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

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Date:   Thu, 02 Apr 2020 20:06:41 -0300
From:   Leonardo Bras <leonardo@...ux.ibm.com>
To:     "Oliver O'Halloran" <oohall@...il.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

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.

Best regards,
Leonardo Bras

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