> On Jan 28, 2018, at 12:21 PM, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> wrote: > >> On January 28, 2018 2:29:10 PM EST, KarimAllah Ahmed <karahmed@xxxxxxxxx> wrote: >> Add direct access to MSR_IA32_SPEC_CTRL for guests. This is needed for >> guests >> that will only mitigate Spectre V2 through IBRS+IBPB and will not be >> using a >> retpoline+IBPB based approach. >> >> To avoid the overhead of atomically saving and restoring the >> MSR_IA32_SPEC_CTRL >> for guests that do not actually use the MSR, only add_atomic_switch_msr >> when a >> non-zero is written to it. > > > We tried this and found that it was about 3% slower that doing the old way of rdmsr and wrmsr. > 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. 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?
