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

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Message-ID: <20241121111221.GE24774@noisy.programming.kicks-ass.net>
Date: Thu, 21 Nov 2024 12:12:21 +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 Wed, Nov 20, 2024 at 06:24:56PM +0100, Valentin Schneider wrote:

> > Oh gawd, just having looked at xen_write_cr3() this might not be
> > entirely trivial to mark noinstr :/
> 
> ... I hadn't even seen that.
> 
> AIUI the CR3 RMW is not "enough" if we have PGE enabled, because then
> global pages aren't flushed.
> 
> The question becomes: what is held in global pages and do we care about
> that when it comes to vmalloc()? I'm starting to think no, but this is x86,
> I don't know what surprises are waiting for me.
> 
> 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.