----- 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
