Re: [Qemu-devel] [PATCH 2/2] docs: add memory-hotplug.txt

[Luiz Capitulino <lcapitulino@redhat.com>]

On Wed, 04 Mar 2015 09:16:53 +0100
Paulo Ricardo Paz Vital <paulo.vital@profitbricks.com> wrote:

> On Tue, 2015-03-03 at 11:16 -0500, Luiz Capitulino wrote:
> > On Tue, 03 Mar 2015 16:12:58 +0100
> > Paulo Ricardo Paz Vital <paulo.vital@profitbricks.com> wrote:
> > 
> > > On Tue, 2015-03-03 at 09:47 -0500, Luiz Capitulino wrote:
> > > > This document describes how to use memory hotplug in QEMU.
> > > > 
> > > > Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
> > > > ---
> > > >  docs/memory-hotplug.txt | 76 +++++++++++++++++++++++++++++++++++++++++++++++++
> > > >  1 file changed, 76 insertions(+)
> > > >  create mode 100644 docs/memory-hotplug.txt
> > > > 
> > > > diff --git a/docs/memory-hotplug.txt b/docs/memory-hotplug.txt
> > > > new file mode 100644
> > > > index 0000000..f70571d
> > > > --- /dev/null
> > > > +++ b/docs/memory-hotplug.txt
> > > > @@ -0,0 +1,76 @@
> > > > +QEMU memory hotplug
> > > > +===================
> > > > +
> > > > +This document explains how to use the memory hotplug feature in QEMU,
> > > > +which is present since v2.1.0.
> > > > +
> > > > +Please, note that memory hotunplug is not supported yet. This means
> > > > +that you're able to add memory, but you're not able to remove it.
> > > > +Also, proper guest support is required for memory hotplug to work.
> > > > +
> > > > +Basic RAM hotplug
> > > > +-----------------
> > > > +
> > > > +In order to be able to hotplug memory, QEMU has to be told how many
> > > > +hotpluggable memory slots to create and what is the maximum amount of
> > > > +memory the guest can grow. This is done at startup time by means of
> > > 
> > > Igor suggested to s/grow/have in the line below, but I think you can
> > > change here also.
> > 
> > Right. If this is the only change required, then the maintainer could
> > change it when applying the series.
> > 
> 
> Ok!
> Should I reply the original patch with the reviewed-by mark?

That's always welcome.

> 
> > > 
> > > > +the -m command-line option, which has the following format:
> > > > +
> > > > + -m [size=]megs[,slots=n,maxmem=size]
> > > > +
> > > > +Where,
> > > > +
> > > > + - "megs" is the startup RAM. It is the RAM the guest will boot with
> > > 
> > > Here, for me, the sentence "It is the RAM the guest will boot with" is
> > > not necessary, since the first sentence ("is the startup RAM") already
> > > explain that VM will start with this amount of RAM.
> > 
> > I'm not strong about this, but I think that the term "startup RAM"
> > might not be obvious to everyone at first so I think it doesn't hurt
> > to define what it means in this context.
> > 
> 
> No problem! As I said, "for me the sentence is not necessary" :-D
> Nice work, BTW!
> 
> > > 
> > > > + - "slots" is the number of hotpluggable memory slots
> > > > + - "maxmem" is the maximum RAM size the guest can have
> > > > +
> > > > +For example, the following command-line:
> > > > +
> > > > + qemu [...] 1G,slots=3,maxmem=4G
> > > > +
> > > > +Creates a guest with 1GB of memory and three hotpluggable memory slots.
> > > > +The hotpluggable memory slots are empty when the guest is booted, so all
> > > > +memory the guest will see after boot is 1GB. The maximum memory the
> > > > +guest can reach is 4GB. This means that three additional gigabytes can be
> > > > +hotplugged by using any combination of the available memory slots.
> > > > +
> > > > +Two monitor commands are used to hotplug memory:
> > > > +
> > > > + - "object_add": creates a memory backend object
> > > > + - "device_add": creates a front-end pc-dimm device and inserts it
> > > > +                 into the first empty slot
> > > > +
> > > > +For example, the following commands add another 1GB to the guest
> > > > +discussed earlier:
> > > > +
> > > > +  (qemu) object_add memory-backend-ram,id=mem1,size=1G
> > > > +  (qemu) device_add pc-dimm,id=dimm1,memdev=mem1
> > > > +
> > > > +Using the file backend
> > > > +----------------------
> > > > +
> > > > +Besides basic RAM hotplug, QEMU also supports using files as a memory
> > > > +backend. This is useful for using hugetlbfs in Linux, which provides
> > > > +access to bigger page sizes.
> > > > +
> > > > +For example, assuming that the host has 1GB hugepages available in
> > > > +the /mnt/hugepages-1GB directory, a 1GB hugepage could be hotplugged
> > > > +into the guest from the previous section with the following commands:
> > > > +
> > > > +  (qemu) object_add memory-backend-file,id=mem1,size=1G,mem-path=/mnt/hugepages-1GB
> > > > +  (qemu) device_add pc-dimm,id=dimm1,memdev=mem1
> > > > +
> > > > +It's also possible to start a guest with memory cold-plugged into the
> > > > +hotpluggable memory slots. This might seem counterintuitive at first,
> > > > +but this allows for a lot of flexibility when using the file backend.
> > > > +
> > > > +In the following command-line example, a 8GB guest is created where 6GB
> > > > +comes from regular RAM, 1GB is a 1GB hugepage page and 256MB is from
> > > > +2MB pages. Also, the guest has additional memory slots to hotplug more
> > > > +2GB if needed:
> > > > +
> > > > + qemu [...] -m 6GB,slots=4,maxmem=10G \
> > > > +   -object memory-backend-file,id=mem1,size=1G,mem-path=/mnt/hugepages-1G \
> > > > +   -device pc-dimm,id=dimm1,memdev=mem1 \
> > > > +   -object memory-backend-file,id=mem2,size=256M,mem-path=/mnt/hugepages-2MB \
> > > > +   -device pc-dimm,id=dimm2,memdev=mem2
> > > 
> > 
>