On 13/02/2018 09:15, David Woodhouse wrote: >>> >>> - if (!data) >>> + if (!data && !spectre_v2_ibrs_all()) >>> break; >> This should check the value of IBRS_ALL in the VM, not in the host. > No, it's host we want. If IBRS_ALL is set in the host, we set the > actual hardware MSR once at boot time and never touch it again. The > SPEC_CTRL MSR we expose to guests is purely a no-op fiction. > > If spectre_v2_ibrs_all() is true then KVM should *never* actually pass > through or touch the real MSR. That would be nice but unfortunately it's not possible. :( The VM might actually not have IBRS_ALL, as usual the reason is migration compatibility. In that case, that no-op fiction would be very slow because the VM will actually do a lot of SPEC_CTRL writes. So the right logic is: - if the VM has IBRS_ALL, pass through the MSR when it is zero and intercept writes when it is one (no writes should happen) - if the VM doesn't have IBRS_ALL, do as we are doing now, independent of what the host spectre_v2_ibrs_all() setting is. Paolo
