Subject: Re: [PATCH 03/26] KVM: arm64: Factor out stage 2 page table data from struct kvm

From: Marc Zyngier <[email-masked]>
Date: Tue, 05 May 2020 19:10:50 +0100

On Tue, 05 May 2020 18:23:51 +0100,
Andrew Scull <[email-masked]> wrote:
> 
> > > > +	/* VTCR_EL2 value for this VM */
> > > > +	u64    vtcr;
> > > 
> > > VTCR seems quite strongly tied to the MMU config. Is it not controlled
> > > independently for the nested MMUs and so remains in this struct?
> > 
> > This particular instance of VTCR_EL2 is the host's version. Which
> > means it describes the virtual HW for the EL1 guest. It constraints,
> > among other things, the number of IPA bits for the guest, for example,
> > and is configured by the VMM.
> > 
> > Once you start nesting, each vcpu has its own VTCR_EL2 which is still
> > constrained by the main one (no nested guest can have a T0SZ bigger
> > than the value imposed by userspace for this guest as a whole).
> > 
> > Does it make sense?
> 
> It does up to my ignorance of the spec in this regard.
> 
> Simliar to James's question, should `vtcr` live inside the mmu struct
> with the top level `kvm::mmu` field containing the host's version and
> the nested mmus containing the nested version of vtcr to be applied to
> the vCPU? I didn't noticed there being a vtcr for the nested version in
> the ~90-patch series so maybe that just isn't something that needs
> thinking about?

They serve two very different purposes. One defines the virtual HW,
the other one is the view that a guest hypervisor gives to its own
guests. The latter is also per-vcpu, and not per VM (yes, NV implies
the "de-facto CnP", for those who remember an intense whiteboard
session).  It thus lives in the system register file (being private to
each vcpu). Another reason is that the HW can directly access the
in-memory version of VTCR_EL2 when ARMv8.4-NV is present.

Thanks,

	M.

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

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