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

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 23 Feb 2022 09:16:51 +0000
"Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:

> * Igor Mammedov (imammedo@redhat.com) wrote:
> > 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?  
> 
> The hook in kvm/kvm-cpu.c kvm_cpu_realizefn:
> 
>     /*
>      * The realize order is important, since x86_cpu_realize() checks if
>      * nothing else has been set by the user (or by accelerators) in
>      * cpu->ucode_rev and cpu->phys_bits, and updates the CPUID results in
>      * mwait.ecx.
>      * This accel realization code also assumes cpu features are already expanded.
>      *
>      * realize order:
>      *
>      * x86_cpu_realize():
>      *  -> x86_cpu_expand_features()
>      *  -> cpu_exec_realizefn():
>      *            -> accel_cpu_realizefn()
>      *               kvm_cpu_realizefn() -> host_cpu_realizefn()
>      *  -> check/update ucode_rev, phys_bits, mwait
>      */

Thanks,
I didn't expect host_cpu_realizefn being called from elsewhere
beside of cpu model it belongs to or models inherited from it.

> 
> Dave
> 
> > > 
> > > take care,
> > >   Gerd
> > >   
> >