Messages in this thread | | | Subject | Re: [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future CPUs | From | Paolo Bonzini <> | Date | Tue, 13 Feb 2018 10:58:00 +0100 |
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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
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