linux-kernel - Re: [PATCH 3/5] vhost: support upto 509 memory regions

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Date:	Wed, 17 Jun 2015 08:34:26 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Igor Mammedov <imammedo@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	pbonzini@...hat.com
Subject: Re: [PATCH 3/5] vhost: support upto 509 memory regions

On Wed, Jun 17, 2015 at 12:00:56AM +0200, Igor Mammedov wrote:
> On Tue, 16 Jun 2015 23:14:20 +0200
> "Michael S. Tsirkin" <mst@...hat.com> wrote:
> 
> > On Tue, Jun 16, 2015 at 06:33:37PM +0200, Igor Mammedov wrote:
> > > since commit
> > >  1d4e7e3 kvm: x86: increase user memory slots to 509
> > > 
> > > it became possible to use a bigger amount of memory
> > > slots, which is used by memory hotplug for
> > > registering hotplugged memory.
> > > However QEMU crashes if it's used with more than ~60
> > > pc-dimm devices and vhost-net since host kernel
> > > in module vhost-net refuses to accept more than 65
> > > memory regions.
> > > 
> > > Increase VHOST_MEMORY_MAX_NREGIONS from 65 to 509
> > 
> > It was 64, not 65.
> > 
> > > to match KVM_USER_MEM_SLOTS fixes issue for vhost-net.
> > > 
> > > Signed-off-by: Igor Mammedov <imammedo@...hat.com>
> > 
> > Still thinking about this: can you reorder this to
> > be the last patch in the series please?
> sure
> 
> > 
> > Also - 509?
> userspace memory slots in terms of KVM, I made it match
> KVM's allotment of memory slots for userspace side.

Maybe KVM has its reasons for this #. I don't see
why we need to match this exactly.

> > I think if we are changing this, it'd be nice to
> > create a way for userspace to discover the support
> > and the # of regions supported.
> That was my first idea before extending KVM's memslots
> to teach kernel to tell qemu this number so that QEMU
> at least would be able to check if new memory slot could
> be added but I was redirected to a more simple solution
> of just extending vs everdoing things.
> Currently QEMU supports upto ~250 memslots so 509
> is about twice high we need it so it should work for near
> future

Yes but old kernels are still around. Would be nice if you
can detect them.

> but eventually we might still teach kernel and QEMU
> to make things more robust.

A new ioctl would be easy to add, I think it's a good
idea generally.

> > 
> > 
> > > ---
> > >  drivers/vhost/vhost.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > > index 99931a0..6a18c92 100644
> > > --- a/drivers/vhost/vhost.c
> > > +++ b/drivers/vhost/vhost.c
> > > @@ -30,7 +30,7 @@
> > >  #include "vhost.h"
> > >  
> > >  enum {
> > > -	VHOST_MEMORY_MAX_NREGIONS = 64,
> > > +	VHOST_MEMORY_MAX_NREGIONS = 509,
> > >  	VHOST_MEMORY_F_LOG = 0x1,
> > >  };
> > >  
> > > -- 
> > > 1.8.3.1
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