Message-ID: <20241209154252.4f8fa5a8@mordecai.tesarici.cz>
Date: Mon, 9 Dec 2024 15:42:52 +0100
From: Petr Tesarik <ptesarik@...e.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Valentin Schneider <vschneid@...hat.com>, Dave Hansen
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Subject: Re: [RFC PATCH v3 13/15] context_tracking,x86: Add infrastructure
 to defer kernel TLBI

On Mon, 9 Dec 2024 13:12:49 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> On Mon, Dec 09, 2024 at 01:04:43PM +0100, Valentin Schneider wrote:
> 
> > > But I wonder what exactly was the original scenario encountered by
> > > Valentin. I mean, if TLB entry invalidations were necessary to sync
> > > changes to kernel text after flipping a static branch, then it might be
> > > less overhead to make a list of affected pages and call INVLPG on them.  
> 
> No; TLB is not involved with text patching (on x86).
> 
> > > Valentin, do you happen to know?  
> > 
> > So from my experimentation (hackbench + kernel compilation on housekeeping
> > CPUs, dummy while(1) userspace loop on isolated CPUs), the TLB flushes only
> > occurred from vunmap() - mainly from all the hackbench threads coming and
> > going.  
> 
> Right, we have virtually mapped stacks.

Wait... Are you talking about the kernel stac? But that's only 4 pages
(or 8 pages with KASAN), so that should be easily handled with INVLPG.
No CR4 dances are needed for that.

What am I missing?

Petr T

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