Re: [PATCH] KVM: Veirfy memory slot only for readability

Sasha Levin
2011-12-04T19:10:22Z

On Sun, 2011-12-04 at 19:34 +0200, Avi Kivity wrote:
> On 12/04/2011 07:29 PM, Sasha Levin wrote:
> > On Sun, 2011-12-04 at 18:53 +0200, Avi Kivity wrote:
> > > On 12/02/2011 07:46 AM, Sasha Levin wrote:
> > > > > Do you want to create read only memory slots for kvm tool?
> > > >
> > > > What KVM tool currently does is copy the kernel into guest memory and
> > > > run it from there. An idea raised recently was instead of copying it we
> > > > should mmap it into the memory to reduce footprint.
> > > >
> > > > This is why I'm looking into adding a read only memory slot. The KVM
> > > > code doesn't have to know it's read only.
> > >
> > > The kernel will patch itself very early. You need to use MAP_PRIVATE
> > > (and thus have a read/write area). It will be interesting to see what
> > > fraction of the memory is modified.
> > >
> > > Note that mapping will remove benefits like huge page support, and that
> > > you can get page sharing by using ksm. Still, it's interesting to see
> > > where this goes.
> >
> > Why would I lose hugepage if the kernel gets it's own memory slot?
>
> (transparent) hugepages only work on anonymous memory. Hopefully later
> it will be extended to work on mapped memory as well.

So I'll lose hugepages just for the kernel memory slot, I wonder how it
will matter performance-wise.

--

Sasha.