Re: [patch 2/3] KVM: x86: disallow KVM_{SET,GET}_LAPIC without in kernel irqchip

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Oct 28, 2009 at 12:30:39PM +0200, Gleb Natapov wrote:
> On Wed, Oct 28, 2009 at 12:20:42PM +0200, Michael S. Tsirkin wrote:
> > On Wed, Oct 28, 2009 at 11:32:00AM +0200, Avi Kivity wrote:
> > > On 10/27/2009 07:50 PM, Michael S. Tsirkin wrote:
> > >> Can the value of irqchip_in_kernel be changed by userspace
> > >> after we have checked it? If yes, this check won't help ...
> > >>    
> > >
> > > A change from false to true is possible, but not the reverse.
> > 
> > Hmm. If we want to rely on this, we have to play with
> > memory barriers to write/read it. Doable, but hard to get right.
> Why? If userspace is so racy that it tries to use vpic while other
> thread creates it let it fail and burn in hell.

Yes but reading uninitialized memory in kernel can
lead to host kernel crashes.

> > Can we always have the irqchip object exist?
> > It doesn't use a lot of memory, does it?
> > Maybe have it inline, save an extra indirection on
> > fastpath ...
> > 
> > > -- 
> > > error compiling committee.c: too many arguments to function
> > --
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> --
> 			Gleb.