Re: [PATCH v4] x86: add etc/phys-bits fw_cfg file

[Gerd Hoffmann]

  Hi,

> > Given newer processors have more than 40 and for older ones we know
> > the possible values for the two relevant x86 vendors we could do
> > something along the lines of:
> >
> >    phys-bits >= 41                   -> valid
> >    phys-bits == 40    + AuthenticAMD -> valid
> >    phys-bits == 36,39 + GenuineIntel -> valid
> >    everything else                   -> invalid
> >
> > Does that look sensible to you?
> >
> 
> Yes, it does! Is phys-bits == 36 the same as invalid?

'invalid' would continue to use the current guesswork codepath for
phys-bits.  Which will end up with phys-bits = 36 for smaller VMs, but
it can go beyond that in VMs with alot (32G or more) of memory.  That
logic assumes that physical machines with enough RAM for 32G+ guests
have a physical address space > 64G.

'phys-bits = 36' would be a hard limit.

So, it's not exactly the same but small VMs wouldn't see a difference.

take care,
  Gerd