Date:   Tue, 19 Apr 2022 12:45:03 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Tony Luck <tony.luck@...el.com>,
        Hans de Goede <hdegoede@...hat.com>, markgross@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Jonathan Corbet <corbet@....net>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "Joseph, Jithu" <jithu.joseph@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        platform-driver-x86@...r.kernel.org, patches@...ts.linux.dev,
        Ravi V Shankar <ravi.v.shankar@...el.com>
Subject: Re: [PATCH v3 02/11] Documentation: In-Field Scan

On Tue, Apr 19, 2022 at 9:48 AM Greg KH <gregkh@...uxfoundation.org> wrote:
>
> On Tue, Apr 19, 2022 at 09:38:50AM -0700, Tony Luck wrote:
> > Add documentation for In-Field Scan (IFS). This documentation
> > describes the basics of IFS, the loading IFS image, chunk
> > authentication, running scan and how to check result via sysfs
> > as well as tunable parameters.
> >
> > The CORE_CAPABILITIES MSR enumerates whether IFS is supported.
> >
> > The full  github location for distributing the IFS images is
> > still being decided. So just a placeholder included for now
> > in the documentation.
> >
> > Future CPUs will support more than one type of test. Plan for
> > that now by using a ".0" suffix on the ABI directory names.
> > Additional test types will use ".1", etc.
> >
> > Reviewed-by: Dan Williams <dan.j.williams@...el.com>
> > Signed-off-by: Tony Luck <tony.luck@...el.com>
> > ---
> >  Documentation/x86/ifs.rst   | 101 ++++++++++++++++++++++++++++++++++++
> >  Documentation/x86/index.rst |   1 +
> >  2 files changed, 102 insertions(+)
> >  create mode 100644 Documentation/x86/ifs.rst
> >
> > diff --git a/Documentation/x86/ifs.rst b/Documentation/x86/ifs.rst
> > new file mode 100644
> > index 000000000000..62f3c07d433a
> > --- /dev/null
> > +++ b/Documentation/x86/ifs.rst
> > @@ -0,0 +1,101 @@
> > +.. SPDX-License-Identifier: GPL-2.0
> > +
> > +=============
> > +In-Field Scan
> > +=============
> > +
> > +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.
> > +
> > +
> > +IFS Image
> > +---------
> > +
> > +Intel provides a firmware file containing the scan tests via
> > +github [#f1]_.  Similar to microcode there is a separate file for each
> > +family-model-stepping.
> > +
> > +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/bus/platform/drivers/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
> > +
> > +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:
> > +
> > +1) All tests have completed.
> > +2) Execution was interrupted.
> > +3) A test detected a problem.
> > +
> > +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/platform/intel_ifs.0/
> > +to control execution:
> > +
> > +Test a specific core::
> > +
> > +  # echo <cpu#> > /sys/devices/platform/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. It is only necessary to test one
> > +thread.
> > +
> > +For e.g. to test core corresponding to cpu5
> > +
> > +  # echo 5 > /sys/devices/platform/intel_ifs.0/run_test
> > +
> > +Results of the last test is provided in /sys::
> > +
> > +  $ cat /sys/devices/platform/intel_ifs.0/status
> > +  pass
>
> sysfs documentation belongs in Documentation/ABI/
>
> And why not just include this whole thing in the driver itself and suck
> the documentation out of that?  No need to have a separate file.

At a minimum a separate file is needed to house the
---
 .. kernel-doc:: $source_file
   :doc: $header
---
...statements, but ok, I'll recommend that going forward to
de-emphasize shipping content directly from Documentation/ when it can
be ingested from "DOC:" source. I had been assuming DOC: blocks in the
code were more for augmenting kernel-doc on driver internal ABIs and
not longer theory of operation documentation that is an awkward fit
for Documentation/ABI/.

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