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Fri, 13 Dec 2024 04:22:48 -0500 (EST) X-Mailer: MessagingEngine.com Webmail Interface MIME-Version: 1.0 Date: Fri, 13 Dec 2024 10:22:28 +0100 From: "Arnd Bergmann" To: "Paolo Bonzini" , "Arnd Bergmann" , kvm@vger.kernel.org Cc: "Thomas Bogendoerfer" , "Huacai Chen" , "Jiaxun Yang" , "Michael Ellerman" , "Nicholas Piggin" , "Christophe Leroy" , "Naveen N Rao" , "Madhavan Srinivasan" , "Alexander Graf" , "Crystal Wood" , "Anup Patel" , "Atish Patra" , "Paul Walmsley" , "Palmer Dabbelt" , "Albert Ou" , "Sean Christopherson" , "Thomas Gleixner" , "Ingo Molnar" , "Borislav Petkov" , "Dave Hansen" , x86@kernel.org, "H. Peter Anvin" , "Vitaly Kuznetsov" , "David Woodhouse" , "Paul Durrant" , "Marc Zyngier" , linux-kernel@vger.kernel.org, linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org Message-Id: <062f7269-580e-4008-904a-919ca0bda482@app.fastmail.com> In-Reply-To: <6bc05f3c-f473-480d-b9a2-c8cd3c80a66c@redhat.com> References: <20241212125516.467123-1-arnd@kernel.org> <20241212125516.467123-6-arnd@kernel.org> <6bc05f3c-f473-480d-b9a2-c8cd3c80a66c@redhat.com> Subject: Re: [RFC 5/5] x86: kvm drop 32-bit host support X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20241213_012255_391268_39C3C1D2 X-CRM114-Status: GOOD ( 15.55 ) X-BeenThere: linux-riscv@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org >n Thu, Dec 12, 2024, at 17:27, Paolo Bonzini wrote: > On 12/12/24 13:55, Arnd Bergmann wrote: >> From: Arnd Bergmann >> >> There are very few 32-bit machines that support KVM, the main exceptions >> are the "Yonah" Generation Xeon-LV and Core Duo from 2006 and the Atom >> Z5xx "Silverthorne" from 2008 that were all released just before their >> 64-bit counterparts. > > Unlike other architectures where you can't run a "short bitness" kernel > at all, or 32-bit systems require hardware enablement that simply does > not exist, the x86 situation is a bit different: 32-bit KVM would not be > used on 32-bit processors, but on 64-bit kernels running 32-bit kernels; > presumably on a machine with 4 or 8 GB of memory, above which you're > hurting yourself even more, and for smaller guests where the limitations > in userspace address space size don't matter. > > Apart from a bunch of CONFIG_X86_64 conditionals, the main issue that > KVM has with 32-bit x86 is that they cannot read/write a PTE atomically > (i.e. without tearing) and therefore they can't use the newer and more > scalable page table management code. So no objections from me for > removing this support, but the justification should be the truth, i.e. > developers don't care enough. Right, I should have updated the description based on the comments for the first version, especially after separating it from the patches that make it harder to run 32-bit kernels on 64-bit hardware. I've updated the changelog now to x86: kvm drop 32-bit host support There are very few 32-bit machines that support KVM, the main exceptions are the "Yonah" Generation Xeon-LV and Core Duo from 2006 and the Atom Z5xx "Silverthorne" from 2008 that were all released just before their 64-bit counterparts. The main usecase for KVM in x86-32 kernels these days is to verify that 32-bit KVM is still working, by running it on 64-bit hardware. With KVM support on other 32-bit architectures going away, and x86-32 kernels on 64-bit hardware becoming more limited in available RAM, this usecase becomes much less interesting. Remove this support to make KVM exclusive to 64-bit hosts on all architectures, and stop testing 32-bit host mode. Link: https://lore.kernel.org/all/Z1B1phcpbiYWLgCD@google.com/ Signed-off-by: Arnd Bergmann which assumes that we end up going ahead with the powerpc patches. Does that work for you? Arnd _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv