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SubjectRE: [PATCH] x86: vmx: Allow direct access to MSR_IA32_SPEC_CTRL

>> On Sun, 2018-01-28 at 12:40 -0800, Andy Lutomirski wrote:
>> >
>> > Do you mean that the host would intercept the guest WRMSR and do
>> > WRMSR itself?  I would suggest that doing so is inconsistent with the
>> > docs.  As specified, doing WRMSR to write 1 to IBRS does *not*
>> > protect the guest.
>> I believe it does. Guest kernel is protected from any guest userspace
>> predictions learned before IBRS was last set to 1 in *any* mode,
>> including host.
> the specification requires you to write a 1 on each transition to higher
> privilege.

Right. Andy's concern was about VMX non-root (i.e. guest) ring 0
attempting to write IBRS but it's trapped and actually happens in the

As long as it *remains* set when the host re-enters the vCPU that should
be fine.


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