[PATCH v6 2/6] KVM: arm64: Reject attempts to set invalid debug arch version

[Jing Zhang]

Hi Oliver,

On Fri, Jul 21, 2023 at 2:18 PM Oliver Upton <oliver.upton at linux.dev> wrote:
>
> On Tue, Jul 18, 2023 at 04:45:18PM +0000, Jing Zhang wrote:
> > From: Oliver Upton <oliver.upton at linux.dev>
> >
> > The debug architecture is mandatory in ARMv8, so KVM should not allow
> > userspace to configure a vCPU with less than that. Of course, this isn't
> > handled elegantly by the generic ID register plumbing, as the respective
> > ID register fields have a nonzero starting value.
> >
> > Add an explicit check for debug versions less than v8 of the
> > architecture.
> >
> > Signed-off-by: Oliver Upton <oliver.upton at linux.dev>
> > Signed-off-by: Jing Zhang <jingzhangos at google.com>
>
> This patch needs to be broken up. You're doing a couple things:
>
>  1) Forcing the behavior of the DebugVer field to be FTR_LOWER_SAFE, and
>    adding the necessary check for a valid version
>
>  2) Changing KVM's value for the field to expose up to Debugv8p8 to the
>    guest.
>
> The latter isn't described in the changelog at all, and worse yet the
> ordering of the series is not bisectable. Changing the default value of
> the field w/o allowing writes breaks migration.
>
> So, please split this patch in two and consider stacking like so:
>
>  - Change #1 above (field sanitization)
>
>  - "KVM: arm64: Enable writable for ID_AA64DFR0_EL1 and ID_DFR0_EL1"
>
>  - Change #2 above (advertise up to v8p8)
>
> --
> Thanks,
> Oliver
Sure, I'll split it as you suggested.

Thanks,
Jing