksource/clocksource0/available_clocksource for a list of available clock sources. For virtual machines there is an additional clock source named kvmclock, which uses the host clock and the TSC: The host exports its current system time (plus some configurable offset) and a snapshot value of TSC register when doing so. Than the guest can interpolate the current time by using the exported_system_time + scale * (current_TSC_value-snapshot_TSC_value). This kvmclock doesn't have anything to do with the RTC clock as far as I know. Now to your problem: You should check the value of /sys/devices/system/clocksource/clocksource0/current_clocksource in your guest. If it is somethong other than kvmclock, you should if using hwclock --hctosys --utc re-synchronizes your guest clock to the host. Sincerely Philipp -- Philipp Hahn Open Source Software Engineer h...@univention.de Univention GmbHLinux for Your Businessfon: +49 421 22 232- 0 Mary-Somerville-Str.1 D-28359 Bremen fax: +49 421 22 232-99 http://www.univention.de/ Treffen Sie Univention auf der ITBusiness vom 20. bis 22. September 2011 auf dem Gemeinschaftsstand der Open Source Business Alliance in Stuttgart in Halle 3 Stand 3D27-7. signature.asc Description: This is a digitally signed message part.
Re: [Qemu-devel] Question on kvm_clock working ...
Thanks Phillip. My current source is kvm-clock. WHen I start my guest, it is in sync with the host clock. Then, I chance the time on my host - using date --set I don't see the guest update its time. I was expecting that the guest will detect host time change and change it? So, when the host is exporting its system time and TSC values, does it go into the emulated RTC of the guest and the guest checks it only once? Or does the guest resync its clock with the host's value periodically? I can try to do: hwclock --hctosys --utc --- this is just to check. (I have kvm-clock as my clock source though). Thanks -a On Tue, Sep 13, 2011 at 2:49 AM, Philipp Hahn h...@univention.de wrote: Hello Al, I just debugged a kvmclock bug, so I claim to have some knowledge in this area now, but please take my answer with a grain of doubt. On Monday 12 September 2011 15:21:25 al pat wrote: Still seeking your guidance on this. Appreciate any pointers you may have. You have to distiguish between the real-time-clock (RTC), which in hardware is a battery powered clock running even when your PC is powered off. Since it's slow to access, most Linux distributions read out its value once during boot using hwclock --hctosys --utc and than don't care about that clock any more until shutdown, when they write back the system time to the RTC using ... --systohc During runtime, other methods are used for time keeping: Either by counting regular interrupts, using the ACPI-PM clock, or the High Performance Event Timer (HPET), or the Time Stamp Counter (TSC) register, or ...; see /sys/devices/system/clocksource/clocksource0/available_clocksource for a list of available clock sources. For virtual machines there is an additional clock source named kvmclock, which uses the host clock and the TSC: The host exports its current system time (plus some configurable offset) and a snapshot value of TSC register when doing so. Than the guest can interpolate the current time by using the exported_system_time + scale * (current_TSC_value-snapshot_TSC_value). This kvmclock doesn't have anything to do with the RTC clock as far as I know. Now to your problem: You should check the value of /sys/devices/system/clocksource/clocksource0/current_clocksource in your guest. If it is somethong other than kvmclock, you should if using hwclock --hctosys --utc re-synchronizes your guest clock to the host. Sincerely Philipp -- Philipp Hahn Open Source Software Engineer h...@univention.de Univention GmbHLinux for Your Businessfon: +49 421 22 232- 0 Mary-Somerville-Str.1 D-28359 Bremen fax: +49 421 22 232-99 http://www.univention.de/ Treffen Sie Univention auf der ITBusiness vom 20. bis 22. September 2011 auf dem Gemeinschaftsstand der Open Source Business Alliance in Stuttgart in Halle 3 Stand 3D27-7.
Re: [Qemu-devel] Question on kvm_clock working ...
On 2011-09-13 13:38, al pat wrote: Thanks Phillip. My current source is kvm-clock. WHen I start my guest, it is in sync with the host clock. Then, I chance the time on my host - using date --set I don't see the guest update its time. I was expecting that the guest will detect host time change and change it? That's not what kvmclock is supposed to provide. Besides a monotonic clock source, it basically offers an alternative persistent clock, to some degree replacing the emulated RTC. So updates of the host system time are only recognized by the guest kernel when it reboots or suspends/resumes. So, when the host is exporting its system time and TSC values, does it go into the emulated RTC of the guest and the guest checks it only once? Or does the guest resync its clock with the host's value periodically? I can try to do: hwclock --hctosys --utc --- this is just to check. (I have kvm-clock as my clock source though). Re-reading the RTC is a brute-force approach to re-synchronize with the host. If potential clock jumps are OK for your use case, you can go that way. Alternatives are using NTP against a time server or writing a RTC plugin for NTP to synchronize against that local source (a colleague once wrote such a plugin for a special guest, but it never made it into public AFAIK). Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux
Re: [Qemu-devel] Question on kvm_clock working ...
Hi Folks, Still seeking your guidance on this. Appreciate any pointers you may have. Thanks much. -a On Fri, Sep 9, 2011 at 11:28 AM, al pat alps@gmail.com wrote: We are doing an experiment with kvm-clock to validate its effectiveness, particularly when running NTP on the host to make sure the host’s clock stays properly sync. Our observations leads us to a few unanswered questions, including the possibility of a bug (our our misunderstanding of how kvm_clock should work). Our understanding is that kvm_clock will help sync the clock between the host and the guest. We do not observe this to happen in reality and thus this question. We are using Ubuntu 11.04 on the host and the guest. The command we issue to launch the VM is the following: $ sudo kvm -m 500 -rtc clock=host guestos.img We also arranged for Ubuntu to show the seconds on the clock displayed in the menu. Observation 1: Upon launching the VM, we see a time difference between the 2 clock ranging from 1 to 2 seconds. Observation 2: If we change the date on the host (with a command such as “date --set 10:00:00 AM Sep 9, 2011”), the time on the guest remains the same, unaffected. Observation 3: After running for a while without NTP on the host, we run “ntpdate” to sync up the host, but the guest stick with whatever previous time. Another test we will run is to have ntpd on the host and wait for an extended time to see if the guest drifts away from that original 1 or 2 second lag. In the meantime, we are asking you for some input in this regards: Questions -What does the “–rtc clock” option is supposed to mean exactly? According to the man page, the guest should get its time from the host, but neither date nor an “ntpdate” affected the clock on the guest. -What are the other options that we should use? -rtc [base=utc|localtime|date][,clock=host|vm][,driftfix=none|slew] Specify base as utc or localtime to let the RTC start at the current UTC or local time, respectively. localtime is required for correct date in MS-DOS or Windows. To start at a specific point in time, provide date in the format 2006-06-17T16:01:21 or 2006-06-17. The default base is UTC. By default the RTC is driven by the host system time. This allows to use the RTC as accurate reference clock inside the guest, specifically if the host time is smoothly following an accurate external reference clock, e.g. via NTP. If you want to isolate the guest time from the host, even prevent it from progressing during suspension, you can set clock to vm instead. Enable driftfix (i386 targets only) if you experience time drift problems, specifically with Windows' ACPI HAL. This option will try to figure out how many timer interrupts were not processed by the Windows guest and will re-inject them. Can someone shed light on what we are missing? Any pointers will be helpful. Thanks -a
