4 ms·

For the benefit of anyone reading this, KVM and VMWare virtualization generally work. Xen has problems because of a stupid Xen workaround for a stupid Intel ha

by khuey 9y ago

For the benefit of anyone reading this, KVM and VMWare virtualization generally work. Xen has problems because of a stupid Xen workaround for a stupid Intel hardware bug from a decade ago. I can provide more details about that via email (in my profile) if desired.

paulie_a 9y ago
Can you please just post the info. Intel deserves to be shamed
- voidmain0001 9y ago
  Is this what khuey is referring to?: https://support.citrix.com/article/CTX136003 https://support.citrix.com/article/CTX136003
- khuey 9y ago
  One of the things the performance monitoring unit (PMU) is capable of doing is triggering an interrupt (the PMI) when a counter overflows. When combined with the ability to write to the counters, this lets you program the PMU to interrupt after a certain number of counted events. Nehalem supposedly had a bug where the PMI fires not on overflow but instead whenever the counter is zero. Xen added a workaround to set the value to 1 whenever it would instead be 0. Later this was observed on microarchitectures other than Nehalem and Xen broadened the workaround to run on every x86 CPU. Intel never provided any help in narrowing it down and there don't seem to be official errata for this behavior too. This behavior is ok for statistically profiling frequent events but if you depend on exact counts (as rr does) or are profiling infrequent events it can mess up your day. https://lists.xen.org/archives/html/xen-devel/2017-07/msg02242.html https://lists.xen.org/archives/html/xen-devel/2017-07/msg022... goes a little deeper and has citations.
irishcoffee 9y ago
Seconding paulie_a, We're running a Xen stack right now and I haven't heard of this. We've worked around a few nasty bugs with Xen and linux doms already, but I'm wondering if we have this problem you're referring to and don't even know it.

4 ms·

For the benefit of anyone reading this, KVM and VMWare virtualization generally work. Xen has problems because of a stupid Xen workaround for a stupid Intel ha

by khuey 9y ago

paulie_a 9y ago
Can you please just post the info. Intel deserves to be shamed
- voidmain0001 9y ago
  Is this what khuey is referring to?: https://support.citrix.com/article/CTX136003 https://support.citrix.com/article/CTX136003
- khuey 9y ago
  One of the things the performance monitoring unit (PMU) is capable of doing is triggering an interrupt (the PMI) when a counter overflows. When combined with the ability to write to the counters, this lets you program the PMU to interrupt after a certain number of counted events. Nehalem supposedly had a bug where the PMI fires not on overflow but instead whenever the counter is zero. Xen added a workaround to set the value to 1 whenever it would instead be 0. Later this was observed on microarchitectures other than Nehalem and Xen broadened the workaround to run on every x86 CPU. Intel never provided any help in narrowing it down and there don't seem to be official errata for this behavior too. This behavior is ok for statistically profiling frequent events but if you depend on exact counts (as rr does) or are profiling infrequent events it can mess up your day. https://lists.xen.org/archives/html/xen-devel/2017-07/msg02242.html https://lists.xen.org/archives/html/xen-devel/2017-07/msg022... goes a little deeper and has citations.
irishcoffee 9y ago
Seconding paulie_a, We're running a Xen stack right now and I haven't heard of this. We've worked around a few nasty bugs with Xen and linux doms already, but I'm wondering if we have this problem you're referring to and don't even know it.