Re: [PATCH v6 1/2] arm/kvm: add support for MTE

[Peter Maydell]

On Thu, 2 Mar 2023 at 14:29, Cornelia Huck <cohuck@redhat.com> wrote:
>
> On Thu, Mar 02 2023, Peter Maydell <peter.maydell@linaro.org> wrote:
> > I think having MTE in the specific case of KVM behave differently
> > to how we've done all these existing properties and how we've
> > done MTE for TCG would be confusing. The simplest thing is to just
> > follow the existing UI for TCG MTE.
> >
> > The underlying reason for this is that MTE in general is not a feature
> > only of the CPU, but also of the whole system design. It happens
> > that KVM gives us tagged RAM "for free" but that's an oddity
> > of the KVM implementation -- in real hardware there needs to
> > be system level support for tagging.
>
> Hm... the Linux kernel actually seems to consider MTE to be a cpu
> feature (at least, it lists it in the cpu features).
>
> So, is your suggestion to use the 'mte' prop of the virt machine to mean
> "enable all prereqs for MTE, i.e. allocate tag memory for TCG and enable
> MTE in the kernel for KVM"? For TCG, we'll get MTE for the max cpu
> model; for KVM, we'd get MTE for host (== max), but I'm wondering what
> should happen if we get named cpu models and the user specifies one
> where we won't have MTE (i.e. some pre-8.5 one)?

I think we can probably cross that bridge when we get to it,
but I imagine the semantics would be "cortex-foo plus MTE"
(in the same way that -cpu cortex-foo,+x,-y can add and
subtract features from a baseline).

thanks
-- PMM