Re: [PATCH 2/2] vhost: increase default limit of nregions from 64 to 509

From: "Michael S. Tsirkin" <mst@redhat.com>
To: Igor Mammedov <imammedo@redhat.com>
Cc: linux-kernel@vger.kernel.org, pbonzini@redhat.com, kvm@vger.kernel.org
Subject: Re: [PATCH 2/2] vhost: increase default limit of nregions from 64 to 509
Date: Thu, 30 Jul 2015 09:33:57 +0300	[thread overview]
Message-ID: <20150730093011-mutt-send-email-mst@redhat.com> (raw)
In-Reply-To: <20150730082603.7c578979@nial.brq.redhat.com>

On Thu, Jul 30, 2015 at 08:26:03AM +0200, Igor Mammedov wrote:
> On Wed, 29 Jul 2015 18:28:26 +0300
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > On Wed, Jul 29, 2015 at 04:29:23PM +0200, Igor Mammedov wrote:
> > > although now there is vhost module max_mem_regions option
> > > to set custom limit it doesn't help for default setups,
> > > since it requires administrator manually set a higher
> > > limit on each host. Which complicates servers deployments
> > > and management.
> > > Rise limit to the same value as KVM has (509 slots max),
> > > so that default deployments would work out of box.
> > > 
> > > Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> > > ---
> > > PS:
> > > Users that would want to lock down vhost could still
> > > use max_mem_regions option to set lower limit, but
> > > I expect it would be minority.
> > 
> > I'm not inclined to merge this.
> > 
> > Once we change this we can't take it back. It's not a decision
> > to be taken lightly.
> considering that continuous HVA idea has failed, why would you
> want to take limit back in the future if we rise it now?

I'm not sure.

I think you merely demonstrated it's a big change for userspace -
not that it's unfeasible.

Alternatively, if we want an unlimited size table, we should keep it
in userspace memory.

> > 
> > And memory hotplug users are a minority.  Out of these, users with a
> > heavily fragmented PA space due to hotplug abuse are an even smaller
> > minority.
> > 
> > > ---
> > >  include/uapi/linux/vhost.h | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
> > > index 2511954..92657bf 100644
> > > --- a/include/uapi/linux/vhost.h
> > > +++ b/include/uapi/linux/vhost.h
> > > @@ -140,7 +140,7 @@ struct vhost_memory {
> > >  #define VHOST_MEM_MAX_NREGIONS_NONE 0
> > >  /* We support at least as many nregions in VHOST_SET_MEM_TABLE:
> > >   * for use on legacy kernels without VHOST_GET_MEM_MAX_NREGIONS support. */
> > > -#define VHOST_MEM_MAX_NREGIONS_DEFAULT 64
> > > +#define VHOST_MEM_MAX_NREGIONS_DEFAULT 509
> > >  
> > >  /* VHOST_NET specific defines */
> > >  
> > > -- 
> > > 1.8.3.1