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([2a01:e0a:59e:9d80:527b:9dff:feef:3874]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4327d698144sm162050515e9.39.2024.11.04.07.52.54 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 04 Nov 2024 07:52:56 -0800 (PST) Message-ID: <0007b45a-0278-4dad-8b8f-a65bd0506dc2@redhat.com> Date: Mon, 4 Nov 2024 16:52:53 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC 00/21] kvm/arm: Introduce a customizable aarch64 KVM host model Content-Language: en-US To: Kashyap Chamarthy Cc: eric.auger.pro@gmail.com, cohuck@redhat.com, qemu-devel@nongnu.org, qemu-arm@nongnu.org, kvmarm@lists.linux.dev, peter.maydell@linaro.org, richard.henderson@linaro.org, alex.bennee@linaro.org, maz@kernel.org, oliver.upton@linux.dev, sebott@redhat.com, shameerali.kolothum.thodi@huawei.com, armbru@redhat.com, berrange@redhat.com, abologna@redhat.com, jdenemar@redhat.com, shahuang@redhat.com, mark.rutland@arm.com, philmd@linaro.org, pbonzini@redhat.com References: <20241025101959.601048-1-eric.auger@redhat.com> From: Eric Auger In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=170.10.129.124; envelope-from=eric.auger@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -23 X-Spam_score: -2.4 X-Spam_bar: -- X-Spam_report: (-2.4 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.34, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham 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: , Reply-To: eric.auger@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Hi Kashyap, On 10/25/24 16:51, Kashyap Chamarthy wrote: > On Fri, Oct 25, 2024 at 12:17:19PM +0200, Eric Auger wrote: > > Hi Eric, > > I'm new to Arm, so please bear with my questions :) > >> This RFC series introduces a KVM host "custom" model. > (a) On terminology: as we know, in the x86 world, QEMU uses these > terms[1]: > > - Host passthrough > - Named CPU models > - Then there's the libvirt abstraction, "host-model", that aims to > provide the best of 'host-passthrough' + named CPU models. > > Now I see the term "host 'custom' model" here. Most > management-layer tools and libvirt users are familiar with the > classic terms "host-model" or "custom". If we now say "host > 'custom' model", it can create confusion. I hope we can settle on > one of the existing terms, or create a new term if need be. > > (I'll share one more thought on how layers above libvirt tend to use > the term "custom", as a reply to patch 21/21, "arm/cpu-features: > Document custom vcpu model".) agreed, as replied earlier, custom terminology most probably will be dropped. > > (b) The current CPU features doc[2] for Arm doesn't mention "host > passthrough" at all. It is only implied by the last part of this > paragraph, from the section titled "A note about CPU models and > KVM"[3]: > > "Named CPU models generally do not work with KVM. There are a few > cases that do work [...] but mostly if KVM is enabled the 'host' > CPU type must be used." indeed > > Related: in your reply[4] to Dan in this series, you write: "Having > named models is the next thing". So named CPU models will be a > thing in Arm, too? Then the above statement in the Arm > 'cpu-features' will need updating :-) Yes named models implementing a baseline are the end goal. > > [...] > >> - the QEMU layer does not take care of IDREG field value consistency. >> The kernel neither. I imagine this could be the role of the upper >> layer to implement a vcpu profile that makes sure settings are >> consistent. Here we come to "named" models. What should they look >> like on ARM? > Are there reasons why they can't be similar to how x86 reports in > `qemu-system-x86 -cpu help`? > > E.g. If it's an NVIDIA "Grace A02" (Neoverse-V2) host, it can report: > > [gracehopper] $> qemu-kvm -cpu help > Available CPUs: > gracehopper-neoverse-v2 > cortex-a57 (deprecated) > host > max > > Or whatever is the preferred nomenclature for ARM. It also gives users > of both x86 and ARM deployments a consistent expectation. > > Currently on a "Grace A02" ("Neoverse-V2") machine, it reports: > > [gracehopper] $> qemu-kvm -cpu help > Available CPUs: > cortex-a57 (deprecated) > host > max > > I see it's because there are no named models yet on ARM :-) yes this is definitively because on ARM there is no such named KVM model besides cortex-a57 on aarch64. on x86 does it return the closest named model? Thanks Eric > > [...] > > [1] https://www.qemu.org/docs/master/system/i386/cpu.html > [2] https://www.qemu.org/docs/master/system/arm/cpu-features.html > [3] https://www.qemu.org/docs/master/system/arm/cpu-features.html#a-note-about-cpu-models-and-kvm > [4] https://lists.nongnu.org/archive/html/qemu-arm/2024-10/msg00891.html > >