linux-kernel - Re: [PATCH v14 05/13] x86/mm: use INVLPGB in flush_tlb

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Message-ID: <f57b0b8c-1df4-4c6c-820e-20940aee7d0d@intel.com>
Date: Fri, 28 Feb 2025 11:18:04 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Rik van Riel <riel@...riel.com>, x86@...nel.org
Cc: linux-kernel@...r.kernel.org, bp@...en8.de, peterz@...radead.org,
 dave.hansen@...ux.intel.com, zhengqi.arch@...edance.com,
 nadav.amit@...il.com, thomas.lendacky@....com, kernel-team@...a.com,
 linux-mm@...ck.org, akpm@...ux-foundation.org, jackmanb@...gle.com,
 jannh@...gle.com, mhklinux@...look.com, andrew.cooper3@...rix.com,
 Manali.Shukla@....com, mingo@...nel.org
Subject: Re: [PATCH v14 05/13] x86/mm: use INVLPGB in flush_tlb_all

On 2/25/25 19:00, Rik van Riel wrote:
>  void flush_tlb_all(void)
>  {
>  	count_vm_tlb_event(NR_TLB_REMOTE_FLUSH);
> +
> +	/* First try (faster) hardware-assisted TLB invalidation. */
> +	if (cpu_feature_enabled(X86_FEATURE_INVLPGB)) {
> +		guard(preempt)();
> +		invlpgb_flush_all();
> +		return;
> +	}

We haven't talked at all about the locking rules for
invlpgb_flush_all(). It was used once in this series without any
explicit preempt twiddling. I assume that was because it was used in a
path where preempt is disabled.

If it does need a universal rule about preempt, can we please add an:

	lockdep_assert_preemption_disabled()

along with a comment about why it needs preempt disabled?

Also, the previous code did:

	if (cpu_feature_enabled(X86_FEATURE_INVLPGB))
		invlpgb_foo();
	else
		old_foo();

Is there a reason to break with that pattern? It would be nice to be
consistent.