linux-kernel - Re: [PATCH v1 1/2] x86/cpufeature: Add facility to match microcode revisions

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Date:   Sat, 6 Oct 2018 11:15:07 -0700
From:   Andi Kleen <andi@...stfloor.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andi Kleen <andi@...stfloor.org>, peterz@...radead.org,
        x86@...nel.org, linux-kernel@...r.kernel.org, eranian@...gle.com,
        kan.liang@...el.com, isaku.yamahata@...el.com, kvm@...r.kernel.org,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH v1 1/2] x86/cpufeature: Add facility to match microcode
 revisions

On Sat, Oct 06, 2018 at 04:14:54PM +0200, Thomas Gleixner wrote:
> On Fri, 5 Oct 2018, Andi Kleen wrote:
> > +/*
> > + * Match specific microcodes or steppings.
> 
> What means microcodes or steppings? If you mean microcode revisions then
> please spell it out and use it all over the place. steppings is confusing
> at best as its associated to the CPU stepping.

The matcher can be used to match specific hardware steppings by setting
the min/max_ucode to 0 or specific microcode revisions 
(which are associated with steppings) 

> > +const struct x86_ucode_id *x86_match_ucode_all(const struct x86_ucode_id *match)
> 
> Can you please name that so it's obvious that this checks all cpus, but
> aside of that, why would a system ever end up with different microcode
> revisions at all? The changelog is not mentioning any reason for this
> function and "/* Check all CPUs */" is not helpful either.

We still support the old microcode interface that allows updates
per CPU, and also it could happen during CPU hotplug.

> 
> > +	int cpu;
> > +	const struct x86_ucode_id *all_m = NULL;
> > +	bool first = true;
> > +
> > +	for_each_online_cpu(cpu) {
> 
> What guarantees that CPUs cannot be plugged? You either need to have
> cpus_read_lock() in this function or a lockdep_assert_cpus_held().

In my case the caller, but yes should be documented.

-Andi