Re: [Qemu-devel] [PATCH 13/27] acpi: memory hotplug ACPI hardware implementation

[Igor Mammedov <imammedo@redhat.com>]

On Mon, 25 Nov 2013 15:31:22 +0100
Paolo Bonzini <pbonzini@redhat.com> wrote:

> Il 25/11/2013 15:18, Igor Mammedov ha scritto:
> >> > For example on reset all slots could be disabled by the
> >> > ACPIHotpluggableDimmBus (even coldplugged ones), and scanned + enabled
> >> > by ASL.
> > memory devices are scanned by OSPM during ACPI tables parsing and 
> > guest (can) use present devices right away.
> 
> Yes, what I'm saying is scan them to enable them so that OSPM sees them
> already as online.
> 
> > Currently I don't see necessity for enabling individual memory devices from
> > ASL since it adds one more round trip for QEMU<->guest protocol for no
> > particular benefit. The only use case Michael suggested would be running
> > OLD BIOS (without memory hotplug support) on machine with actively used
> > memory hotplug (if OLD BIOS runs on machine with memory hotplug but there is
> > no hotplugged memory devices, it runs just fine), so case looks like not worth
>      ^^^^^^^^^^
> 
> *Hotpluggable*, not hotplugged.  It would also break just with
> coldplugged memory devices.
> 
> > That would allow later to put present at boot DIMMs into E820 (way before ASL
> > is executed) if that would be necessary and also help to convert initial
> > memory into hotpluggable one as well.
> 
> I am afraid that mixing hot-unplug with E820 could cause some
> unanticipated trouble.  OSes may not expect memory in the E820 ranges to
> disappear.
> 
> But since we're speaking about e820, do guests work if you make the
> hotpluggable memory region reserved in the e820?  That should work with
> old BIOS too using QEMU_CFG_E820_TABLE.  If that works, I see no need
> for an "enabled" bit either.
It's on my to do list to make more code reading/experiments/testing to see if
that is ok with guest. I can't say it right now, that's why E820 changes is
not part of this series.

As for OLD BIOS it should be aware about not continuous memory ranges and
calculate present ram below/above4g via E820, there was Gerd's patch
to introduce new E820 fwcfg from QEMU and I hope to use/extend it for
telling BIOS about present coldplugged DIMMs. But I guess it's not for
OLD BIOS anyway, since it OLD SeaBIOS uses CMOS for getting ram sizes.

> 
> Paolo
> 
> > an effort and complexity it would bring.
>