Re: [edk2-devel] [PATCH v2] OvmfPkg/PlatformInitLib: catch QEMU's CPU hotplug reg block regression

["Gerd Hoffmann" <kraxel@redhat.com>]

On Tue, Jan 17, 2023 at 05:43:53PM +0100, Ard Biesheuvel wrote:
> On Tue, 17 Jan 2023 at 13:37, Gerd Hoffmann <kraxel@redhat.com> wrote:
> >
> >   Hi,
> >
> > > >> In particular the firmware makes no further decisions based on
> > > >> whether QEMU advertized some of these features.
> > > >
> > > > I was thinking the other way around:  When cpu hotplug is disabled in
> > > > qemu it should be safe to skip the whole cpu hotplug checking dance.
> > > > See test patch below.
> > > >
> > > > That would give us a config switch (turn off cpu hotplug support)
> > > > which would allow edk2 run on qemu versions with broken cpu hotplug.
> > > >
> > > > Does the idea look sane or do I miss something?
> >
> > > This would be wrong.
> > >
> > > [ detailed description snipped here (but stored for later reference,
> > >   thanks for all the details) ]
> >
> > So, the tl;dr version:  cpu hotplug is older than smi feature
> > negotiation, so smi hotplug feature bit being off doesn't imply
> > qemu wouldn't hotplug cpus.
> >
> > So, no easy way out.  Luckily this affects tcg only.
> >
> > For edk2 ci doing (tcg) efi shell test boots switching to Oliver's
> > latest containers with fixed qemu included should handle things
> > (latest series just posted).  So once this is in we should be able to
> > merge this patch without breaking CI.
> 
> My head is spinning.
> 
> What about running QEMU with only a single CPU, and without any of
> these features? Is there really no way we can make that work without
> turning OVMF into the timebomb that Laszlo describes?

I can't see any way :(

ovmf seeing only a single cpu does not imply cpu hotplug can't happen,
it could be "qemu -smp cpus=1,maxcpus=4".  Figuring the maxcpus number
depends on the broken cpu hotplug registers.

> It's just very annoying that on a non-KVM host and a given QEMU
> binary, you might simply be out of luck entirely, and there is no way
> you can run OVMF with the fix applied. I would like to avoid that if
> possible.

Indeed.

take care,
  Gerd