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

[Michael S. Tsirkin]

On Fri, Sep 23, 2022 at 08:23:12AM +0200, Gerd Hoffmann wrote:
>   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

I dropped the patch for now.

-- 
MST