Re: [PATCH 2/7] hw/boards: Introduce 'kvm_supported' field to MachineClass

[Daniel P . Berrangé]

On Fri, Feb 19, 2021 at 12:08:05PM +0000, Peter Maydell wrote:
> On Fri, 19 Feb 2021 at 11:58, Daniel P. Berrangé <berrange@redhat.com> wrote:
> > Is the behaviour reported really related to KVM specifically, as opposed
> > to all hardware based virt backends ?
> >
> > eg is it actually a case of some machine types being  "tcg_only" ?
> 
> Interesting question. At least for Arm the major items are:
>  * does the accelerator support emulation of EL3/TrustZone?
>    (KVM doesn't; this is the proximate cause of the assertion
>    failure if you try to enable KVM for the raspi boards.)
>  * does the board type require a particular CPU type which
>    KVM doesn't/can't support?
> Non-KVM accelerators could at least in theory have different answers
> to those questions, though in practice I think they do not.
> 
> I think my take is that we probably should mark the boards
> as 'tcg-only' vs 'not-tcg-only', because in practice that's
> the interesting distinction. Specifically, our security policy
> https://qemu.readthedocs.io/en/latest/system/security.html
> draws a boundary between "virtualization use case" and
> "emulated", so it's really helpful to be able to say clearly
> "this board model does not support virtualization, and therefore
> any bugs in it or its devices are simply outside the realm of
> being security issues" when doing analysis of the codebase or
> when writing or reviewing new code.

Oh, yes, that is useful to correlate with.

> If we ever have support for some new accelerator type where there's
> a board type distinction between KVM and that new accelerator and
> it makes sense to try to say "this board is supported by the new
> thing even though it won't work with KVM", the folks interested in
> adding that new accelerator will have the motivation to look
> into exactly which boards they want to enable support for and
> can add a funky_accelerator_supported flag or whatever at that time.
> 
> Summary: we should name this machine class field
> "virtualization_supported" and check it in all the virtualization
> accelerators (kvm, hvf, whpx, xen).

Agreed.

Regards,
Daniel
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