Compute Pressure Level 1

The new PressureObserver(callback) constructor steps are:

1. Set this.[[Callback]] to callback.

The observe(source, options) method steps are:

1. Let relevantGlobal be this's relevant global object.
2. For each document in relevantGlobal's owning document set:
   1. If document is not allowed to use the policy-controlled feature token "compute-pressure", return a promise rejected with NotAllowedError.
   Note
3. Set this.[[SampleIntervalMap]][source] to options's sampleInterval.
4. Let promise be a new promise.
5. Let pendingPromiseTuple be (source, promise).
6. Append pendingPromiseTuple to this.[[PendingObservePromises]].
7. React to promise:
   • If promise was fulfilled or rejected, then:
     1. Remove tuple from this.[[PendingObservePromises]].
8. Run the following steps in parallel:
   1. Let platformCollector be null.
   2. If relevantGlobal's platform collector mapping contains source:
      1. Set platformCollector to relevantGlobal's platform collector mapping[source].
   3. Otherwise:
      1. Let newCollector be a new platform collector whose associated pressure source is null.
      2. Let virtualPressureSource be the result of invoking get a virtual pressure source with source and relevantGlobal.
      3. If virtualPressureSource is not null:
         1. If virtualPressureSource's can provide samples is true:
            1. Set newCollector's associated pressure source to virtualPressureSource.
            2. Append newCollector to virtualPressureSource's connected platform collectors.
      4. Otherwise:
         1. Let realPressureSource be an implementation-defined pressure source that provides telemetry data about source, or null if none exists.
         2. Set newCollector's associated pressure source to realPressureSource.
      5. If newCollector's associated pressure source is not null:
         1. Set platformCollector to newCollector.
         2. Set relevantGlobal's platform collector mapping[source] to platformCollector.
   4. If platformCollector is null, queue a global task on the PressureObserver task source given relevantGlobal to reject promise NotSupportedError and abort these steps.
   5. Invoke activate data collection with source and relevantGlobal.
   6. Queue a global task on the PressureObserver task source given relevantGlobal to run these steps:
      1. If promise was rejected, run the following substeps:
         1. If relevantGlobal's registered observer list for source is empty, invoke deactivate data collection with source and relevantGlobal.
         2. Return.
      2. Append a new registered observer whose observer is this to relevantGlobal's registered observer list for source.
      3. Resolve promise.
9. Return promise.

The unobserve(source) method steps are:

1. If source is not a supported source type, throw "NotSupportedError".
2. Remove from this.[[QueuedRecords]] all records associated with source.
3. Remove this.[[SampleIntervalMap]][source].
4. Remove this.[[LastRecordMap]][source].
5. Remove this.[[AfterPenaltyRecordMap]][source].
6. For each (promiseSource, pendingPromise) of this.[[PendingObservePromises]], if source is equal to promiseSource, reject pendingPromise with an AbortError.
7. Let relevantGlobal be this's relevant global object.
8. Let registeredObserverList be relevantGlobal's registered observer list for source.
9. Remove any registered observer from registeredObserverList whose observer is this.
10. If registeredObserverList is empty:
    1. Invoke deactivate data collection with source and relevantGlobal.
    2. Remove relevantGlobal's platform collector mapping[source].

The disconnect() method steps are:

1. Empty observer.[[QueuedRecords]].
2. Clear this.[[SampleIntervalMap]].
3. Clear this.[[LastRecordMap]].
4. Clear this.[[AfterPenaltyRecordMap]].
5. For each (promiseSource, pendingPromise) of this.[[PendingObservePromises]], reject pendingPromise with an AbortError.
6. Let relevantGlobal be this's relevant global object.
7. For each source → registeredObserverList of relevantGlobal's registered observer list ordered map:
   1. Remove any registered observer from registeredObserverList whose observer is this.
   2. If registeredObserverList is empty:
      1. Invoke deactivate data collection with source and relevantGlobal.
      2. Remove relevantGlobal's platform collector mapping[source].

The takeRecords() method steps are:

1. Let records be a clone of observer.[[QueuedRecords]].
2. Empty observer.[[QueuedRecords]].
3. Return records.

The knownSources getter steps are:

1. Return user agent's supported source types in alphabetical order.

The source getter steps are to return its [[Source]] internal slot.

The state getter steps are to return its [[State]] internal slot.

The time getter steps are to return its [[Time]] internal slot.

When PressureRecord.toJSON is called, run Web IDL Standard's default toJSON steps.

The sampleInterval member represents the requested sampling interval expressed in milliseconds. In the case the value it set to 0, the system will only call the PressureUpdateCallback in the case there is a change to the PressureState.

The reset observation window steps given the argument observer, are as follows:

• set observer.[[ObservationWindow]] to an implementation-defined randomized integer value in milliseconds within an implementation-defined range.
• set observer.[[MaxChangesThreshold]] to an implementation-defined randomized integer value of maximum allowed changes within the observationWindow within an implementation-defined range.
• set observer.[[PenaltyDuration]] to an implementation-defined randomized integer value in milliseconds, within an implementation-defined range.
• Empty the observer.[[ChangesCountMap]] map.

To determine the owning document set for a relevant global object relevantGlobal:

1. Let owningDocumentSet be an empty set.
2. If relevantGlobal is Window, then append relevantGlobal's associated document to owningDocumentSet.
3. Otherwise, for each owner in WorkerGlobalScope relevantGlobal's owner set:
   1. If owner is a Document, then append owner to owningDocumentSet.
   2. If owner is a WorkerGlobalScope, set owningDocumentSet to the union of owningDocumentSet and owner's owning document set.
4. Return owningDocumentSet.

The document has implicit focus steps given the argument document, are as follows:

1. If document is not fully active, return false.
2. Let relevantGlobal be document's relevant global object.
3. For each origin in initiators of active Picture-in-Picture sessions:
   1. If relevantGlobal's relevant settings object's origin is same origin with origin, return true.
4. If relevantGlobal's browsing context is capturing, return true.
5. Let topLevelBC be relevantGlobal's browsing context's top-level browsing context.
6. If topLevelBC does not have system focus, return false.
7. Let focusedDocument be the topLevelBC's currently focused area's node document.
8. If relevantGlobal's relevant settings object's origin is same origin with focusedDocument's origin, return true.
9. Otherwise, return false.

The may receive data steps given the argument observer are as follows:

1. Let relevantGlobal be observer's relevant global object.
2. If relevantGlobal is a Window object:
   1. Return the result of running document has implicit focus with relevantGlobal's associated Document.
3. If relevantGlobal is a WorkerGlobalScope object:
   1. Let owningDocuments be relevantGlobal's owning document set.
   2. For each document in owningDocuments:
      1. If the result of running document has implicit focus with document is true, return true.
      2. Otherwise, continue.
4. Return false.

1. If observer.[[LastRecordMap]][source] does not exist, return true.
2. Let record be observer.[[LastRecordMap]][source].
3. Let sampleInterval be observer.[[SampleIntervalMap]][source].
4. Let timeDeltaMilliseconds = timestamp - record.[[Time]].
5. If timeDeltaMilliseconds ≥ sampleInterval, return true, otherwise return false.

1. If observer.[[SampleIntervalMap]][source] > 0, return true.
2. If observer.[[LastRecordMap]][source] does not exist, return true.
3. Let record be observer.[[LastRecordMap]][source].
4. If record.[[State]] is not equal to state, return true.
5. Return false.

1. Increment observer.[[ChangesCountMap]][source].
2. Return observer.[[ChangesCountMap]][source] ≤ observer.[[MaxChangesThreshold]].

To get a virtual pressure source, given a source type source and relevantGlobal, perform the following steps. They return a virtual pressure source or null.

1. Let topLevelTraversable be null.
2. If relevantGlobal is a Window object:
   1. Set topLevelTraversable to relevantGlobal's navigable's top-level traversable.
3. If relevantGlobal is a DedicatedWorkerGlobalScope object:
   1. Let owningDocuments be relevantGlobal's owning document set.
   2. If owningDocuments is empty, return null.
   3. Assert: owningDocuments's size is 1.
   4. Set topLevelTraversable to owningDocuments[0]'s node navigable's top-level traversable.
4. If topLevelTraversable is null, return null.
5. Let topLevelVirtualPressureSourceMapping be the topLevelTraversable's virtual pressure source mapping.
6. Let virtualPressureSource be null.
7. If topLevelVirtualPressureSourceMapping contains source:
   1. Set virtualPressureSource to topLevelVirtualPressureSourceMapping[source].
8. Return virtualPressureSource.

To activate data collection given a source type source and relevantGlobal, perform the following steps:

1. If relevantGlobal's platform collector mapping does not contain source, abort these steps.
2. Let platformCollector be relevantGlobal's platform collector mapping[source].
3. If platformCollector's activated is true, abort these steps.
4. Set platformCollector's activated to true.
5. In an implementation-defined manner, start running the data collection steps with relevantGlobal, source, and platformCollector.
   Note

To deactivate data collection given a source type source and relevantGlobal, perform the following steps:

1. If relevantGlobal's platform collector mapping does not contain source, abort these steps.
2. Let platformCollector be relevantGlobal's platform collector mapping[source].
3. If platformCollector's activated is false, abort these steps.
4. In an implementation-defined manner, stop running the data collection steps with relevantGlobal, source, and platformCollector.
5. Set platformCollector's activated to false.
6. If platformCollector's associated pressure source is a virtual pressure source:
   1. Remove platformCollector from its associated pressure source's connected platform collectors.
7. Otherwise, perform any implementation-defined steps to signal to platformCollector's associated pressure source to stop retrieving telemetry data.

