On Mon, Jan 7, 2019 at 10:20 AM Andi Kleen <a...@linux.intel.com> wrote: > > > The issue is compatibility. Prior to your change, reading this MSR > > from a VM would raise #GP. After your change, it won't. That means > > that if you have a VM migrating between hosts with kernel versions > > before and after this change, the results will be inconsistent. In the > > No it will not be. All Linux kernel uses of this MSR are guarded > by a CPUID check.
Linux usage is irrelevant to the architected behavior of the virtual CPU. According to volume 4 of the SDM, this MSR is only supported when CPUID.01H:ECX.PDCM [bit 15] is set. Therefore, kvm should raise #GP whenever a guest tries to read this MSR and the guest's CPUID.01H:ECX.PDCM [bit 15] is clear.
