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[91.219.240.2]) by smtp.gmail.com with ESMTPSA id hy24sm3822381ejc.40.2021.02.11.01.34.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 11 Feb 2021 01:34:16 -0800 (PST) From: Vitaly Kuznetsov To: =?utf-8?Q?Daniel_P=2E_Berrang=C3=A9?= Subject: Re: [PATCH v4 00/19] i386: KVM: expand Hyper-V features early and provide simple 'hv-default=on' option In-Reply-To: <20210211091430.GB1302824@redhat.com> References: <20210210164033.607612-1-vkuznets@redhat.com> <20210210165606.GM1240644@redhat.com> <20210210174652.GE1420762@habkost.net> <87ft23m2f6.fsf@vitty.brq.redhat.com> <20210211091430.GB1302824@redhat.com> Date: Thu, 11 Feb 2021 10:34:15 +0100 Message-ID: <87czx7lzhk.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=vkuznets@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Received-SPF: pass client-ip=63.128.21.124; envelope-from=vkuznets@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -33 X-Spam_score: -3.4 X-Spam_bar: --- X-Spam_report: (-3.4 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.568, 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_H4=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: Paolo Bonzini , Igor Mammedov , Marcelo Tosatti , Eduardo Habkost , qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Daniel P. Berrang=C3=A9 writes: > On Thu, Feb 11, 2021 at 09:30:53AM +0100, Vitaly Kuznetsov wrote: >> Eduardo Habkost writes: >>=20 >> > On Wed, Feb 10, 2021 at 04:56:06PM +0000, Daniel P. Berrang=C3=A9 wrot= e: >> >> On Wed, Feb 10, 2021 at 05:40:12PM +0100, Vitaly Kuznetsov wrote: >> >> > Changes since v3: >> >> > - Make 'hv-default' override 'hv-*' options which were already set= =20 >> >> > (e.g. 'hv-feature=3Don,hv-default' case) [Igor]. Make 'hv-passthr= ough' >> >> > behave the same way. >> >> > - Add "i386: be more picky about implicit 'hv-evmcs' enablement" pa= tch to avoid >> >> > enabling 'hv-evmcs' with hv-default/hv-passthrough when guest CPU= lacks VMX. >> >> > - Add "i386: support 'hv-passthrough,hv-feature=3Doff' on the comma= nd line" patch >> >> > to make 'hv-passthrough' semantics match the newly introduced 'hv= -default'. >> >> > - Add "i386: track explicit 'hv-*' features enablement/disablement"= patch to >> >> > support the above mentioned changes. >> >> > - Expand qtest to check the above mentioned improvements. >> >> >=20 >> >> > Original description: >> >> >=20 >> >> > Upper layer tools like libvirt want to figure out which Hyper-V fea= tures are >> >> > supported by the underlying stack (QEMU/KVM) but currently they are= unable to >> >> > do so. We have a nice 'hv_passthrough' CPU flag supported by QEMU b= ut it has >> >> > no effect on e.g. QMP's=20 >> >> >=20 >> >> > query-cpu-model-expansion type=3Dfull model=3D{"name":"host","props= ":{"hv-passthrough":true}} >> >> >=20 >> >> > command as we parse Hyper-V features after creating KVM vCPUs and n= ot at >> >> > feature expansion time. To support the use-case we first need to ma= ke=20 >> >> > KVM_GET_SUPPORTED_HV_CPUID ioctl a system-wide ioctl as the existin= g >> >> > vCPU version can't be used that early. This is what KVM part does. = With >> >> > that done, we can make early Hyper-V feature expansion (this series= ). >> >> >=20 >> >> > In addition, provide a simple 'hv-default' option which enables (an= d >> >> > requires from KVM) all currently supported Hyper-V enlightenments. >> >> > Unlike 'hv-passthrough' mode, this is going to be migratable. >> >>=20 >> >> How is it going to be migratable if the semantics vary depending on >> >> the host kernel KVM reporting features, because different kernels >> >> will expose different features ? >> > >> > "all currently supported" in this context means "all features >> > supported when the machine type was added", not "all features >> > supported by the host kernel". >>=20 >> Yes, exactly. >>=20 >> 'hv-passthrough' enables 'everything supported by the host' and this is >> not migratable. >>=20 >> 'hv-default' requires a certain set of features (depending on the >> machine type) so the VM won't start if the host lacks something. > > Ok, so I presume HV features will only be added to hv-default when we > know they are available in the oldest kernel we are targetting ? Upsteam > is more conservative in this respect than downstreams, the latter can > guarantee much more modern kernels. > Yes, it is kind of an open question when a feature gets 'promoted' to 'hv-default'. Currently, the latest feature we include is 'HYPERV_FEAT_STIMER_DIRECT' which dates back to Linux 5.0. It is also possible to use something like 'hv-default,hv-stimer-direct=3Doff,...' when running on an older kernel (and this is still migratable). --=20 Vitaly