linux-kernel - Re: [PATCH v3 2/4] x86/speculation: Support "Enhanced IBRS" on future CPUs

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Date:   Tue, 20 Feb 2018 11:42:07 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     David Woodhouse <dwmw2@...radead.org>
cc:     karahmed@...zon.de, x86@...nel.org, kvm@...r.kernel.org,
        torvalds@...ux-foundation.org, pbonzini@...hat.com,
        linux-kernel@...r.kernel.org, bp@...en8.de, peterz@...radead.org,
        jmattson@...gle.com, rkrcmar@...hat.com,
        arjan.van.de.ven@...el.com, dave.hansen@...el.com, mingo@...nel.org
Subject: Re: [PATCH v3 2/4] x86/speculation: Support "Enhanced IBRS" on future
 CPUs

On Tue, 20 Feb 2018, Thomas Gleixner wrote:
> On Tue, 20 Feb 2018, David Woodhouse wrote:
> > On Tue, 2018-02-20 at 09:31 +0100, Thomas Gleixner wrote:
> > > > @@ -3387,13 +3387,14 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> > > >  
> > > >  		vmx->spec_ctrl = data;
> > > >  
> > > > -		if (!data)
> > > > +		if (!data && !spectre_v2_ibrs_all())
> > > >  			break;
> > > >  
> > > >  		/*
> > > >  		 * For non-nested:
> > > >  		 * When it's written (to non-zero) for the first time, pass
> > > > -		 * it through.
> > > > +		 * it through unless we have IBRS_ALL and it should just be
> > > > +		 * set for ever.
> > >
> > > A non zero write is going to disable the intercept only when IBRS_ALL
> > > is on. The comment says is should be set forever, i.e. not changeable by
> > > the guest. So the condition should be:
> > > 
> > > 		if (!data || spectre_v2_ibrs_all())
> > > 			break;
> > > Hmm?
> > 
> > Yes, good catch. Thanks.
> > 
> > However, Paolo is very insistent that taking the trap every time is
> > actually a lot *slower* than really frobbing IBRS on certain
> > microarchitectures, so my hand-waving "pfft, what did they expect?" is
> > not acceptable.
> > 
> > Which I think puts us back to the "throwing the toys out of the pram"

There are no more toys in the pram. I threw them all out weeks ago ...

> > solution; demanding that Intel give us a new feature bit for "IBRS_ALL,
> > and the bit in the MSR is a no-op". Which was going to be true for
> > *most* new CPUs anyway. (Note: a blacklist for those few CPUs on which
> > it *isn't* true might also suffice instead of a new feature bit.)
> >
> > Unless someone really wants to implement the atomic MSR save and
> > restore on vmexit, and make it work with nesting, and make the whole
> > thing sufficiently simple that we don't throw our toys out of the pram
> > anyway when we see it?
> 
> That whole stuff was duct taped into microcode in a rush and the result is
> that we have only the choice between fire and frying pan. Whatever we
> decide to implement is not going to be a half baken hack.

  s/not// of course

> 
> I fully agree that Intel needs to get their act together and implement
> IBRS_ALL sanely.
> 
> The right thing to do is to allow the host to lock down the MSR once it
> enabled IBRS_ALL so that any write to it will just turn into NOOPs. That
> removes all worries about guests and in future generations of CPUs this bit
> might just be hardwired to one and the MSR just a dummy for compability
> reasons.
> 
> Thanks,
> 
> 	tglx
>