linux-kernel - Re: [PATCH 1/4] x86/mm: Add a few comments

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Date:   Fri, 11 Nov 2022 12:50:47 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     x86@...nel.org, dave.hansen@...el.com,
        linux-kernel@...r.kernel.org, oliver.sang@...el.com
Subject: Re: [PATCH 1/4] x86/mm: Add a few comments

On Thu, Nov 10, 2022 at 07:02:31PM +0100, Peter Zijlstra wrote:

> > Do we need any of those alias passes at all for pure protection bit
> > changes? I thought we only did these because things like cacheability
> > bits have to be in sync due to machine checks etc?
> > 
> > Or am I missing some case where writability matters too?
> 
> I _think_, but I'm not actually sure, that it matters in exactly that
> case dhansen mentions, where we do a physical to virtual address
> translation and expect access to match whatever alias we originally came
> from.

That of course only covers the directmap; for giggles I did the below
patch on top of these and the testcase at hand boots and finishes just
fine...

So yeah, no sodding clue why we do that :/

diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
index f275605892df..c63e6117221a 100644
--- a/arch/x86/mm/pat/set_memory.c
+++ b/arch/x86/mm/pat/set_memory.c
@@ -1656,7 +1676,7 @@ static int cpa_process_alias(struct cpa_data *cpa)
 			return ret;
 	}
 
-#ifdef CONFIG_X86_64
+#if 0 // def CONFIG_X86_64
 	/*
 	 * If the primary call didn't touch the high mapping already
 	 * and the physical address is inside the kernel map, we need