Re: [Qemu-devel] [PATCH v5 2/3] pc & q35: Add new machine opt max-ram-below-4g

[Gerd Hoffmann <kraxel@redhat.com>]

  Hi,

> > So instead of default 0, it would be preferable to set the default to the
> > actual value, and let user override it.
> >
> > Or if that's too hard, set max_ram_below_4g instead of setting
> > gigabyte_align. gigabyte_align switches everywhere is messy
> > enough, adding max_ram_below_4g into mix is just too messy.
> >
> 
> I do not see a way to encode the default since for QEMU 2.0 it depends on the specified ram size:
> 
> > This is intentional.
> 
> > If we can fit all ram into low memory, because it is less than 3.5G,
> > we'll do that (pc machine type, q35 numbers are different but logic is
> > the same).  This way 32bit (+non-PAE) guests can continue to have up to
> > 3.5G memory.
> 
> > If we can't fit all ram into low memory (thus the guest should be able
> > to access ram above 4G anyway), then we'll cut off at a gigabyte
> > boundary (3G for pc machine type).  This way our ram is nicely
> > gigabyte-aligned and we can get best performance benefits from huge
> > pages.
> 
> > The size of the pci hole changing in the second case is only a side
> > effect, it's not the main reason for the change.
> 
> > cheers,
> >   Gerd
> 
> So migration of a QEMU 2.0 pc to QEMU 2.1 without gigabyte_align would require the user
> to specify the correct value of max-ram-below-4g.

You are expected to use the same machine type on both ends for live
migration.  That is the whole point why the gigabyte alignment logic is
activated for new machine types only:  Just -M pc-<version> should be
enough to make the machine config and the vmstate wire format compatible
even with different qemu versions on both ends, without requiring the
user manually specifying obscure parameters.

> When you add xen into the mix I do not see a way to get right of gigabyte_align.
> 
> You have 3 cases:
> 
> 1) old xen (without max-ram-below-4g), QEMU 2.1 or later.
>      Will expect that QEMU acts as if max-ram-below-4g=3.75G was specified.
>      I.E. gigabyte_align is ignored. Note: xen 4.4 asks for "pc,accel=xen" in some cases.

IMO xen should use a versioned machine type to make live migration more
reliable.  IIRC this was discussed anyway for other reasons (see
xen-platform-pci discussions, picking pc-i440fx-1.6 IIRC).  That should
also make qemu use the memory layout expected by old xen.

> 3) new xen (with max-ram-below-4g), QEMU 2.1 or later.
>       Expects that max-ram-below-4g works.

Newer xen versions can switch to a newer machine type, once it knows how
to deal with the changes: other memory layout, new config switches,
whatever else might have changed ...

cheers,
  Gerd