Re: [PATCH kvmtool 2/3] arm64: Initial nested virt support

[Marc Zyngier <maz@kernel.org>]

On Fri, 20 Jun 2025 12:09:38 +0100,
Alexandru Elisei <alexandru.elisei@arm.com> wrote:
> 
> Hi Andre,
> 
> Thanks for doing this, it was needed. Haven't given this a proper look (I'm
> planning to do that though!), but something jumped at me, below.
> 
> On Fri, Jun 20, 2025 at 11:44:53AM +0100, Andre Przywara wrote:
> > The ARMv8.3 architecture update includes support for nested
> > virtualization. Allow the user to specify "--nested" to start a guest in
> 
> './vm help run' shows:
> 
> --pmu             Create PMUv3 device
> --disable-mte     Disable Memory Tagging Extension
> --no-pvtime       Disable stolen time
> 
> Where:
> 
> --pmu checks for KVM_CAP_ARM_PMU_V3.
> --disable-mte is there because MTE is enabled automatically for a guest when
> KVM_CAP_ARM_MTE is present.
> --no-pvtime is there because pvtime is enabled automatically; no capability
> check is needed, but the control group for pvtime is called
> KVM_ARM_VCPU_PVTIME_CTRL.
> 
> What I'm trying to get at is that the name for the kvmtool command line option
> matches KVM's name for the capability. What do you think about naming the
> parameter --el2 to match KVM_CAP_ARM_EL2 instead of --nested?
> 
>  Also, I seem to remember that the command line option for enabling
>  KVM_CAP_ARM_EL2_E2H0 in Marc's repo is --e2h0, so having --el2 instead of
>  --nested looks somewhat more consistent to me.
> 
>  Thoughts?

I think --el2 describes the wrong thing. We don't only expose EL2 to a
guest, but we also expose FEAT_NV2 by default. So "nested" is IMO
closer to the effects of the capability. If anything, it is
KVM_CAP_ARM_EL2 that is badly named (yes, there is some history here,
but I'm not going to entertain changing the #define after 8 years).

Similarly, QEMU has "virtualization=on" as an indication that it
should engage NV, and not "el2=on".

If you wanted a pure --el2 flag, then it should engage NV just like
--nested does, but disable FEAT_NV2 in the idregs. This would give you
EL2 without recursive NV and HCR_EL2.E2H RES1.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.