** Description changed: [Impact] When kexec'ing into a crash kernel with ncpus > 1, VMs can raise a KVM emulation failure. This will cause the VM to go into the "paused" state, and prevents it from being restored without a full VM restart. This happens only when there are multiple enabled CPUs in the crash kernel command-line, regardless of whether `nr_cpus` or `maxcpus` is being used. Due to the vCPU MMU state not being cleaned up correctly, the secondary CPUs try to access virtual addresses with a faulty MMU context that will result in the emulation failure. This shows up with a similar spew as below: $ sudo tail -n20 /var/log/libvirt/qemu/focal-vm.log KVM internal error. Suberror: 1 emulation failure EAX=0000de8f EBX=00000000 ECX=0000008f EDX=00000600 ESI=00000000 EDI=00000000 EBP=00000000 ESP=0000f90c EIP=0000cdb1 EFL=00000002 [-------] CPL=0 II=0 A20=1 SMM=0 HLT=0 ES =0000 00000000 0000ffff 00009300 CS =f000 000f0000 0000ffff 00009b00 SS =de00 000de000 0000ffff 00009300 DS =de00 000de000 0000ffff 00009300 FS =0000 00000000 0000ffff 00009300 GS =0000 00000000 0000ffff 00009300 LDT=0000 00000000 0000ffff 00008200 TR =0000 00000000 0000ffff 00008b00 GDT= 00000000 0000ffff IDT= 00000000 0000ffff CR0=60000010 CR2=00000000 CR3=290b8001 CR4=00000000 DR0=0000000000000000 DR1=0000000000000000 DR2=0000000000000000 DR3=0000000000000000 DR6=00000000ffff0ff0 DR7=0000000000000400 EFER=0000000000000000 Code=66 83 c4 28 66 5b 66 c3 66 56 66 53 66 52 b1 8f 88 c8 e6 70 <e4> 71 66 0f b6 f0 66 89 f2 67 88 54 24 03 88 c8 e6 70 66 31 db 88 d8 e6 71 66 56 66 68 1a [Test Plan] 1. Boot an Ubuntu guest VM with e.g. multipass: $ multipass launch daily:focal -c8 -m16g -n focal-vm 2. Configure guest crash kernel command-line with `nr_cpus=8`: ubuntu@focal-vm:~$ grep CMDLINE_APPEND /etc/default/kdump-tools # KDUMP_CMDLINE_APPEND - Additional arguments to append to the command line KDUMP_CMDLINE_APPEND="reset_devices systemd.unit=kdump-tools-dump.service nr_cpus=8 irqpoll nousb ata_piix.prefer_ms_hyperv=0" 3. Crash guest VM and watch for the KVM emulation failure: ubuntu@focal-vm:~$ echo c | sudo tee /proc/sysrq-trigger [Where problems could occur] As we're resetting MMU context on vCPUs, potential regressions would show up in workloads relying on KVM guests. We should properly test the scenario mentioned in the bug to make sure secondary CPUs are being cleaned up properly, and that no other regressions have been introduced when rebooting or kexec'ing into different kernels. Since we're adding an MMU reset at kvm_vcpu_reset(), the overall regression potential should be fairly low and contained to starting/resetting vCPUs (i.e. VM start and reboot). [Other info] This has been fixed by upstream commit: 0aa1837533e5 KVM: x86: Properly reset MMU context at vCPU RESET/INIT And the two follow up commits, which revert the vendor-specific resets: 5d2d7e41e3b8 KVM: SVM: Drop explicit MMU reset at RESET/INIT 61152cd907d5 KVM: VMX: Remove explicit MMU reset in enter_rmode() - These commits have been introduced during the upstream 5.14 and 5.15 release candidates, and as such should be backported to previous supported kernels. + These commits have been introduced during the upstream 5.14 and 5.15 release candidates. They have been picked up by upstream stable up until 5.11, so backports are only needed for Bionic and Focal (4.15 and 5.4 kernels). $ git describe --contains 0aa1837533e5 5d2d7e41e3b8 61152cd907d5 v5.14-rc1~166^2~58 v5.15-rc1~65^2~119 v5.15-rc1~65^2~120
-- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1948862 Title: KVM emulation failure when booting into VM crash kernel with multiple CPUs To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1948862/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
