linux-kernel - Re: [PATCH] x86/cacheinfo: Don't use cpu_llc_shared_map for !CONFIG

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Date:   Wed, 17 Aug 2022 21:40:29 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Saurabh Sengar <ssengar@...ux.microsoft.com>
Cc:     ssengar@...rosoft.com, mikelley@...rosoft.com, tglx@...utronix.de,
        mingo@...hat.com, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.com, peterz@...radead.org, tim.c.chen@...ux.intel.com,
        will@...nel.org, song.bao.hua@...ilicon.com,
        suravee.suthikulpanit@....com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/cacheinfo: Don't use cpu_llc_shared_map for
 !CONFIG_SMP

On Wed, Aug 10, 2022 at 09:15:15AM -0700, Saurabh Sengar wrote:
> cpu_llc_shared_map is always declared and defined, but populated in
> arch/x86/kernel/smpboot.c which only builds for CONFIG_SMP=y. For
> UniProcessor builds this mask is never populated and hence invalid.
> Current code doesn't handle the case of AMD UniProcessor correctly,

What is "AMD UniProcessor"?

> which results "shared_cpu_map" and "shared_cpu_list" files missing from
> sysfs entries for l3 cache. This patch fixes this issue.

What issue exactly?

What is the real life use case for this?

> This code used to work because of a another bug in 'cpumask_next',
> where in the CONFIG_SMP=n case the cpumask_next() ignores empty mask
> that results as if CPU 0 was set. This bug in 'cpumask_next' was fixed by
> following commit, which exposes the cpu_llc_shared_map bug.
> 
> b81dce77ced ("cpumask: Fix invalid uniprocessor mask assumption")
> 
> Fixes: 2b83809a5e ("x86/cpu/amd: Derive L3 shared_cpu_map from cpu_llc_shared_mask")

Add

---
[core]
        abbrev = 12

[alias]
        one = show -s --pretty='format:%h (\"%s\")'
---

to your git .config so that when you do

$ git one <commit id>

you can get the proper formatting and abbreviated sha1 for commits.

Thx.

-- 
Regards/Gruss,
    Boris.

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