Message-ID: <20241205183111.12dc16b3@mordecai.tesarici.cz>
Date: Thu, 5 Dec 2024 18:31:11 +0100
From: Petr Tesarik <ptesarik@...e.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Dave Hansen <dave.hansen@...el.com>, Valentin Schneider
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Subject: Re: [RFC PATCH v3 13/15] context_tracking,x86: Add infrastructure
 to defer kernel TLBI

On Thu, 21 Nov 2024 16:30:16 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> On Thu, Nov 21, 2024 at 07:07:44AM -0800, Dave Hansen wrote:
> > On 11/21/24 03:12, Peter Zijlstra wrote:  
> > >> I see e.g. ds_clear_cea() clears PTEs that can have the _PAGE_GLOBAL flag,
> > >> and it correctly uses the non-deferrable flush_tlb_kernel_range().  
> > > 
> > > I always forget what we use global pages for, dhansen might know, but
> > > let me try and have a look.
> > > 
> > > I *think* we only have GLOBAL on kernel text, and that only sometimes.  
> > 
> > I think you're remembering how _PAGE_GLOBAL gets used when KPTI is in play.  
> 
> Yah, I suppose I am. That was the last time I had a good look at this
> stuff :-)
> 
> > Ignoring KPTI for a sec... We use _PAGE_GLOBAL for all kernel mappings.
> > Before PCIDs, global mappings let the kernel TLB entries live across CR3
> > writes. When PCIDs are in play, global mappings let two different ASIDs
> > share TLB entries.  
> 
> Hurmph.. bah. That means we do need that horrible CR4 dance :/

In general, yes.

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.

AFAIK there is currently no such IPI function for doing that, but if we
could add one. If the list of invalidated global pages is reasonably
short, of course.

Valentin, do you happen to know?

Petr T

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