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

[Paolo Bonzini]

----- 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