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From:   "Oliver O'Halloran" <oohall@gmail.com>
Date:   Fri, 3 Apr 2020 10:31:28 +1100
Message-ID: <CAOSf1CHSCMsQgJTcMPiRUFDxBF=WVpTRk7-bzyg6iit8bmm7rg@mail.gmail.com>
Subject: Re: [PATCH v3 1/1] powerpc/kernel: Enables memory hot-remove after
 reboot on pseries guests
To:     Leonardo Bras <leonardo@linux.ibm.com>
Cc:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Thomas Gleixner <tglx@linutronix.de>,
        Nathan Fontenot <nfont@linux.vnet.ibm.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Allison Randal <allison@lohutok.net>,
        Bharata B Rao <bharata@linux.ibm.com>,
        Claudio Carvalho <cclaudio@linux.ibm.com>,
        Hari Bathini <hbathini@linux.ibm.com>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Fri, Apr 3, 2020 at 10:07 AM Leonardo Bras <leonardo@linux.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