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

From: Peter Maydell
Subject: Re: [Qemu-devel] [PATCH v7 01/16] hw/cpu: introduce CPU clusters
Date: Tue, 4 Dec 2018 19:16:39 +0000

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).

-- PMM

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