31.1.Introduction¶

In Field Scan (IFS) is a hardware feature to run circuit level tests ona 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 upwith a new platform-device instance-id.

31.2.IFS Image¶

Intel provides firmware files containing the scan tests via the webpage[1].Look under “In-Field Scan Test Images Download” section towards theend of the page. Similar to microcode, there are separate files for eachfamily-model-stepping. IFS Images are not applicable for some test types.Wherever applicable the sysfs directory would provide a “current_batch” file(see below) for loading the image.

https://intel.com/InFieldScan

31.3.IFS Image Loading¶

The driver loads the tests into memory reserved BIOS local to each CPUsocket in a two step process using writes to MSRs to first load theSHA hashes for the test. Then the tests themselves. Status MSRs providefeedback on the success/failure of these steps.

The test files are kept in a fixed location: /lib/firmware/intel/ifs_<n>/For e.g if there are 3 test files, they would be named in the followingfashion:ff-mm-ss-01.scanff-mm-ss-02.scanff-mm-ss-03.scan(where ff refers to family, mm indicates model and ss indicates stepping)

A different test file can be loaded by writing the numerical portion(e.g 1, 2 or 3 in the above scenario) into the curent_batch file.To load ff-mm-ss-02.scan, the following command can be used:

# echo 2 > /sys/devices/virtual/misc/intel_ifs_<n>/current_batch

The above file can also be read to know the currently loaded image.

31.4.Running tests¶

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

1. All tests have completed.

2. Execution was interrupted.

3. A test detected a problem.

Note that ALL THREADS ON THE CORE ARE EFFECTIVELY OFFLINE FOR THEDURATION OF THE TEST. This can be up to 200 milliseconds. If the systemis running latency sensitive applications that cannot tolerate aninterruption of this magnitude, the system administrator must arrangeto 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 corresponding test’s STATUS MSR provides details on whathappened. The driver makes the value of this MSR visible to applicationsvia the “details” file (see below). Interrupted tests may be restarted.

The IFS driver provides sysfs interfaces via /sys/devices/virtual/misc/intel_ifs_<n>/to control execution:

Test a specific core:

# echo <cpu#> > /sys/devices/virtual/misc/intel_ifs_<n>/run_test

when HT is enabled any of the sibling cpu# can be specified to testits corresponding physical core. Since the tests are per physical core,the result of testing any thread is same. All siblings must be onlineto 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_<n>/run_test

Results of the last test is provided in /sys:

$ cat /sys/devices/virtual/misc/intel_ifs_<n>/statuspass

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_<n>/details0x8081

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

31.5.Driver design choices¶

1) The ACTIVATE_SCAN MSR allows for running any consecutive subrange ofavailable tests. But the driver always tries to run all tests and onlyuses 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.

31.6.Structural Based Functional Test at Field (SBAF):¶

SBAF is a new type of testing that provides comprehensive core testcoverage complementing Scan at Field (SAF) testing. SBAF mimics themanufacturing screening environment and leverages the same test suite.It makes use of Design For Test (DFT) observation sites and featuresto maximize coverage in minimum time.

Similar to the SAF test, SBAF isolates the core under test from therest of the system during execution. Upon completion, the coreseamlessly resets to its pre-test state and resumes normal operation.Any machine checks or hangs encountered during the test are confined tothe isolated core, preventing disruption to the overall system.

Like the SAF test, the SBAF test is also divided into multiple batches,and each batch test can take hundreds of milliseconds (100-200 ms) tocomplete. If such a lengthy interruption is undesirable, it isrecommended to relocate the time-sensitive applications to other cores.

structifs_test_msrs¶

MSRs used in IFS tests

struct ifs_test_msrs {    u32 copy_hashes;    u32 copy_hashes_status;    u32 copy_chunks;    u32 copy_chunks_status;    u32 test_ctrl;};

structifs_data¶

attributes related to intel IFS driver

struct ifs_data {    int loaded_version;    bool loaded;    bool loading_error;    int valid_chunks;    int status;    u64 scan_details;    u32 cur_batch;    u32 generation;    u32 chunk_size;    u32 array_gen;    u32 max_bundle;};