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[149.14.88.106]) by smtp.gmail.com with ESMTPSA id n10-20020a5d67ca000000b0020c5253d8cfsm16087878wrw.27.2022.05.10.02.31.04 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 10 May 2022 02:31:05 -0700 (PDT) Message-ID: Date: Tue, 10 May 2022 11:31:04 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.9.0 Subject: Re: QEMU 32-bit vs. 64-bit binaries Content-Language: en-US To: "Dr. David Alan Gilbert" , Peter Maydell Cc: Markus Armbruster , qemu-devel@nongnu.org, qemu-arm , Richard Henderson , Mark Cave-Ayland , Fabiano Rosas , muriloo@linux.ibm.com, Paolo Bonzini , =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= , Daniel Henrique Barboza , mopsfelder@gmail.com, qemu-ppc@nongnu.org, =?UTF-8?Q?C=c3=a9dric_Le_Goater?= , qemu-RISC-V References: <9ec244e0-4c7c-69ff-08f8-da451f6da449@linux.ibm.com> <87sfpqaey7.fsf@linux.ibm.com> <2ab9e2b3-5dba-4e18-ed2e-6063a2716f4c@ilande.co.uk> <87ilql9xww.fsf@linux.ibm.com> <472e45e8-319b-ad48-3afa-0dfa74e6ad20@redhat.com> <877d6tzs2e.fsf@pond.sub.org> <32e5877d-ba45-ac63-d24e-1f9f8676c6bb@redhat.com> From: Thomas Huth In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=170.10.129.124; envelope-from=thuth@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -28 X-Spam_score: -2.9 X-Spam_bar: -- X-Spam_report: (-2.9 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.082, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_LOW=-0.7, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 10/05/2022 11.22, Dr. David Alan Gilbert wrote: > * Peter Maydell (peter.maydell@linaro.org) wrote: >> On Tue, 10 May 2022 at 10:01, Thomas Huth wrote: >>> >>> On 10/05/2022 10.54, Markus Armbruster wrote: >>>> Thomas Huth writes: >>>> >>>> [...] >>>> >>>>> I once suggested in the past already that we should maybe get rid of >>>>> the 32-bit variants in case the 64-bit variant is a full superset, so >>>>> we can save compile- and test times (which is quite a bit for QEMU), >>>>> but I've been told that the 32-bit variants are mostly still required >>>>> for supporting KVM on 32-bit host machines. >>>> >>>> Do we still care for 32-bit host machines? >>> >>> As long as the Linux kernel still supports 32-bit KVM virtualization, I >>> think we have to keep the userspace around for that, too. >>> >>> But I wonder why we're keeping qemu-system-arm around? 32-bit KVM support >>> for ARM has been removed with Linux kernel 5.7 as far as I know, so I think >>> we could likely drop the qemu-system-arm nowadays, too? Peter, Richard, >>> what's your opinion on this? >> >> Two main reasons, I think: >> * command-line compatibility (ie there are lots of >> command lines out there using that binary name) >> * nobody has yet cared enough to come up with a plan for what >> we want to do differently for these 32-bit architectures, >> so the default is "keep doing what we always have" >> >> In particular, I don't want to get rid of qemu-system-arm as the >> *only* 32-bit target binary we drop. Either we stick with what >> we have or we have a larger plan for sorting this out consistently >> across target architectures. > > To my mind, qemu-system-arm makes a lot of sense, and I'd rather see the > 32 bit guests disappear from qemu-system-aarch64. > It's difficult to justify to someone running their aarch virt stack why > their binary has the security footprint that includes a camera or PDA. I'm not very familiar with KVM on ARM - but is it possible to use KVM there with an arbitrary machine? If that's the case, a user might want to use KVM on their 64-bit host to run a 32-bit guest machine, and then you need to keep the 32-bit machines in the -aarch64 binary. Something like that is at least theoretically possible with ppc64, I think: Using KVM-PR, it should be possible to run a g3beige (i.e. 32-bit) machine on a 64-bit host. Not sure whether anybody has tried that in recent times (afaik KVM-PR is in a rather bad shape nowadays), but it might have been possible at one point in time in the past. (PPC folks, please correct me if I'm wrong) Thomas