In-Field Scan

29. In-Field Scan¶

29.1. Introduction¶

In Field Scan (IFS) is a hardware feature to run circuit level tests on a CPU core to detect problems that are not caught by parity or ECC checks. Future CPUs will support more than one type of test which will show up with a new platform-device instance-id, for now only .0 is exposed.

29.2. IFS Image¶

Intel provides a firmware file containing the scan tests via github [1]. Similar to microcode there is a separate file for each family-model-stepping.

29.3. IFS Image Loading¶

The driver loads the tests into memory reserved BIOS local to each CPU socket in a two step process using writes to MSRs to first load the SHA hashes for the test. Then the tests themselves. Status MSRs provide feedback on the success/failure of these steps. When a new test file is installed it can be loaded by writing to the driver reload file:

# echo 1 > /sys/devices/virtual/misc/intel_ifs_0/reload

Similar to microcode, the current version of the scan tests is stored in a fixed location: /lib/firmware/intel/ifs.0/family-model-stepping.scan

29.4. Running tests¶

Tests are run by the driver synchronizing execution of all threads on a core and then writing to the ACTIVATE_SCAN MSR on all threads. Instruction execution continues when:

All tests have completed.
Execution was interrupted.
A test detected a problem.

Note that ALL THREADS ON THE CORE ARE EFFECTIVELY OFFLINE FOR THE DURATION OF THE TEST. This can be up to 200 milliseconds. If the system is running latency sensitive applications that cannot tolerate an interruption of this magnitude, the system administrator must arrange to migrate those applications to other cores before running a core test. It may also be necessary to redirect interrupts to other CPUs.

In all cases reading the SCAN_STATUS MSR provides details on what happened. The driver makes the value of this MSR visible to applications via the “details” file (see below). Interrupted tests may be restarted.

The IFS driver provides sysfs interfaces via /sys/devices/virtual/misc/intel_ifs_0/ to control execution:

Test a specific core:

# echo <cpu#> > /sys/devices/virtual/misc/intel_ifs_0/run_test

when HT is enabled any of the sibling cpu# can be specified to test its corresponding physical core. Since the tests are per physical core, the result of testing any thread is same. All siblings must be online to run a core test. It is only necessary to test one thread.

For e.g. to test core corresponding to cpu5

# echo 5 > /sys/devices/virtual/misc/intel_ifs_0/run_test

Results of the last test is provided in /sys:

$ cat /sys/devices/virtual/misc/intel_ifs_0/status
pass

Status can be one of pass, fail, untested

Additional details of the last test is provided by the details file:

$ cat /sys/devices/virtual/misc/intel_ifs_0/details
0x8081

The details file reports the hex value of the SCAN_STATUS MSR. Hardware defined error codes are documented in volume 4 of the Intel Software Developer’s Manual but the error_code field may contain one of the following driver defined software codes:

0xFD	Software timeout
0xFE	Partial completion

29.5. Driver design choices¶

1) The ACTIVATE_SCAN MSR allows for running any consecutive subrange of available tests. But the driver always tries to run all tests and only uses the subrange feature to restart an interrupted test.

2) Hardware allows for some number of cores to be tested in parallel. The driver does not make use of this, it only tests one core at a time.

struct ifs_data¶: attributes related to intel IFS driver

Definition

struct ifs_data {
  int integrity_cap_bit;
  int loaded_version;
  bool loaded;
  bool loading_error;
  int valid_chunks;
  int status;
  u64 scan_details;
};

Members

integrity_cap_bit: MSR_INTEGRITY_CAPS bit enumerating this test
loaded_version: stores the currently loaded ifs image version.
loaded: If a valid test binary has been loaded into the memory
loading_error: Error occurred on another CPU while loading image
valid_chunks: number of chunks which could be validated.
status: it holds simple status pass/fail/untested
scan_details: opaque scan status code from h/w