Re: [PATCH] arm64/kvm: Add generic v8 KVM target

[Christoffer Dall <christoffer.dall@linaro.org>]

On Wed, Jun 24, 2015 at 09:29:56AM +0100, Marc Zyngier wrote:
> On 22/06/15 09:44, Peter Maydell wrote:
> > On 17 June 2015 at 10:00, Suzuki K. Poulose <suzuki.poulose@arm.com> wrote:
> >> From: "Suzuki K. Poulose" <suzuki.poulose@arm.com>
> >>
> >> This patch adds a generic ARM v8 KVM target cpu type for use
> >> by the new CPUs which eventualy ends up using the common sys_reg
> >> table. For backward compatibility the existing targets have been
> >> preserved. Any new target CPU that can be covered by generic v8
> >> sys_reg tables should make use of the new generic target.
> > 
> > How do you intend this to work for cross-host migration?
> 
> It is not meant to work for cross migration at all.
> 
> > Is the idea that the kernel guarantees that "generic" looks
> > 100% the same to the guest regardless of host hardware? I'm
> > not sure that can be made to work, given impdef differences
> > in ID register values, bp/wp registers, and so on.
> > 
> > Given that, it seems to me that we still need to provide
> > KVM_ARM_TARGET_$THISCPU defines so userspace can request
> > a specific guest CPU flavour; so what does this patch
> > provide that isn't already provided by just having userspace
> > query for the "preferred" CPU type as it does already?
> 
> The way I see this working is that a "generic" CPU cannot be migrated
> (because we don't know anything about it). If it can be identified as a
> known (non generic) implementation, then we can migrate it.
> 
Concretely, how should this work?  Be enforced by userspace or should we
deny certain SET_ONE_REG operations from working on this target?

Also, can we imagine any scenario where the generic CPU cannot me
modeled for a VM on a specific piece of hardware (current or future)?

-Christoffer