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

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 17 Jun 2015 08:34:26 +0200
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> 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@redhat.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@redhat.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.
np, I can cap it at safe 300 slots but it's unlikely that it
would take cut off 1 extra hop since it's capped by QEMU
at 256+[initial fragmented memory]

> 
> > > 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.
I can try to do something like this on top of this series.

> 
> > > 
> > > 
> > > > ---
> > > >  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