This change has been discarded.

Better solution proposed in r3619

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Submitter:	Michael LeBeane
Repository:	gem5
Branch:	default
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Description

Changeset 11554:57b008eb65c3
---------------------------
ruby: convert atomic accesses to functional in RubyPort
Instead of panicing when receiving an atomic access in ruby, convert it to a
functional access and warn the user.  This makes device models that issue
atomic accesses (such as KVM when performing a MMIO access) work correctly in
Ruby.

Testing Done

This is very dangerous.

The "functional" accessor functions are really only for debug access, and any writes happening on the functional interfaces do not really have a well-defined consistency model with respect to the "real" memory accessrs using atomic/timing. This is a really dangerous hack...

Michael LeBeane Aug. 4, 2016, 10:01 a.m. (Aug. 4, 2016, 10:01 a.m.)

The motivation for this is to make KVM work in ruby (it currently issues an atomic access to do MMIO, and atomic is not supported in Ruby). We could add ruby specific code in KVM to do a functional access when using Ruby instead. Do you have any other suggestions?

Andreas Hansson Aug. 4, 2016, 10:05 a.m. (Aug. 4, 2016, 10:05 a.m.)

Who is accessing what, and under what conditions? Why is the atomic access going via Ruby?

If all the Ruby blocks are truly idle, is implementing atomic not trivial?

Andreas Sandberg Aug. 10, 2016, 9:30 a.m. (Aug. 10, 2016, 9:30 a.m.)

Have you considered adding timing support to KVM instead?

I don't think it would be that hard, you could just have to add a new state to BaseKvmCPU to indicate that an MMIO access is in flight (e.g., RunningMMIOPending). When you receive a response, you transition into the RunningServiceCompletion state and wake the CPU. You'd end up with these state transitions Runnning -> RunningService -> RunningMMIOPending -> RunningServiceCompletion for a typical MMIO.

Obviously, you'd still have to issue atomic accesses if the system is in atomic mode.

Michael LeBeane Aug. 16, 2016, 3:29 p.m. (Aug. 16, 2016, 3:29 p.m.)

Thanks for the feedback! I have implemented timing accesses in KVM as you suggested in r3619 and will close this patch.

Status: Discarded

Change Summary:

Better solution proposed in r3619

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This change has been discarded.

ruby: convert atomic accesses to functional in RubyPort

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Status: Discarded

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