linux-kernel - Re: [RFC PATCH v3 13/15] context_tracking,x86: Add infrastructure to defer kernel TLBI

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Message-ID: <20241120152216.GM19989@noisy.programming.kicks-ass.net>
Date: Wed, 20 Nov 2024 16:22:16 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Valentin Schneider <vschneid@...hat.com>
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Subject: Re: [RFC PATCH v3 13/15] context_tracking,x86: Add infrastructure to
 defer kernel TLBI

On Tue, Nov 19, 2024 at 04:35:00PM +0100, Valentin Schneider wrote:

> +void noinstr __flush_tlb_all_noinstr(void)
> +{
> +	/*
> +	 * This is for invocation in early entry code that cannot be
> +	 * instrumented. A RMW to CR4 works for most cases, but relies on
> +	 * being able to flip either of the PGE or PCIDE bits. Flipping CR4.PCID
> +	 * would require also resetting CR3.PCID, so just try with CR4.PGE, else
> +	 * do the CR3 write.
> +	 *
> +	 * XXX: this gives paravirt the finger.
> +	 */
> +	if (cpu_feature_enabled(X86_FEATURE_PGE))
> +		__native_tlb_flush_global_noinstr(this_cpu_read(cpu_tlbstate.cr4));
> +	else
> +		native_flush_tlb_local_noinstr();
> +}

Urgh, so that's a lot of ugleh, and cr4 has that pinning stuff and gah.

Why not always just do the CR3 write and call it a day? That should also
work for paravirt, no? Just make the whole write_cr3 thing noinstr and
voila.