[SeaBIOS] [RFC PATCH 0/9] ACPI memory hotplug

Vasilis Liaskovitis vasilis.liaskovitis at profitbricks.com
Tue Apr 24 10:24:51 CEST 2012

On Tue, Apr 24, 2012 at 10:52:24AM +0300, Gleb Natapov wrote:
> On Mon, Apr 23, 2012 at 02:31:15PM +0200, Vasilis Liaskovitis wrote:
> > The 440fx spec mentions: "The address range from the top of main DRAM to 4
> > Gbytes (top of physical memory space supported by the 440FX PCIset) is normally
> > mapped to PCI. The PMC forwards all accesses within this address range to PCI."
> > 
> > What we probably want is that the initial memory map creation takes into account
> > all dimms specified (both populated/unpopulated) 
> Yes.
> > So "-m 1G, -device dimm,size=1G,populated=true -device dimm,size=1G,populated=false"
> > would create a system map with top of memory and start of PCI-hole at 2G. 
> > 
> What -m 1G means on this command line? Isn't it redundant?
yes, this was redundant with the original concept.

> May be we should make -m create non unplaggable, populated slot starting
> at address 0. Ten you config above will specify 3G memory with 2G
> populated (first of which is not removable) and 1G unpopulated. PCI hole
> starts above 3G.

I agree -m should mean one big unpluggable slot.

So in the new proposal,"-device dimm populated=true" means a hot-removable dimm
that has already been hotplugged. 

A question here is when exactly should the initial hot-add event for this dimm
be played? If the relevant OSPM has not yet been initialized (e.g. acpi_memhotplug
module in a linux guest needs to be loaded), the guest may not see the event. 
This is a general issue of course, but with initially populated hot-removable
dimms it may be a bigger issue. Can ospm acpi initialization be detected?

Or maybe you are suggesting "populated=true" is part of initial memory (i.e. not
hot-added, but still hot-removable). Though in that case guestOS may use it for
bootmem allocations, making hot-remove more likely to fail at the memory
offlining stage.

> > This may require some shifting of physical address offsets around
> > 3.5GB-4GB - is this the minimum PCI hole allowed?
> Currently it is 1G in QEMU code.

> > 
> > E.g. if we specify 4x1GB DIMMs (onlt the first initially populated)   
> > -m 1G, -device dimm,size=1G,populated=true -device dimm,size=1G,populated=false
> > -device dimm,size=1G,populated=false -device dimm,size=1G,populated=false
> > 
> > we create the following memory map:
> > dimm0: [0,1G)
> > dimm1: [1G, 2G)
> > dimm2: [2G, 3G)
> > dimm3: [4G, 5G) or dimm3 is split into [3G, 3.5G) and [4G, 4.5G)
> > 
> > does either of these options sound reasonable?
> > 
> We shouldn't split dimms IMO. Just unnecessary complication. Better make
> bigger PCI hole.



- Vasilis

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