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[Xen-devel] Xen's per CPU time calibration

Hi all, 

I have a question about CPU time calibration. In timer ISR, each physical CPU read the platform time to a calibration data structure and then raise the TIME_CALIBRATE_SOFTIRQ. Later the physical CPU will do time calibration through the softirq handler local_time_calibration() based on the platform time it read before. Strictly speaking, this time is NOT the current accurate platform time already because there is a latency between read and use. So why does it work in this way? Why not directly do calibration in timer interrupt?

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