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Re: [PATCH 1/1] spapr/rtas: Add MinMem to ibm,get-system-parameter RTAS

From: David Gibson
Subject: Re: [PATCH 1/1] spapr/rtas: Add MinMem to ibm,get-system-parameter RTAS call
Date: Tue, 24 Mar 2020 14:44:04 +1100

On Mon, Mar 23, 2020 at 07:07:21PM -0300, Leonardo Bras wrote:
> On Mon, 2020-03-23 at 14:24 +1100, David Gibson wrote:
> > On Fri, Mar 20, 2020 at 09:39:22PM -0300, Leonardo Bras wrote:
> > > Add support for MinMem SPLPAR Characteristic on emulated
> > > RTAS call ibm,get-system-parameter.
> > > 
> > > MinMem represents Minimum Memory, that is described in LOPAPR as:
> > > The minimum amount of main store that is needed to power on the
> > > partition. Minimum memory is expressed in MB of storage.
> > 
> > Where exactly does LoPAPR say that?  The version I'm looking at
> > doesn't describe MinMem all that clearly, other than to say it must be
> > <= DesMem, which currently has the same value here.
> Please look for "Minimum Memory". It's on Table 237. SPLPAR Terms. 

Ah, thanks.

Hm, yes.  In the way we manage VMs with KVM and qemu, I don't think we
cal really draw any meaningful distinction between MinMem and DesMem,
so it's reasonble for them to have the same value.

> > > This  provides a way for the OS to discern hotplugged LMBs and
> > > LMBs that have started with the VM, allowing it to better provide
> > > a way for memory hot-removal.
> > 
> > This seems a bit dubious.  Surely we should have this information from
> > the dynamic-reconfiguration-memory stuff already?  Trying to discern
> > this from purely a number seems very fragile - wouldn't that mean
> > making assumptions about how qemu / the host is laying out it's fixed
> > and dynamic memory which might not be justified?
> I agree. 
> I previously sent a RFC proposing the usage of a new flag for this same
> reason [1], which I thank you for positive feedback.
> Even though I think using a flag is a better solution, I am also
> working in this other option (MinMem), that would use parameter already
> available on the platform, in case the new flag don't get approved.
> So far, using MinMem looks like a very complex solution on kernel side,
> and I think it's a poor solution.
> I decided to send this patch because it's a simple change to the
> platform support, that should cause no harm and could even be useful to
> other OSes.

Hm, ok.

David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!

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