Re: [PATCH RESEND v2 0/6] target/arm: Add nested virtualization support

[Andrew Jones]

On Tue, Apr 27, 2021 at 04:01:10PM +0100, Peter Maydell wrote:
> On Tue, 27 Apr 2021 at 15:58, Peter Maydell <peter.maydell@linaro.org> wrote:
> >
> > On Tue, 27 Apr 2021 at 15:48, Andrew Jones <drjones@redhat.com> wrote:
> >
> > > Since these types of features seem to blur the line between being a CPU
> > > and board property, then I think I'd prefer we call them CPU properties,
> > > as they come from the CPU manual.
> >
> > Conversely, I prefer to call them board properties, because that's
> > the way it works in hardware: the hardware board has the necessary
> > support for the system-level feature, and part of that is that it
> > has an SoC or CPU which has been configured to have the properties
> > that are needed for the board to support the feature. Having a CPU
> > that nominally supports a feature is useless if the system as a whole
> > doesn't handle it.
> 
> ...this also means that we're consistent between boards: some board
> models unconditionally have support for a feature (and always set it
> on the CPU, GIC, etc), some don't ever support the feature (and always
> disable it), and a few offer the user the choice. Having the user
> use CPU properties suggests that they can, for instance, plug a
> has-el3 CPU into any board model, which in general won't work.
>

I feel like this can be looked at both ways. A board can have a supporting
CPU or a CPU can have a supporting board. While a CPU is physically
mounted in a board, giving it a natural "physical member of" relationship
to the board, a board's design is driven by the features of the CPU, which
leads to the board having a "implements dependencies of" relationship to
the CPU.

I think the way we look at it depends on what we consider our top level
requirements to be. If it's a board specification that we're implementing,
then we clearly look at it from the board perspective. However, for mach-
virt, we don't have much of a board specification. We want it to be a
minimal board that supports VIRTIO and CPU features.

Maybe this is a place where our perspective and interface should diverge
between mach-virt and the emulated board models?

All that said, if we'd rather start thinking about system features, then
should we rework 'pmu' and perhaps other CPU features, which might better
be considered system features? Also, is the '-M virt,\?' type of probing
sufficient? Don't we need some sort of probing that considers both the
board support and the CPU support? 'virt' might support a system feature
that '-M virt -cpu xyz' does not support, right? How do users (libvirt)
know if they need to probe both the board and the CPU for feature support?
How do we probe the CPU's support for the feature if we don't want to
expose the feature as a CPU property? And, if we do expose the feature
as a CPU property, then what should be the response to enabling it without
the board property? Error out or imply its enablement when it's available?

I think we need some sort of system feature document to explain what a
system feature is and how it combines board properties together with CPU
features (which may or may not be exposed as properties). We'll need
to document how to properly do the probing and we'll also need tests to
check all our {board-property, cpu-type, cpu-property} misconfiguration
possibilities for proper error handling.

Thanks,
drew