Note A corresponding flavor extra spec will not be introduced since enabling HPET is really a per-image concern rather than a resource concern for capacity planning. A few options to use Traits were considered as described in the next section, but we end up choosing the simpler approach due to the following reasons: HPET is provided by qemu via emulation, so there are no security implications as there are already better clock sources available. See nova commit ba3fd The HPET device is only supported for x86 architectures, so in a cloud with a mix of architectures the image would have to be specific to ensure the instance is scheduled on an x86 host. Initially we would only support enabling HPET on qemu. Specifying the hypervisor type will ensure the instance is scheduled on a host using the qemu hypervisor.
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Brall A benchmark in that environment for 10 million event counts found that TSC took about 0. When a corresponding timer interrupt is active, this bit is set. This page is not meant as a full description of HPET, only as a lightweight introduction.
If it is set, software can clear it by writing 1 to this bit. The difficulties are exacerbated if the comparator value is not synchronized with the timer immediately, but delayed by one or two ticks, as some chipsets do. INI file to enforce its use. But this device has no driver and is not used at all. These comparators are or bit-wide. In standard mapping, each timer has its own interrupt routing control.
The implicit transformation of a special extra spec into placement-isms is arcane. Determine if timer N is periodic capable, save that information to avoid re-reading it every time. This means that next write to timer N comparator register will have the usual meaning, while second next write will write directly to the accumulator.
This page has been accessed 35, times. Be aware of this when choosing interrupt routing for timers. A popular value is It is recommended to use 32 bit counter when on bit only software. The following is the procedure you need to perform to initialize main counter and comparators in order to receive interrupts. Views Read Edit View history. A corresponding flavor extra spec will not be introduced since enabling HPET is really a per-image concern rather than a resource concern for capacity planning.
If another interrupt occurs before that bit is cleared, the interrupt will remain active. HPET is a continuously running timer that counts upward, not a one-shot device that counts down to zero, causes one interrupt and then stops.
You can help by adding to it. This has the express advantage of being independent of specificatioh CPU frequency and still provides a very reasonable sub-microsecond resolution and accuracy. Determine allowed interrupt routing for current timer and allocate an interrupt for it.
End users can indicate their desire to have HPET in the guest by uploading their own images with ypet same trait. Otherwise it has no effect. The comparators can be put into one-shot mode or periodic mode, with at least one comparator supporting periodic mode and all of them supporting one-shot mode. Bit 3 is also quite straightforward — 1 means periodic timer.
Writes of 0 have no effect. Example Spec — The title of your blueprint. If 32 bit reads are performed on 64 bit hept, consult 2. Comparators are NOT required to support this mode; you must detect this capability when initializing a comparator. Unsourced material may be challenged and removed. Most Related.
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These comparators are or bit-wide. The comparators can be put into one-shot mode or periodic mode, with at least one comparator supporting periodic mode and all of them supporting one-shot mode. Comparators can be driven by the operating system, e. Comparison to predecessors[ edit ] You can help by adding to it. Compared to these older timer circuits, the HPET has higher frequency and wider bit counters although they can be driven in bit mode. A popular value is While and RTC can be put into an HPET-like one-shot mode, the set-up process is so slow that their one-shot mode is not used in practice for tasks requiring precise scheduling.
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This article needs additional citations for verification. I hope the above code is obvious. The comparators can be put into one-shot mode or periodic mode, with at least one comparator supporting periodic mode and all of them supporting one-shot mode. The functionality is dependent of whether edge or level-triggered mode is used for timer n. If she forgets hw: The following operating systems are known to be able to use HPET: But this device has no driver and is not used at all. This has the express advantage of being independent of the CPU frequency and specificxtion provides a very reasonable sub-microsecond resolution and accuracy.