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Message-ID: <acbb25cdc30dc38a54eb6fb4874cc418b002c1df.camel@linux.ibm.com>
Subject: Re: [PATCH v3 1/1] powerpc/kernel: Enables memory hot-remove after
 reboot on pseries guests
From:   Leonardo Bras <leonardo@linux.ibm.com>
To:     "Oliver O'Halloran" <oohall@gmail.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>
Date:   Thu, 02 Apr 2020 20:41:05 -0300
In-Reply-To: <CAOSf1CHSCMsQgJTcMPiRUFDxBF=WVpTRk7-bzyg6iit8bmm7rg@mail.gmail.com>
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On Fri, 2020-04-03 at 10:31 +1100, Oliver O'Halloran wrote:
> On Fri, Apr 3, 2020 at 10:07 AM Leonardo Bras <leonardo@linux.ibm.com> wr=
ote:
> > Hello Oliver, thank you for the feedback.
> > Comments inline:
> >=20
> > 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-remove=
d
> > > 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.
> >=20
> > All LMBs are listed in DR memory, even the base memory.
> >=20
> > 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.
> >=20
> > 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).
> >=20
> > To create a solution that doesn't break PowerVM, this new flag was made
> > necessary.
>=20
> 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?
>=20

Humm, wouldn't that assume that all unmovable memory should be at the
first LMBs?=20

If we use the recently approved flag, there would be no such
limitation, and there would be possible to do some other things, like
hot-adding more unmovable memory to kernel usage.

What do you think?

Best regards,

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