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

[Keir Fraser <keir@xxxxxxxxxxxxx>]

On 10/11/06 15:33, "Glauber de Oliveira Costa" <gcosta@xxxxxxxxxx> 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.

 -- Keir


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