Re: [PATCH] Make ballooning work with maxmem > mem (i386 version)

["Daniel P. Berrange" <berrange@redhat.com>]

On Fri, Nov 10, 2006 at 03:43:10PM +0000, Keir Fraser wrote:
> On 10/11/06 15:33, "Glauber de Oliveira Costa" <gcosta@redhat.com> wrote:
> 
> >> I took both patches and then changed my mind and immediately reverted them.
> >> There is a better way: we should support the XENMEM_memory_map hypercall.
> >> We should provide a hypercall (domctl) to set a memory_map_limit parameter
> >> and then Xen can use that to fake a memory map when XENMEM_memory_map is
> >> called. The tools can set that parameter from config['maxmem'].
> > 
> > And what happens when the hypercall ever returns ENOSYS, like a kernel
> > running in a bit old Hypervisor?
> > 
> > IMHO,If we have to ever fallback into default assumptions, it seems wiser
> > to extend the physicall map to maximum_reservation, not current_reservation.
> 
> Maxmem will in future be fixed to track tot_pages. That was its original
> purpose: to cap what memory the guest is allowed *now*, not to tell it the
> max that it will ever be allowed. Also on e.g., dom0 the max is implemented
> as ~0UL (no limit). "Fortunately" this looks like a -ve return code from the
> max_mem query hypercall, but if it didn't (for example we masked off the
> high bit to make sure the return value was +ve; a fix I will probably
> implement) then dom0 boot will be broken as it will try to allocate a
> much-too-big p2m table.
> 
> We have a way to tell the guest its maximum-ever memory allowance, it's just
> not implemented on the tools and hypervisor side yet. That should be fixed.

This is not going to be any help for guests running on existing 3.0.3 tree
though. Glauber's patch makes the maxmem stuff work correctly for guest
kernels without requiring an upgrade of the HV which is much nicer for
compatability allowing both current & future guests to work on both current
and future HVs. With the increasingly widespread deployment of Xen I think
this kind of compatability is very important.

Regards,
Dan.
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