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

[Konrad Rzeszutek Wilk]

On Tue, Jun 17, 2014 at 10:43:29PM +0300, Michael S. Tsirkin wrote:
> On Tue, Jun 17, 2014 at 02:44:41PM -0400, Don Slutz wrote:
> > On 06/17/14 14:22, Michael S. Tsirkin wrote:
> > >On Fri, Jun 06, 2014 at 01:52:05PM -0400, Don Slutz wrote:
> > >>This is a pc & q35 only machine opt.  One use is to allow for more
> > >>ram in a 32bit guest for example:
> > >>
> > >>-machine pc,max-ram-below-4g=3.75G
> > >>
> > >>If you add enough PCI devices then all mmio for them will not fit
> > >>below 4G which may not be the layout the user wanted. This allows
> > >>you to increase the below 4G address space that PCI devices can use
> > >>(aka decrease ram below 4G) and therefore in more cases not have any
> > >>mmio that is above 4G.
> > >>
> > >>For example using "-machine pc,max-ram-below-4g=2G" on the command
> > >>line will limit the amount of ram that is below 4G to 2G.
> > >I'm not sure I get it.
> > >
> > >All this only has effect if you have >4G RAM, right?
> > 
> > Nope.  When it takes effect includes RAM > max-ram-below-4g. I.E.
> > 
> >    -machine pc,max-ram-below-4g=2G -m 3G
> > 
> > Will have 2G in 32bit space (below 4G) and 1G in 64bit space (above 4G).
> > 
> > >Presumably you then have a 64 bit guest so why does it
> > >care about memory/MMIO being below 4G?
> > 
> > It is not the guest that matters, it is all the PCI devices in use. There 
> > are
> > ones (all old hardware) that only support 32bit addresses.
> 
> Emulated devices? Let's just teach them to support 64 bit BARs.
> Looks like a nicer solution than asking user to make
> this decision.

I presume real normal PCI devices.
> 
> >  When using
> > these you may need more room.
> > 
> > Also pci-passthru of real hardware that is 32bit only may require this.
> >    -Don Slutz
> 
> Guest and host BARs are unrelated so it seems we could allow 64 bit BARs
> even when they are 32 bit on host.

That assumes that the devices are OK with having their BARs remmaped.
That is not always the case and some of them might not like that.

Also OSes might want to only use 32-bit BARs as that is what they
had been written for. I presume since Don is working for a telecom
that this might be some ancient networking hardware with super-fast
BSD OS. 
> 
> Though PCI Express spec requires 64 bit BARs since version 1.0,
> are there really many devices like this out there?
> Care giving examples?

I think he mentioned PCI devices, not PCIe.