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Re: [PATCH] q35: catch invalid cpu hotplug configuration


From: Gerd Hoffmann
Subject: Re: [PATCH] q35: catch invalid cpu hotplug configuration
Date: Thu, 12 Aug 2021 13:07:00 +0200

  Hi,

> > +    if ((lpc->smi_host_features & 
> > BIT_ULL(ICH9_LPC_SMI_F_CPU_HOT_UNPLUG_BIT)) &&
> > +        !(lpc->smi_host_features & 
> > BIT_ULL(ICH9_LPC_SMI_F_CPU_HOTPLUG_BIT))) {
> > +        /*
> > +         * smi_features_ok_callback() throws an error on this.
> > +         *
> > +         * So bail out here instead of advertizing the invalid
> > +         * configuration and get obscure firmware failures from that.
> > +         */
> > +        error_setg(errp, "cpu hot-unplug requires cpu hot-plug");
> > +        return;
> > +    }
> 
> What does this actually mean in practice ? AFAIK from the libvirt side
> we don't do anything special to enable/disable CPU hotplug. What QEMU
> config setting is wrong to lead to this scenario, thta the user/mgmt
> app has to fix ?

It's ICH9-LPC.x-smi-cpu-{hotplug,hotunplug}.  user/mgmt should not mess
with that, but given it is possible I think printing an error is better
than going for an assert().

If this triggers it is most likely qemu getting compat properties wrong.

Full story at https://bugzilla.redhat.com/show_bug.cgi?id=1985924 ;)

take care,
  Gerd




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