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Re: [Qemu-ppc] [Qemu-devel] [RFC PATCH] powerpc: add PVR mask support

From: Andreas Färber
Subject: Re: [Qemu-ppc] [Qemu-devel] [RFC PATCH] powerpc: add PVR mask support
Date: Thu, 15 Aug 2013 17:11:40 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130620 Thunderbird/17.0.7

Am 15.08.2013 15:12, schrieb Anthony Liguori:
> Everyone is talking past each other and no one is addressing the real
> problem.  There are two distinct issues here:
> 1) We have two ABIs that cannot be changed unless there's a very good
>    reason to.  Alexey's original patch breaks both.  The guest ABI
>    cannot change given a fixed command line.
>    IOW, the exposed PVR value for -cpu POWER7 cannot change across
>    versions of QEMU or when running on different hardware.  This breaks
>    live migration and save/resume.
>    We also cannot break the command line interface.  If the last version
>    of QEMU supported -cpu POWER7_v2.1, then we must continue to support
>    that.

1a) How should -cpu 0xDEADBEEF or -cpu DEADBEEF behave.

    I expect it to error out as before
    rather than applying the same fuzz/mask that -cpu host might.

    That would let us implement our own fuzz logic in kvm.c,
    operating on a GSList of ObjectClasses to handle multiple matches.


>    If there's a good reason to break either of these, that's fine but
>    that justification needs be up front in the patch commit message.
> 2) The only "-cpu" that makes sense is "-cpu host" for KVM on HV (or
>    whatever ya'll call it).  POWER does not have the ability to
>    virtualize the hardware PVR value.  There is a virtual PVR in the
>    device tree but that's orthogonal to what we think of as the VCPU (it
>    essentially means IIUC that the cpu is compatible with that PVR).
>    We should explicitly disallow any -cpu value when KVM on HV is
>    enabled other than host.
>    The implementation of "-cpu host" is also goofy on PPC.  -cpu host
>    does a match on existing CPU models meaning that we have to define a
>    CPU model for any possible CPU we run on.  This would require having
>    every possible CPU model implemented in QEMU which is silly.
>    Instead, we should have a passthrough CPU model for use with "-cpu
>    host" which is essentially what Alexey's patch turns -cpu POWER7
>    into.

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