----- pbonzini@xxxxxxxxxx wrote: > ----- Original Message ----- > > From: "David Woodhouse" <dwmw2@xxxxxxxxxxxxx> > > To: "Paolo Bonzini" <pbonzini@xxxxxxxxxx>, > linux-kernel@xxxxxxxxxxxxxxx, kvm@xxxxxxxxxxxxxxx > > Cc: jmattson@xxxxxxxxxx, aliguori@xxxxxxxxxx, "thomas lendacky" > <thomas.lendacky@xxxxxxx>, bp@xxxxxxxxx > > Sent: Monday, January 8, 2018 8:41:07 PM > > Subject: Re: [PATCH 3/7] kvm: vmx: pass MSR_IA32_SPEC_CTRL and > MSR_IA32_PRED_CMD down to the guest > > > > On Mon, 2018-01-08 at 19:08 +0100, Paolo Bonzini wrote: > > > > > > + if (have_spec_ctrl && vmx->spec_ctrl != 0) > > > + wrmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl); > > > + > > > > I think this one probably *is* safe even without an 'else lfence', > > which means that the CPU can speculate around it, but it wants a > > comment explaining that someone has properly analysed it and saying > > precisely why. > > This one is okay as long as there are no indirect jumps until > vmresume. But the one on vmexit is only okay because right now > it's *disabling* IBRS. Once IBRS is used by Linux, we'll need an > lfence there. I'll add a comment. > > Paolo That is true but from what I understand, there is an indirect branch from this point until vmresume. That indirect branch resides in atomic_switch_perf_msrs() immediately called after this WRMSR: atomic_switch_perf_msrs() -> perf_guest_get_msrs() -> x86_pmu.guest_get_msrs(). -Liran
