linux-kernel - Re: [RFC PATCH 15/26] x86/alternatives: Non-emulated text poking

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Date:   Wed, 8 Apr 2020 13:23:29 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ankur Arora <ankur.a.arora@...cle.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, hpa@...or.com,
        jpoimboe@...hat.com, namit@...are.com, mhiramat@...nel.org,
        jgross@...e.com, bp@...en8.de, vkuznets@...hat.com,
        pbonzini@...hat.com, boris.ostrovsky@...cle.com,
        mihai.carabas@...cle.com, kvm@...r.kernel.org,
        xen-devel@...ts.xenproject.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [RFC PATCH 15/26] x86/alternatives: Non-emulated text poking

On Tue, Apr 07, 2020 at 10:03:12PM -0700, Ankur Arora wrote:
> +static int __maybe_unused text_poke_late(patch_worker_t worker, void *stage)
> +{
> +	int ret;
> +
> +	lockdep_assert_held(&text_mutex);
> +
> +	if (system_state != SYSTEM_RUNNING)
> +		return -EINVAL;
> +
> +	text_poke_state.stage = stage;
> +	text_poke_state.num_acks = cpumask_weight(cpu_online_mask);
> +	text_poke_state.head = &alt_modules;
> +
> +	text_poke_state.patch_worker = worker;
> +	text_poke_state.state = PATCH_SYNC_DONE; /* Start state */
> +	text_poke_state.primary_cpu = smp_processor_id();
> +
> +	/*
> +	 * Run the worker on all online CPUs. Don't need to do anything
> +	 * for offline CPUs as they come back online with a clean cache.
> +	 */
> +	ret = stop_machine(patch_worker, &text_poke_state, cpu_online_mask);

This.. that on its own is almost a reason to NAK the entire thing. We're
all working very hard to get rid of stop_machine() and you're adding
one.

Worse, stop_machine() is notoriously crap on over-committed virt, the
exact scenario where you want it.

> +
> +	return ret;
> +}