Steven Price writes:

Introduce a paravirtualization interface for KVM/arm64 based on the
"Arm Paravirtualized Time for Arm-Base Systems" specification DEN 0057A.

This only adds the details about "Stolen Time" as the details of "Live
Physical Time" have not been fully agreed.

[...]

+
+Stolen Time
+-----------
+
+The structure pointed to by the PV_TIME_ST hypercall is as follows:
+
+  Field       | Byte Length | Byte Offset | Description
+  ----------- | ----------- | ----------- | --------------------------
+  Revision    |      4      |      0      | Must be 0 for version 0.1
+  Attributes  |      4      |      4      | Must be 0
+  Stolen time |      8      |      8      | Stolen time in unsigned
+              |             |             | nanoseconds indicating how
+              |             |             | much time this VCPU thread
+              |             |             | was involuntarily not
+              |             |             | running on a physical CPU.

I know very little about the topic, but I don't understand how the spec
as proposed allows an accurate reading of the relation between physical
time and stolen time simultaneously. In other words, could you draw
Figure 1 of the spec from within the guest? Or is it a non-objective?

For example, if you read the stolen time before you read CNTVCT_EL0,
isn't it possible for a lengthy event like a migration to occur between
the two reads, causing the stolen time to be obsolete and off by seconds?

--
Cheers,
Christophe de Dinechin (IRC c3d)

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