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Re: [Qemu-devel] Dropped CPU feature names and backward compatibility

From: Eduardo Habkost
Subject: Re: [Qemu-devel] Dropped CPU feature names and backward compatibility
Date: Wed, 19 Sep 2018 13:36:17 -0300
User-agent: Mutt/1.9.2 (2017-12-15)

On Tue, Sep 18, 2018 at 05:35:20PM +0200, Paolo Bonzini wrote:
> On 18/09/2018 16:22, Eduardo Habkost wrote:
> > On Tue, Sep 18, 2018 at 04:02:54PM +0200, Paolo Bonzini wrote:
> >> On 18/09/2018 15:14, Eduardo Habkost wrote:
> >>> If it broke something, we should restore the option names and
> >>> declare them as deprecated.
> >>
> >> I think in this particular case it's okay to add them back as no-ops,
> >> especially we'd actually want them to be customizable for user-mode
> >> emulation.
> > 
> > We can make the flag work on user-mode emulation, but I wouldn't
> > like to keep QEMU silent if the option was explicitly set in the
> > command-line in system emulation, because it means the user or
> > some software component is really confused and trying to do
> > something that is never going to work.
> They just want to copy the host flags blindly into the guest.  osxsave
> and ospke require some collaboration from the guest OS, but it should be
> pretty harmless to have them on the command line, because in the end the
> apps _should_ anyway be checking OSPKE.  If somebody is writing such an
> app and is puzzled that OSPKE is missing, they should first of all ask
> themselves if they have installed the right guest OS.

I wouldn't say that a no-op option that would just confuse
apps/users is harmless.  I really don't see any benefit in
keeping it.

I would agree with keeping them if it avoided additional
complexity on the app side.  But if an app is copying host flags
to the QEMU command-line, the app should be already aware that
only a subset of CPUID flags is configurable in QEMU.

The only reason I can see for keeping the options is
compatibility with command-lines generated by current versions of
apps/libvirt.  But that's exactly why we agreed on a deprecation
policy, isn't it?


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