Re: [PATCH 0/2] KVM: arm/arm64: Add VCPU workarounds firmware register

[Marc Zyngier <marc.zyngier@arm.com>]

On Tue, 22 Jan 2019 10:17:00 +0000,
Dave Martin <Dave.Martin@arm.com> wrote:
> 
> On Mon, Jan 07, 2019 at 12:05:35PM +0000, Andre Przywara wrote:
> > Workarounds for Spectre variant 2 or 4 vulnerabilities require some help
> > from the firmware, so KVM implements an interface to provide that for
> > guests. When such a guest is migrated, we want to make sure we don't
> > loose the protection the guest relies on.
> > 
> > This introduces two new firmware registers in KVM's GET/SET_ONE_REG
> > interface, so userland can save the level of protection implemented by
> > the hypervisor and used by the guest. Upon restoring these registers,
> > we make sure we don't downgrade and reject any values that would mean
> > weaker protection.
> 
> Just trolling here, but could we treat these as immutable, like the ID
> registers?  
> 
> We don't support migration between nodes that are "too different" in any
> case, so I wonder if adding complex logic to compare vulnerabilities and
> workarounds is liable to create more problems than it solves...

And that's exactly the case we're trying to avoid. Two instances of
the same HW. One with firmware mitigations, one without. Migrating in
one direction is perfectly safe, migrating in the other isn't.

It is not about migrating to different HW at all.

	M.

-- 
Jazz is not dead, it just smell funny.