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Re: [Qemu-devel] [PATCH v7 01/16] hw/cpu: introduce CPU clusters

From: Eduardo Habkost
Subject: Re: [Qemu-devel] [PATCH v7 01/16] hw/cpu: introduce CPU clusters
Date: Tue, 4 Dec 2018 17:45:32 -0200
User-agent: Mutt/1.9.2 (2017-12-15)

On Tue, Dec 04, 2018 at 07:16:39PM +0000, Peter Maydell wrote:
> On Tue, 4 Dec 2018 at 19:05, Eduardo Habkost <address@hidden> wrote:
> > On Tue, Dec 04, 2018 at 06:24:19PM +0000, Peter Maydell wrote:
> > > A cluster is a group of CPUs which are all identical and have
> > > the same view of the rest of the system.
> > With that definition in mind, why can't QEMU cluster CPUs
> > automatically by looking at CPU models and address space objects?
> That sounds like it is in theory feasible and in practice
> quite tricky. You would have to look not just at the CPU
> model name but also introspect all its properties for
> ones which change features of the CPU and are set differently
> on different CPUs (and I don't think there's any way to
> automatically tell which properties are ones which make
> the CPU different for which-cluster purposes and which aren't).
> And if we automatically checked whether address space objects
> were the same it would rule out implementing devices with
> per-cpu banked memory mapped registers by mapping different
> things into the AS for each CPU (though that's a hypothetical
> at the moment -- I've thought about implementing stuff that
> way but we tend to implement that sort of thing by looking
> at current_cpu->cpu_index at the moment).

I see.

Can't we at least do something to make sure the cluster objects
make sense?  e.g. by ensuring at least QOM CPU type is the same,
and that cpu->address_space somehow points to

CCing Paolo, so he can correct me if this doesn't make sense.


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