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

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Message-ID: <20241121153016.GL39245@noisy.programming.kicks-ass.net>
Date: Thu, 21 Nov 2024 16:30:16 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Valentin Schneider <vschneid@...hat.com>, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org, kvm@...r.kernel.org, linux-mm@...ck.org,
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	Dave Hansen <dave.hansen@...ux.intel.com>,
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Subject: Re: [RFC PATCH v3 13/15] context_tracking,x86: Add infrastructure to
 defer kernel TLBI

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 :/