Re: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option

[Eduardo Habkost <ehabkost@redhat.com>]

On Wed, Jan 20, 2016 at 03:55:33PM +0100, Gerd Hoffmann wrote:
>   Hi,
> 
> > > +     *    qemu -M pc,max-ram-below-4g=2G -m 4G     -> 2048M low, 2048M high
> 
> > I assume max-ram-below-4g > 3.5G was unsupported before, and we
> > are breaking compatibility intentionally.
> 
> max-ram-below-4g did only reduce memory, so max-ram-below-4g > 3.5G (or
> max-ram-below-4g > 3G with gigabyte align) had no effect and therefore
> is something pretty pointless.

I see. So, if max-ram-below-4g > 3G with gigabyte align was
unsupported too, my example below is also not relevant regarding
compatibility.

> 
> I'd expect the one case quoted above to be the only case relevant in
> practice (i.e. move split from 3G to 2G for more PCI I/O space),
> especially given that the option was added after gigabyte alignment
> support.
> 
> > Because this patch also
> > changes the resulting memory layout when
> >   3G < max_ram_below_4g < ram_size < 3.5G
> > e.g.:
> >   qemu -M pc,max-ram-below-4g=3200M -m 3328M
> 
> Ah, I see.  With the patch applied the gigabyte align option is weighed
> higher, so qemu wouldn't give you 3200M lowmem.  I would be highly
> surprised to see such a configuration in the wild ...

The new behavior looks OK, considering that the option is just
"_max_ RAM below 4G", not "split lowmem at this exact address".
But it is inconsistent with the behavior when
max-ram-below-4g < 3G (where gigabyte_align is ignored), and
something we will need to keep compatibility for a long time.

Considering that we never supported
  gigabyte_align && max_ram_below_4g > 3G ||
  max_ram_below_4g > 3.5G
before, we could simply remove the MachineClass::gigabyte_align
field from pc_piix, and just do the following:

* pc > 1.7: max_ram_below_4g = 3G
  (equivalent to gigabyte_align=true)
* pc <= 1.7: max_ram_below_4g = 3.5G
  (equivalent to gigabyte_align=false)

-- 
Eduardo