Re: [PATCH 3/7] kvm: vmx: pass MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD down to the guest

[Liran Alon]

----- 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:
> > > 
> > > +ÂÂÂÂÂÂÂif (have_spec_ctrl && vmx->spec_ctrl != 0)
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂwrmsrl(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