Re: [PATCH RFCv2 2/4] i386/pc: relocate 4g start to 1T where applicable

[Igor Mammedov <imammedo@redhat.com>]

On Tue, 22 Feb 2022 10:42:55 +0100
Gerd Hoffmann <kraxel@redhat.com> wrote:

>   Hi,
> 
> > > And the upstream code is now pretty much identical except for the
> > > default;  note that for TCG you do need to keep to 40 I think.  
> > 
> > will TCG work with 40bits on host that supports less than that?  
> 
> When I understand things correctly the problem is that the phys-bits
> limit applies to the npt/ept tables too, effectively restricting guest
> physical address space to host physical address space.
> 
> TCG is not affected by that and should work just fine.
> 
> Not sure what happens if you turn off npt/ept and run on softmmu.
> Possibly that works fine too.
> 
> > Also quick look at host-phys-bits shows that it affects only 'host'
> > cpu model and is NOP for all other models.  
> 
> I don't think so.  microvm forces host-phys-bits=on and that works with
> all cpu models.

I just don't see how host-phys-bits can work for other than 'host' cpu model.
It's true that property is available for all cpu models, but the field it sets
is only used in target/i386/host-cpu.c, the same applies to host-phys-bits-limit.
Am I missing something?

> 
> take care,
>   Gerd
>