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

["Michael S. Tsirkin" <mst@xxxxxxxxxx>]

On Tue, Jun 17, 2014 at 04:08:54PM -0400, Konrad Rzeszutek Wilk wrote:
> 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.

OK that's a small niche but it makes some sense.
So let's focus on this case, it can be addressed
cleanly:

        default: max-ram-below-4g=4g
        pc:
                low = MIN(low, max-ram-below-4g)

no special handling for any corner cases.

For bonus points detect and warn if value chosen causes
alignment issues.

-- 
MST

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