[Bug 213155] possible kernel regression when running 11.x-RELEASE on KVM on AMD Opterons

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=213155

--- Comment #25 from Bennett <FREEBSD at bfeitell.users.panix.com> ---
Thank you all for continuing to chase this down.  I expect that using QEMU64 as
the CPU type, and adding the additional features is the best stopgap fix until
this bug is actually resolved.

I have been reading through the Linux Kernel mailing list as they work out the
patches for Meltdown and Spectre, and suspect that misidentifying the CPU as an
Intel Westmere device to gain AES support will cause problems once the new
mitigation code goes live.  There still might be a performance hit for FreeBSD
guests with this solution in the future, as the QEMU64 cpu exposed by KVM will
probably not show the new MSRs of a microcode patched Opeteron 4 or Opteron 5
(K8)  CPU.  This will cause the guest to default to the expensive "retpoline"
mitigation for Spectre, rather than the more efficient microcode patched AMD
specific "MFence" mitigation that is in the works.

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