Comment 3 for bug 1764956

Hi Vincent, thanks for your report - really good research, even indicating a potential fix.
I manage to reproduce this issue using the following steps:

1) Be sure to run Ubuntu 16.04 on both guest and host, both
running kernel 4.4.0 (could be latest version of this kernel)

2) In the guest, install the package "msr-tools", like:
"sudo apt install msr-tools"

3) Still on guest, run: "sudo modprobe msr; sudo wrmsr 0x48 1"
This will enable the MSR bit for IBRS inside the guest

4) Now the host shows bad performance

If we run "sudo wrmsr 0x48 0" in the guest, host gets its performance back.

I'll investigate some commits upstream, including the one you suggested, and once we figure the exact fix for this, will request SRU to the kernel team.
Thanks,

Guilherme