linux-kernel - Re: [PATCH v2 12/14] platform/x86/intel/ifs: Add current

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Message-ID: <Y2/x/vdtE0ciuOhE@zn.tnic>
Date:   Sat, 12 Nov 2022 20:20:30 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Thiago Macieira <thiago.macieira@...el.com>
Cc:     Jithu Joseph <jithu.joseph@...el.com>, hdegoede@...hat.com,
        markgross@...nel.org, tglx@...utronix.de, mingo@...hat.com,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        gregkh@...uxfoundation.org, ashok.raj@...el.com,
        tony.luck@...el.com, linux-kernel@...r.kernel.org,
        platform-driver-x86@...r.kernel.org, patches@...ts.linux.dev,
        ravi.v.shankar@...el.com, athenas.jimenez.gonzalez@...el.com,
        sohil.mehta@...el.com
Subject: Re: [PATCH v2 12/14] platform/x86/intel/ifs: Add current_batch sysfs
 entry

On Sat, Nov 12, 2022 at 10:21:35AM -0800, Thiago Macieira wrote:
> Not exactly. That's what this file is there for. It allows the algorithm to 
> read the current batch file, add 1, then echo back. If the load succeeds, the 
> the batch exists; if not, then the algorithm should simply go back to 0.

This sounds to me like there's a special order in which those batches
should be executed?

I thought they're simply collections of test sequences which can be run
in any order...

> First, there's the question of the ability to see into /lib/firmware. I'm not a 
> kernel dev but I'm told that request_firmware() only operates on the root 
> container's filesystem view. We're expecting that the application may get 
> deployed as a container (with full privileges so it can write to /sys, sure), 
> so it won't be able to see the host system's /lib to know what files are 
> available. It could "guess" at the file names, based on the current processor's 
> family/model/stepping and a natural number, but that's sub-optimal.

It is not about seeing - you simply give it the filename -
request_firmware* does the "seeing". Either the file's there or it
isn't.

> Unless the driver were allowed to load any file named by the application, from 
> its own view of the filesystem, permitting the firmware files being distributed 
> inside the container.

There's a reason I wrote:

"There will be no requirement on the naming - only on the filename
length and it should be in that directory /lib/firmware/intel/ifs_0/"

Of course the driver should load only from that directory.

> Second, for electrical reasons, we expect that certain processor generations 
> will need a timeout between tests before testing can be done again on a given 
> core, whether the same batch or the next one. This time out can be in the 
> order of many minutes, which is longer than any hyperscaler is willing to 
> allocate for a system self-test hogging a core or the whole system, just 
> waiting. For example, let's say that the timeout is 15 minutes and there are 4 
> batches: this means the whole testing procedure takes one hour, even though 
> the actual downtime for each core was less than 1 second. This is lost 
> revenue.

All that doesn't matter - if the CPU *must* wait 15 minutes between
batches, then that should be enforced by the driver and not relied upon
by userspace to DTRT.

> Instead, they wish the next available maintenance window to simply resume 
> testing at the point where the last one stopped. These windows need not be 
> scheduled; they can also be opportunistic, when the orchestrator determines 
> the machine or a subset of one is going to be idle. That's what the algorithm 
> in the pull request above implements: if the current_batch's result was 
> "untested", it is attempted again, otherwise it tries the next one, rolling 
> back to 0 if the loading failed. This removes the need to know anything about 
> the timeout on the current processor or even whether there is one, or how many 
> batches there are.242

This all has nothing to do with whether you give it a number or a
filename. How you glue your testing around it together is a userspace
issue - all the kernel driver needs to be able to do is load the
sequence and execute it.

Echoing filenames into sysfs is no different from echoing numbers into
it - former is simpler. If the CPU says it cannot execute the sequence
currently, you have to think about how you retry that sequence. How you
specify it doesn't matter.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette