linux-kernel - Re: [PATCH v3 1/1] powerpc/kernel: Enables memory hot-remove after reboot on pseries guests

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Date:   Fri, 3 Apr 2020 10:31:28 +1100
From:   "Oliver O'Halloran" <oohall@...il.com>
To:     Leonardo Bras <leonardo@...ux.ibm.com>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Thomas Gleixner <tglx@...utronix.de>,
        Nathan Fontenot <nfont@...ux.vnet.ibm.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Allison Randal <allison@...utok.net>,
        Bharata B Rao <bharata@...ux.ibm.com>,
        Claudio Carvalho <cclaudio@...ux.ibm.com>,
        Hari Bathini <hbathini@...ux.ibm.com>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/1] powerpc/kernel: Enables memory hot-remove after
 reboot on pseries guests

On Fri, Apr 3, 2020 at 10:07 AM Leonardo Bras <leonardo@...ux.ibm.com> wrote:
>
> Hello Oliver, thank you for the feedback.
> Comments inline:
>
> On Fri, 2020-04-03 at 09:46 +1100, Oliver O'Halloran wrote:
> >
> > I don't really understand why the flag is needed at all. According to
> > PAPR any memory provided by dynamic reconfiguration can be hot-removed
> > so why aren't we treating all DR memory as hot removable? The only
> > memory guaranteed to be there 100% of the time is what's in the
> > /memory@0 node since that's supposed to cover the real mode area.
>
> All LMBs are listed in DR memory, even the base memory.
>
> The v1 of the patch would work this way, as qemu would configure it's
> DR memory with (DRC_INVALID | RESERVED) flags and the hot-added memory
> with (ASSIGNED) flag. Looking for assigned flag would be enough.
>
> But as of today, PowerVM doesn't seem to work that way.
> When you boot a PowerVM virtual machine with Linux, all memory is added
> with the same flags (ASSIGNED).
>
> To create a solution that doesn't break PowerVM, this new flag was made
> necessary.

I'm still not convinced it's necessary. Why not check memory@0 and use
the size as a clip level? Any memory above that level gets marked as
hotpluggable and anything below doesn't. Seems to me that would work
on all current platforms, so what am I missing here?

>
> Best regards,
> Leonardo Bras