linux-kernel - Re: [RFC v2 7/9] x86/mm: Introduce Remote Action Request

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Message-Id: <4A879001-E213-4239-9D25-CDA8EC3E2CD9@gmail.com>
Date: Tue, 20 May 2025 14:29:44 +0300
From: Nadav Amit <nadav.amit@...il.com>
To: Rik van Riel <riel@...riel.com>
Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
 "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
 the arch/x86 maintainers <x86@...nel.org>,
 kernel-team@...a.com,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 luto@...nel.org,
 peterz@...radead.org,
 Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>,
 "H. Peter Anvin" <hpa@...or.com>,
 Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [RFC v2 7/9] x86/mm: Introduce Remote Action Request

Not a full review, but..

> On 20 May 2025, at 4:02, Rik van Riel <riel@...riel.com> wrote:
> 
> +/*
> + * This is a modified version of smp_call_function_many() of kernel/smp.c,

The updated function names is smp_call_function_many_cond() and it is
not aligned with smp_call_rar_many. I think the new version is (suprisingly)
better, so it’d be beneficial to bring smp_call_rar_many() to be like the
updated one in smp.c.

> + * without a function pointer, because the RAR handler is the ucode.
> + */
> +void smp_call_rar_many(const struct cpumask *mask, u16 pcid,
> +		       unsigned long start, unsigned long end)
> +{
> +	unsigned long pages = (end - start + PAGE_SIZE) / PAGE_SIZE;
> +	int cpu, next_cpu, this_cpu = smp_processor_id();
> +	cpumask_t *dest_mask;
> +	unsigned long idx;
> +
> +	if (pages > RAR_INVLPG_MAX_PAGES || end == TLB_FLUSH_ALL)
> +		pages = RAR_INVLPG_MAX_PAGES;
> +
> +	/*
> +	 * Can deadlock when called with interrupts disabled.
> +	 * We allow cpu's that are not yet online though, as no one else can
> +	 * send smp call function interrupt to this cpu and as such deadlocks
> +	 * can't happen.
> +	 */
> +	WARN_ON_ONCE(cpu_online(this_cpu) && irqs_disabled()
> +		     && !oops_in_progress && !early_boot_irqs_disabled);

I thought you agreed to change it to make it use lockdep instead (so it will
be compiled out without LOCKDEP), like done in smp_call_function_many_cond()

> +
> +	/* Try to fastpath.  So, what's a CPU they want?  Ignoring this one. */
> +	cpu = cpumask_first_and(mask, cpu_online_mask);
> +	if (cpu == this_cpu)
> +		cpu = cpumask_next_and(cpu, mask, cpu_online_mask);
> +

Putting aside the rest of the code, I see you don’t call should_flush_tlb().
I think it is worth mentioning in commit log or comment the rationale behind
it (and maybe benchmarks to justify it).