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. > For that matter, what are the semantics of VMRESUME doing a write to > IBRS as part of its MSR switch? Is it treated as IBRS=1 from guest > context? Why does it matter? We *have* confirmed, FWIW, that VMRESUME writing 1 to IBRS as part of its MSR switch when it was already 1 is not optimised away and *is* treated as writing IBRS=1 again.
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