The data collection steps given relevantGlobal, source and platformCollector are as follows:

1. Let pressureSource be platformCollector's associated pressure source.
2. If pressureSource is null, abort these steps.
3. Let sample be pressureSource's latest sample.
4. If sample is null, abort these steps.
5. Let state be null.
6. If pressureSource is a virtual pressure source:
   1. Set state to the PressureState stored in sample's data.
7. Otherwise:
   1. If change in contributing factors is substantial is false, abort these steps.
   2. Set state to an adjusted pressure state calculated from source and sample's data.
      Note
8. Assert: state is not null.
9. Let rawTimestamp be sample's timestamp.
10. Let timeValue be the relative high resolution time based on rawTimestamp and relevantGlobal.
11. For each observer in relevantGlobal's registered observer list for source:
    1. If running may receive data with observer returns false, continue.
    2. If running passes rate test with observer, source and timeValue returns false, continue.
    3. If running should dispatch with observer, source, state returns false, continue.
    4. Let record be a new PressureRecord object with its [[Source]] set to source, [[State]] set to state, and [[Time]] set to timeValue.
    5. If observer.[[AfterPenaltyRecordMap]][source] exists:
       1. Set observer.[[AfterPenaltyRecordMap]][source] to record.
       2. Continue.
    6. If running passes rate obfuscation test with observer and source returns false:
       1. Set observer.[[AfterPenaltyRecordMap]][source] to record.
       2. Set observer.[[ChangesCountMap]][source] to 0.
       3. Create timer of observer.[[PenaltyDuration]] duration with the following callback:
          1. If observer.[[AfterPenaltyRecordMap]][source] exists:
             1. Let record be observer.[[AfterPenaltyRecordMap]][source].
             2. Remove observer.[[AfterPenaltyRecordMap]][source].
             3. Run queue a record with observer, source, record.
       4. Continue.
    7. Run queue a record with observer, source, record.

To queue a record given the arguments observer, source, record, run these steps:

1. If size of observer.[[QueuedRecords]] is greater than max queued records, then remove the first item.
2. Append record to observer.[[QueuedRecords]].
3. Set observer.[[LastRecordMap]][source] to record.
4. Queue a pressure observer task with observer's relevant global object.

The PressureObserver task source is a task source used for scheduling tasks to 10.6.5 Notify Pressure Observers.

To queue a pressure observer task given relevantGlobal as input, run these steps:

1. If the relevantGlobal's pressure observer task queued is true, then return.
2. Set the relevantGlobal's pressure observer task queued to true.
3. Queue a global task on PressureObserver task source with relevantGlobal to notify pressure observers.

To notify pressure observers given relevantGlobal as input, run these steps:

1. Set relevantGlobal's pressure observer task queued to false.
2. Let notifySet be a new set of all observers in relevantGlobal’s registered observer lists.
3. For each observer of notifySet:
   1. Let records be a clone of observer.[[QueuedRecords]].
   2. Empty observer.[[QueuedRecords]].
   3. If records is not empty, then invoke observer.[[Callback]] with records and observer. If this throws an exception, catch it, and report the exception.

This specification defines the following unloading document cleanup steps given a Document document:

1. Let relevantGlobal be document's relevant global object.
2. For each source key of relevantGlobal's registered observer list ordered map:
   1. Invoke deactivate data collection with source and relevantGlobal.
   2. Remove relevantGlobal's platform collector mapping[source].

Whenever a WorkerGlobalScope relevantGlobal's closing flag is set to true, perform the following steps:

1. For each source key of relevantGlobal's registered observer list ordered map:
   1. Invoke deactivate data collection with source and relevantGlobal.
   2. Remove relevantGlobal's platform collector mapping[source].

It may be possible to identify users across non-same origin sites if unique or very precise values can be accessed at the same time by sites not sharing origin. This attack is mitigated by 11.2.1 Data minimization, 11.2.2 Rate obfuscation, and 11.2.7 Same-origin restriction.

