linux-kernel - Re: [PATCH 2/2] xen: enable BALLOON_MEMORY

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Date:   Tue, 24 Mar 2020 16:16:28 +0100
From:   Roger Pau Monné <roger.pau@...rix.com>
To:     Jürgen Groß <jgross@...e.com>
CC:     <linux-kernel@...r.kernel.org>,
        Ian Jackson <ian.jackson@...citrix.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        <xen-devel@...ts.xenproject.org>
Subject: Re: [PATCH 2/2] xen: enable BALLOON_MEMORY_HOTPLUG by default

On Tue, Mar 24, 2020 at 04:09:35PM +0100, Jürgen Groß wrote:
> On 24.03.20 16:00, Roger Pau Monne wrote:
> > Without it a PVH dom0 is mostly useless, as it would balloon down huge
> > amounts of RAM in order get physical address space to map foreign
> > memory and grants, ultimately leading to an out of memory situation.
> > 
> > Such option is also needed for HVM or PVH driver domains, since they
> > also require mapping grants into physical memory regions.
> > 
> > Suggested-by: Ian Jackson <ian.jackson@...citrix.com>
> > Signed-off-by: Roger Pau Monné <roger.pau@...rix.com>
> > ---
> > Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> > Cc: Juergen Gross <jgross@...e.com>
> > Cc: Stefano Stabellini <sstabellini@...nel.org>
> > Cc: xen-devel@...ts.xenproject.org
> > ---
> >   drivers/xen/Kconfig | 1 +
> >   1 file changed, 1 insertion(+)
> > 
> > diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
> > index 57ddd6f4b729..c344bcffd89d 100644
> > --- a/drivers/xen/Kconfig
> > +++ b/drivers/xen/Kconfig
> > @@ -13,6 +13,7 @@ config XEN_BALLOON
> >   config XEN_BALLOON_MEMORY_HOTPLUG
> >   	bool "Memory hotplug support for Xen balloon driver"
> >   	depends on XEN_BALLOON && MEMORY_HOTPLUG
> > +	default y
> >   	help
> >   	  Memory hotplug support for Xen balloon driver allows expanding memory
> >   	  available for the system above limit declared at system startup.
> > 
> 
> Another variant would be to set: default XEN_BACKEND
> 
> This would match the reasoning for switching it on.

I would rather have it always on if possible, as gntdev or privcmd
(when used to map foreign pages from user-space) will also require it,
and they are not gated on XEN_BACKEND AFAICT.

> Either way would be fine with me, so you can add
> 
> Reviewed-by: Juergen Gross <jgross@...e.com>

Thanks!

Roger.