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

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Message-ID: <20241120153221.GM38972@noisy.programming.kicks-ass.net>
Date: Wed, 20 Nov 2024 16:32:21 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Valentin Schneider <vschneid@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	kvm@...r.kernel.org, linux-mm@...ck.org, bpf@...r.kernel.org,
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Subject: Re: [RFC PATCH v3 13/15] context_tracking,x86: Add infrastructure to
 defer kernel TLBI

On Wed, Nov 20, 2024 at 04:22:16PM +0100, Peter Zijlstra wrote:
> 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.

Oh gawd, just having looked at xen_write_cr3() this might not be
entirely trivial to mark noinstr :/