In computer security a covert channel creates a capability to transfer information between processes that are not supposed to be allowed to communicate. In modern multi-process web engines in the generic case each window or tab resides in its own process (documents that have the same origin or sites that have the same site typically share the same process). Using this API it may be possible to create a cross-site covert channel C where a site A on one tab first broadcasts to the channel C after having manipulated the state of the CPU. Next a site B (that is not same site with site A) on another tab reads the broadcasted data from the channel C by using this API to learn when the state of the CPU has changed. This process is repeated as long as the scripts run on both the sites A and B.

This attack is mitigated by 11.2.2 Rate obfuscation and 11.2.5 Break calibration. Implementers are advised to consider all these mitigations for long-running scripts.

Targeted de-anonymization attacks constitute a critical class of threats that jeopardize a user's anonymity. These attacks allow a malicious or partially compromised website (referred to as the “malicious site”) to ascertain whether a website visitor possesses a specific public identifier, such as an email address or a social media handle.

While anonymity may be a luxury for some, for certain individuals, it is far more than that—it is a matter of survival. Consider for instance those who engage in political protests, work as journalists covering sensitive topics, etc.

As an example, an attacker can privately share a resource with the target for instance using a public resource sharing service (“victim site”), and then measure side-effects (indicating successful access) on loading the resource via side-channels. If the logged in visitor can access the embedded resource successfully, that indicates that the current visit is indeed the intended target.

Specifically, exposing reliable information about the total CPU pressure can let an attacking site understand if a target of a cross-origin navigation (e.g. an iframe or pop-up window from another site) performed a CPU-intensive operation.

Techniques such as pop-under and tab-under can be used to hide the loading from the user.

One possible attack is that the malicious website opens e.g., a popup to a resource on a victim site to which the user is logged in (e.g. a video streaming site or online document editor) pointing to a resource shared with specific users.

Assuming that loading the resource puts increased pressure on the CPU, this would create a side-channel reveals to the attacking site if the user is logged into an account with access to the resource, deanonymizing the user.

Given that modern CPUs recover quickly from high pressure, one possible mitigation strategy could be to temporarily disable readings for a few seconds after loading popup and iframe content.

This specification adheres to the generic data minimization principles to limit exposure of data related to low-level details of the underlying platform to the minimum required to address its high-value use cases. This includes consideration for limiting exposure of identifying information about devices.

The specific application of data minimization principles in the context of this specification are discussed in 11.2.2 Rate obfuscation and 11.2.7 Same-origin restriction.

The specification requires implementing the rate obfuscation mitigation to keep track of the number of pressure changes over an implementation-defined sliding observation window and set a flag if an implementation-defined threshold for the number of pressure changes is exceeded. Similarly, it is also recommended for the implementation to observe any abnormal activity such as a high number of pressure state changes spanning across multiple states, and set this flag similarly.

If this flag is set, the implementation is recommended to give the pressure observer a penalty during which it will not be able to inform scripts of changes in its pressure state as it normally would. The duration of this penalty is implementation-defined and it is recommended to be randomized. When notify pressure observers resumes operation after the penalty, it only reports the latest pressure state and disregards any interim state information received from the platform collector during this penalty.

Based on implementation experience, implementers must use:

• a range in between 50 and 100 changes for PressureObserver's [[MaxChangesThreshold]] internal slot.
• a range in between 5000 milliseconds and 10000 milliseconds for PressureObserver's [[PenaltyDuration]] internal slot.

This section is non-normative.

Based on implementation experience, implementers are advised to use:

• a range in between 300000 milliseconds (5 minutes) and 600000 milliseconds (10 minutes) for PressureObserver's [[ObservationWindow]] internal slot.

In a calibration process an attacker tries to manipulate the CPU so that this API would report a transition into a certain pressure state with the highest probability in response to the pressure exerted by the fabricated workload. This break calibration mitigation solution can slow down or prevent this calibration process from succeeding by slightly changing at runtime the implementation-defined low-level hardware metrics that contribute to these pressure state transitions. Even if the initial calibration would succeed, its results will be invalidated at runtime when this mitigation is running continuously. Any attempts to recalibrate will similarly be mitigated against.

This section is non-normative.

Based on implementation experience, implementers are advised to apply the mitigation to a randomized time value within a range between 120000 milliseconds (2 minutes) and 240000 milliseconds (4 minutes).

By default data delivery is restricted to documents served from the same-origin as an initiator of an active picture-in-picture-session, documents capturing or the document with system focus, if any.

The documents qualifying for data delivery, under the above rules, can delegate it to documents in child navigables.

The feature can be extended to third-party contexts such as iframes only by a declared policy.

Shared workers can be shared across documents, such as top level document and those associated iframes. If one of the documents in the owner set passes the above data delivery requirements, the shared worker will qualify for data delivery. This means that the embedded iframe is able to pass along the data to the embedding document.