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Mon, 22 Sep 2025 10:06:40 -0700 (PDT) X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list MIME-Version: 1.0 References: <4ff89b72-03ff-4447-9d21-dd6a5fe1550f@app.fastmail.com> <20250917125951.GA1390993@nvidia.com> <02b0f383-1c43-4eeb-a76f-830c2970b833@app.fastmail.com> <547dcb81-434d-4910-aa7c-1d69019fcb3d@app.fastmail.com> <20250919143436.GC2132010@nvidia.com> <44f910bf-ac2c-4b2f-8e50-5cfc7dd0761a@app.fastmail.com> In-Reply-To: <44f910bf-ac2c-4b2f-8e50-5cfc7dd0761a@app.fastmail.com> From: Nicolas Schichan Date: Mon, 22 Sep 2025 19:05:49 +0200 X-Gm-Features: AS18NWDJSAue7YE7c8rlG0jvR5lve033dTh_Rnu2Tx6e392cgPO_ZPu-mU9r2No Message-ID: Subject: Re: [TECH TOPIC] Reaching consensus on CONFIG_HIGHMEM phaseout To: Arnd Bergmann Cc: Jason Gunthorpe , Geert Uytterhoeven , ksummit@lists.linux.dev, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, linux-mips@vger.kernel.org, linux-mm@kvack.org, imx@lists.linux.dev, Christophe Leroy , Richard Weinberger , Lucas Stach , Linus Walleij , Geert Uytterhoeven , Ankur Arora , David Hildenbrand , Mike Rapoport , Lorenzo Stoakes , Matthew Wilcox , Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , Suren Baghdasaryan , Ira Weiny , Nishanth Menon , =?UTF-8?Q?Heiko_St=C3=BCbner?= , Alexander Sverdlin , "Chester A. Unal" , Sergio Paracuellos , Andreas Larsson Content-Type: text/plain; charset="UTF-8" > On Fri, Sep 19, 2025, at 16:34, Jason Gunthorpe wrote: > > On Fri, Sep 19, 2025 at 04:22:20PM +0200, Arnd Bergmann wrote: [...] > > I am right it sounds like we can drop vivt cache support with highmem? > > vivt is alot easier to understand if kmap isn't running around > > creating virtual addresses aliases! > > I think so, yes. I've prototyped a patch for it to add a dependency > on !VIVT and the drop the code paths that are clearly no longer > needed without it. This obviously needs proper testing, but the > diffstat is promising: > > diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig > index f14bac913d50..0a31cfaf3818 100644 > --- a/arch/arm/Kconfig > +++ b/arch/arm/Kconfig > @@ -1214,6 +1214,7 @@ config ARCH_SPARSEMEM_ENABLE > config HIGHMEM > bool "High Memory Support" > depends on MMU > + depends on !CPU_CACHE_VIVT > select KMAP_LOCAL > select KMAP_LOCAL_NON_LINEAR_PTE_ARRAY > help > arch/arm/Kconfig | 1 + > arch/arm/configs/gemini_defconfig | 1 - > arch/arm/configs/multi_v5_defconfig | 1 - > arch/arm/configs/mvebu_v5_defconfig | 1 - > arch/arm/include/asm/highmem.h | 56 +----------------------- > arch/arm/mm/cache-feroceon-l2.c | 31 +------------ > arch/arm/mm/cache-xsc3l2.c | 47 +++----------------- > arch/arm/mm/dma-mapping.c | 14 ++---- > arch/arm/mm/flush.c | 19 ++------ > mm/highmem.c | 86 +++++-------------------------------- > 10 files changed, 27 insertions(+), 230 deletions(-) > > I did get an email from Nicolas Schichan (added to Cc here), > and he is still supporting a widely deployed Kirkwood based > platform that uses 1GB RAM configurations. He should get > a chance to test that with CONFIG_VMSPLIT_3G_OPT, but I > would expect that to continue working, possibly with minor > bugfixes. Hello Arnd, We don't use HIGMEM on our Kirkwood platform, we are happy using a 2G/2G WMSPLIT. We don't need a lot of virtual address space for userland, and with the 2G split we don't waste physical memory. I'm happy to test your patch serie with VMSPLIT_3G_OPT and see if it still boots with it once you send it. Regards, -- Nicolas Schichan