Re: [RFC PATCH v2 02/28] KVM: arm64: Save ID registers' sanitized value per vCPU

[Reiji Watanabe <reijiw@google.com>]

On Thu, Nov 4, 2021 at 2:39 PM Reiji Watanabe <reijiw@google.com> wrote:
>
> Hi Oliver,
>
> On Thu, Nov 4, 2021 at 9:14 AM Oliver Upton <oupton@google.com> wrote:
> >
> > Hi Reiji,
> >
> > On Tue, Nov 02, 2021 at 11:24:54PM -0700, Reiji Watanabe wrote:
> > > Extend sys_regs[] of kvm_cpu_context for ID registers and save ID
> > > registers' sanitized value in the array for the vCPU at the first
> > > vCPU reset. Use the saved ones when ID registers are read by
> > > userspace (via KVM_GET_ONE_REG) or the guest.
> >
> > Based on my understanding of the series, it appears that we require the
> > CPU identity to be the same amongst all vCPUs in a VM. Is there any
> > value in keeping a single copy in kvm_arch?
>
> Yes, that's a good point.
> It reminded me that the idea bothered me after we discussed a similar
> case about your counter offset patches, but I didn't seriously
> consider that.
>
> Thank you for bringing this up.
> I will look into keeping it per VM in kvm_arch.

I just remembered that I made the prototype that kept ID registers
per VM as the option B (, which introduced per VM ID register
configuration API though...).

Anyway, I've noticed that requiring the consistency of ID registers
amongst vCPUs in a VM affects KVM_ARM_VCPU_INIT API, with which
userspace can currently configure different features for each vCPUs.
I'm not sure if any existing userspace program practically does that
though.

Now, I think I should rather remove that consistency requirement...
(at least for features that can be configured by KVM_ARM_VCPU_INIT)

Thanks,
Reiji