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Re: [Qemu-devel] [PATCH 10/16] dimm: add busy slot check and slot auto-a

From: Igor Mammedov
Subject: Re: [Qemu-devel] [PATCH 10/16] dimm: add busy slot check and slot auto-allocation
Date: Wed, 24 Jul 2013 13:34:20 +0200

On Wed, 24 Jul 2013 11:41:04 +0200
Paolo Bonzini <address@hidden> wrote:

> Il 24/07/2013 10:36, Igor Mammedov ha scritto:
> > On Tue, 23 Jul 2013 19:09:26 +0200
> > Paolo Bonzini <address@hidden> wrote:
> > 
> >> Il 23/07/2013 18:23, Igor Mammedov ha scritto:
> >>> - if slot property is not specified on -device/device_add command,
> >>> treat default value as request for assigning DimmDevice to
> >>> the first free slot.
> >>
> >> Even with "-m" instead of "-numa mem", I think this is problematic
> >> because we still need to separate the host and guest parts of the DIMM
> >> device.  "-numa mem" (or the QMP command that Wanlong added) will be
> >> necessary to allocate memory on the host side before adding a DIMM.
> > why not do host allocation part at the same time when DIMM is added, is
> > there a real need to separate DIMM device?
> > 
> > I probably miss something but -numa mem option and co aside what problem
> > couldn't be solved during DIMM device initialization and would require
> > a split DIMM device?
> Because otherwise, every option we add to "-numa mem" will have to be
> added to "-device dimm".  For example,
>    -device dimm,policy=interleave
if it's feature of DIMM device sure, if it is not lets find a better
place for it. See below for an alternative approach.

> makes no sense to me.
> In fact, this is no different from having to do drive_add or netdev_add
> before device_add.  First you tell QEMU about the host resources to use,
> then you add the guest device and bind the device to those resources.
> >> So slots will have three states: free (created with "-m"), allocated (a
> >> free slot moves to this state with "-numa mem...,populated=no" when
> >> migrating, or with the QMP command for regular hotplug), populated (an
> >> allocated slot moves to this state with "-device dimm").
> >>
> >> You would be able to plug a DIMM only into an allocated slot, and the
> >> size will be specified on the slot rather than the DIMM device.
> > 'slot' property is there only for migration sake to provide stable
> > numeric ID for QEMU<->ACPI BIOS interface. It's not used for any other
> > purpose and wasn't intended for any other usage..
> How would you otherwise refer to the memory you want to affect in a
> set-mem-policy monitor command?
could be 'id' property or even better a QOM path

> > on baremetal slot has noting to do with size of plugged in DIMM,
> On baremetal slots also belong to a specific NUMA node, for what it's
> worth.  There are going to be differences with baremetal no matter what.
sure we can deviate here, but I don't see full picture yet so I'm trying
to find justification for it first and asking questions. Maybe a better
solution will be found.

> > why we
> > would model it other way if it only brings problems: like predefined size,
> It doesn't have to be predefined.  In the previous discussions (and also
> based on Vasilis and Hu Tao's implementations) I assumed predefined slot
> sizes.  Now I understand the benefit of having a simpler command-line
> with "-m", but then in return you need three slot states instead of just
> unpopulated/populated.
> So you'd just do
>    set-mem-policy 0 size=2G      # free->allocated
>    device_add dimm,slotid=0      # allocated->populated
> to hotplug a 2G DIMM.  And you'll be able to pin it to host NUMA nodes,
> and assign it to guest NUMA nodes, like this:
>    set-mem-policy 0 size=2G,nodeid=1,policy=membind host-nodes=0-1
>    device_add dimm,slotid=0
Do policy and other -numa mem properties belong to a particular DIMM device
or rather to a particular NUMA node?

How about following idea: guest-node maps to a specific host-node, then
when we plug DIMM, guest node provides information on policies and whatever
to the creator of DIMM device (via DimmBus and/or mhc) which allocates
memory, applies policies and binds new memory to a specific host node.
That would eliminate 2 stage approach.

in this case DIMM device only needs to specify where it's plugged in, using
'node' property (now number but could become QOM path to NUMA node object).

Ideally it would be QOM hierarchy:

where even 'node' property would become obsolete, just specify right
bus to attach DIMM device to.

we need a similar QOM hierarchy for CPUs as well to sort out
-numa cpus=ids mess.

> Again, this is the same as drive_add/device_add.
> Paolo
> > allocated, free etc. I think slot should be either free or busy.
> > 
> > 
> >>
> >> In general, I don't think free slots should be managed by the DimmBus,
> >> and host vs. guest separation should be there even if we accept your
> >> "-m" extension (doesn't look bad at all, I must say).
> >>
> >> Paolo
> > 

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