linux-kernel - Re: [TECH TOPIC] Reaching consensus on CONFIG

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Message-ID: <aMGFhuHyQwtwyF7B@casper.infradead.org>
Date: Wed, 10 Sep 2025 15:04:54 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Linus Walleij <linus.walleij@...aro.org>
Cc: Arnd Bergmann <arnd@...db.de>, ksummit@...ts.linux.dev,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linuxppc-dev@...ts.ozlabs.org, linux-mips@...r.kernel.org,
	linux-mm@...ck.org, imx@...ts.linux.dev,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Richard Weinberger <richard@....at>,
	Lucas Stach <l.stach@...gutronix.de>,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	Ankur Arora <ankur.a.arora@...cle.com>,
	David Hildenbrand <david@...hat.com>,
	Mike Rapoport <rppt@...nel.org>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Ira Weiny <ira.weiny@...el.com>, Nishanth Menon <nm@...com>,
	Heiko Stübner <heiko@...ech.de>,
	Alexander Sverdlin <alexander.sverdlin@...il.com>,
	"Chester A. Unal" <chester.a.unal@...nc9.com>,
	Sergio Paracuellos <sergio.paracuellos@...il.com>,
	Andreas Larsson <andreas@...sler.com>
Subject: Re: [TECH TOPIC] Reaching consensus on CONFIG_HIGHMEM phaseout

On Wed, Sep 10, 2025 at 03:10:05PM +0200, Linus Walleij wrote:
> This is is done so that we can unmap *most* of the kernel
> memory but *keep* the VMALLOC area, so that e.g. exceptions
> can still execute and save to stack when they occur, and we can
> call into the kernel to execute said exceptions.
> 
> This in practice means the area 0xf1000000-0xffffffff
> which also includes the high exception vectors that must for
> natural reasons also be kept in the mapping.
> What we have mapped today (all of the time) is
> 0xc0000000-0xffffffff.
> 
> A minimal 256 MB mapping from 0xf0000000-0xffffffff
> should be able to cover this.

Can I ask where the MMIO (PCI?) window lives?