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b=WOSWW30fHWEkzoLuda6WnC6r0uGzfIBZIGZA5Jxs/HtiJ9TcsYi6/6xeofHwDHlzLa OrcmB0Gou+SO2bmi952VRs2TYpK6HHYrlO/JA9liHmL+gIt3dLNu5i0KWQ0Lixwx/2L4 8AfV1WoP1qAUgOBmkp40SjVLbpzUPH52Mqt3Iu8H5KgMZY//ZhdzjxdzL8JLJ4ovQlGy yOX+lhZ2m/ftafo0rJi5ZV+z+LDCJwG6JtElnavRG+HhKeWcC7gJIYaA70Cepy0qr7Et Qd2TG7IORYwWVZlT8U+TNp7phgA1wRanzE2cAQhWGP8kcL11iMqLE7wx/MldbqVSXpL3 OC/g== X-Gm-Message-State: APuU4Z9g8vdEhjopYRq6v/LiWCKlVfvPJN8jqyVXyvgm8M9KrqzjjXUY Nr/qRzjGnA8A7HGZp+7ghvZSxIa/IPWLMnfS3Bem9LLu67+Icdub6OF1o4T/lssLS5DR+UDfhgL CCIYk6w== X-Received: from pjp16.prod.google.com ([2002:a17:90b:55d0:b0:3a8:1de3:a20e]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90a:ec87:b0:3ac:a1c7:20fd with SMTP id 98e67ed59e1d1-3b0db5edad3mr586400a91.4.1793922306059; Thu, 05 Nov 2026 15:45:06 -0800 (PST) Date: Thu, 5 Nov 2026 15:45:05 -0800 In-Reply-To: <58dcd595e3187d335e103a042d196cb7@panayotis.com> X-Mailing-List: kvm@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list Mime-Version: 1.0 References: <58dcd595e3187d335e103a042d196cb7@panayotis.com> Message-ID: Subject: Re: nVMX: Windows 11 with nested Hyper-V fails to boot when MBEC is exposed to L1 (7.2) From: Sean Christopherson To: Panayotis Katsaloulis Cc: kvm@vger.kernel.org, Paolo Bonzini Content-Type: text/plain; charset="us-ascii" On Thu, Nov 05, 2026, Panayotis Katsaloulis wrote: > Hi, > > After updating the host from 7.1.13 to 7.2.9, a Windows 11 guest that > runs its own hypervisor (VBS / Hyper-V, hypervisorlaunchtype Auto) no > longer boots. It fails very early (no bugcheck, no minidump, bootstat > not updated) and ends in Automatic Repair ("Your device ran into a > problem and couldn't be repaired"). Startup Repair reports no root > cause. > > Hiding MBEC from the guest is enough to make it boot again: > > -cpu host,migratable=on,vmx=on,vmx-mbec=off,hv-... > > Host: > CPU: Intel Core i9-13900 (Raptor Lake) > Kernel: 7.2.9 (distro kernel, Manjaro), kvm_intel nested=Y, mbec=Y > QEMU: 11.1.2 (also reproduced with 11.1.1) > libvirt: 12.8.0 > OVMF: edk2 202608, Secure Boot, swtpm 0.10.2 (TPM 2.0, CRB) > Machine: pc-q35-9.2 Paolo was theorizing that might be related to Hyper-V expecting PFEC_SS to be set[1] (Hyper-V appears to enable CET if and only if MBEC/GMET is available). And IIRC, the host and guest hangs from an early bug report[2] were due to bugs in the host's handling of hrtimers that affected only KVM VMX, but I don't think we ever explained the outright failure to boot. Paolo, do we have any patches to try out, or more concrete ideas/theories? [1] https://lore.kernel.org/all/20261012165651.130826-1-pbonzini@redhat.com [2] https://lore.kernel.org/all/85548beb-1486-40f9-beb4-632c78e3360b@proxmox.com > Guest: > Windows 11 24H2 (build 26100), UEFI + Secure Boot, VBS enabled, > hypervisorlaunchtype Auto, 8 GiB, host-passthrough CPU. > Hyper-V enlightenments: relaxed, vapic, spinlocks, vpindex, runtime, > synic, stimer, frequencies, tlbflush, ipi, evmcs, avic, hypervclock. > > Results (host kernel 7.2.9 in all cases): > > vmx vmx-mbec hv-evmcs QEMU result > on default on 11.1.2 fails (Automatic Repair) > off n/a on 11.1.2 boots > on default on 11.1.1 fails > on off on 11.1.1 boots > on default off 11.1.1 fails > > The same guest, with an identical QEMU command line (compared from the > libvirt log; only fds/ids differ), last booted fine on 2026-10-23 on > 7.1.13 with QEMU 11.1.1. So disabling eVMCS does not help, and QEMU > and libvirt do not seem to be involved; the only difference that > matters is whether MBEC is offered to L1. > > What I could not test: > - 7.1.x with the exact same setup again (no longer installable here) > - latest 7.2.y / 7.3.y > - kvm_intel.mbec=0 (I expect it to behave like vmx-mbec=off) > > Someone else reports the same symptom (7.1.8 works, 7.2.x fails, > Windows 11 26100 with nested Hyper-V), without a root cause: > https://discuss.cachyos.org/t/windows-11-kvm-guest-with-nested-hyper-v-boot-loops-on-linux-cachyos-7-2-x-works-on-7-1-8/36134 > > I can test patches or collect traces if that helps. > > Thanks, > Panayotis >