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bh=6cpof6JrdjbWplaOdv2JpUCMgt20PiO0UsNonbenBP8=; b=Vx4UiX7bJj9ClHi9rQ7WHG0Mefr+Uos1qiXWHzkI/rbi8mqgKf/4Q4hcU8yLxzFFQbozRb QAO6vdEnduMt3yX7KJLCdKkPnugbYJnuv7Mz2II97Y/a40SylR5OrOPnYHVdqr0Lpww+YE ma8RHZbeHn8BrGt9aFJKo0RcxutGItY= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-97-1Vy0OEk3Nv-D8e9qEQPDCQ-1; Wed, 06 Jan 2021 12:03:04 -0500 X-MC-Unique: 1Vy0OEk3Nv-D8e9qEQPDCQ-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id B4EDE800D55 for ; Wed, 6 Jan 2021 17:03:03 +0000 (UTC) Received: from localhost (ovpn-116-153.rdu2.redhat.com [10.10.116.153]) by smtp.corp.redhat.com (Postfix) with ESMTP id 71FD75D9D2; Wed, 6 Jan 2021 17:03:00 +0000 (UTC) Date: Wed, 6 Jan 2021 12:02:59 -0500 From: Eduardo Habkost To: Vitaly Kuznetsov Subject: Re: [PATCH 5/5] i386: provide simple 'hyperv=on' option to x86 machine types Message-ID: <20210106170259.GO18467@habkost.net> References: <20201119103221.1665171-6-vkuznets@redhat.com> <20201216205202.GJ3140057@habkost.net> <20201218181340.5e398280@redhat.com> <87r1n0j20n.fsf@vitty.brq.redhat.com> <20210105000435.1cf4c6f6@redhat.com> <87lfd7iowi.fsf@vitty.brq.redhat.com> <20210105170312.32cf0e12@redhat.com> <87a6tnibv4.fsf@vitty.brq.redhat.com> <20210106141303.145790f7@redhat.com> <87sg7egp73.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 In-Reply-To: <87sg7egp73.fsf@vitty.brq.redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=ehabkost@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Received-SPF: pass client-ip=216.205.24.124; envelope-from=ehabkost@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -29 X-Spam_score: -3.0 X-Spam_bar: --- X-Spam_report: (-3.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.252, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=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: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Igor Mammedov , Marcelo Tosatti , qemu-devel@nongnu.org, Paolo Bonzini Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, Jan 06, 2021 at 02:38:56PM +0100, Vitaly Kuznetsov wrote: > Igor Mammedov writes: > > > On Tue, 05 Jan 2021 17:31:43 +0100 > > Vitaly Kuznetsov wrote: > > > >> Igor Mammedov writes: > >> > >> > On Tue, 05 Jan 2021 12:50:05 +0100 > >> > > >> > I think there is a misunderstanding, idea was: > >> > > >> > cpu_initfn() { > >> > //current set > >> > cpu->default_hyperv_cpu_features = ACD > >> > } > >> > > >> > compat_props_5.1 { > >> > cpu.default_hyperv_cpu_features = AB > >> > } > >> > > >> > compat_props_5.2 { > >> > cpu.default_hyperv_cpu_features = ABC > >> > } > >> > > >> > >> ... > >> > >> > I was talking about CPU features/properties only, it doesn't apply to other devices. > >> > It makes sense for machine to have a knob to create onboard hyperv specific > >> > devices if there is any (do we have any?). > >> > > >> > If there aren't any currently, I wouldn't bother with machine knob > >> > and just use -cpu foo,hv_default=on or -device cpu,hv_default=on > >> > like any other cpu feature. > >> > > >> > >> We don't currently have any devices which are not 'CPU features' (in > >> QEMU terminology), however, we already have Vmbus and I can easily > >> imagine us implementing e.g. hartbeat/kvp/vss/... devices on top. We > >> *may* want to enable these 'automatically' and that's what make > >> '-machine' option preferable. It is, however, not a *must* right now and > >> we can indeed wait until these devices appear and be happy with > >> 'hv_default' -cpu option for now. We will, however, need to teach upper > >> layers about the change when/if it happens. > > > > which makes me think we are trying to bite something that we shouldn't. > > Do we really need this patch (QEMU knob) to magically enable subset of > > features and/or devices for a specific OS flavor? I think we really want this, yes. It's not for a specific OS flavor, it is just a machine feature. > > > > It's job of upper layers to abstract low level QEMU details in to coarse > > grained knobs (libvirt/virt-install/virt-manager/...). > > For example virt-install may know that it installing a specific Windows > > version, and can build a tailored for that OS configuration including > > needed hyperv CPU features and hyperv specific devices. > > (if I'm not mistaken libosinfo is used to get metadata for preferred > > configuration, so perhaps this should become a patch for that library > > and its direct users). virt-install/libosinfo/etc can be used to enable a feature automatically, but the coarse grained knob may be provided by QEMU. > > > > What we actually lack is a documentation for preferred configuration > > in docs/hyperv.txt, currently it just enumerates possible features. > > We can just document a recommended 'best practices' there without > > putting it in QEMU code and let upper layers to do their job in > > the stack. > > The problem we're facing here is that when a new enlightenment is > implemented it takes forever to propagate to the whole stack. We don't > have any different recommendations for different Windows versions, > neither does genuine Hyper-V. The 'fine grained' mechanis we have just > contributes to the creation of various Frankenstein configurations > (which look nothing like real Hyper-V), people just google for 'Windows > KVM slow', add something to their scripts and this keeps propagating. Exactly. Requiring new code to be added to all other components in the stack every time we add a low level feature to KVM or QEMU is not working. It's even worse when we require users to manually update their configurations with low level bits. > > Every time I see a configuration with only a few 'hv_*' options I ask > 'why don't you enable the rest?' and I'm yet to receive an answer > different from 'hm, I don't know, I copied it from somewhere and it > worked'. > > Setting 'hv_*' options individually should be considered debug only. They can also be useful in production to work around unexpected issues (not just debugging). I don't think we should prevent other layers from controlling low level knobs. We just shouldn't make the low level knobs necessary for making the feature work. -- Eduardo