Date: Fri, 10 Apr 2015 17:36:49 -0400 From: Ryan Stone <rysto32@gmail.com> To: Allan Jude <allanjude@freebsd.org> Cc: freebsd-virtualization@freebsd.org Subject: Re: bhyve clock problem, solved by kern.timecounter.hardware="TSC-low" in /etc/sysctl.conf Message-ID: <CAFMmRNyp_m4m7H1u5N4O1B35Rt3H_HcL-rUDRMMyY6a6FOtB%2Bw@mail.gmail.com> In-Reply-To: <5528115B.7060509@freebsd.org> References: <552809F4.6070206@redbarn.org> <CAOb7DXGAWiTUvmq5xJT8Z8W9D3hFuWeHQ-vbADw2AP0Fv4e8Lw@mail.gmail.com> <552810CE.7020502@redbarn.org> <5528115B.7060509@freebsd.org>
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Using the TSC as the default timecounter in a VM is dangerous. On some hardware, the TSC is not synchronized across all CPU cores. This means that if a VM migrates from one core to another, it could see the timecounter value go backwards. Time jumping backwards can cause all kinds of hilarity. What's really needed is for bhyve to support KVM clock or a similar scheme (brianv@ might also still need to commit the guest side of KVM clock as well).
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