Re: [PATCH] libxl: create PVH guests with max memory assigned

[David Vrabel <david.vrabel@citrix.com>]

On 05/08/14 09:55, Ian Campbell wrote:
> On Thu, 2014-07-17 at 13:02 +0200, Roger Pau Monne wrote:
> 
> Sorry for the delay replying, this somehow slipped through my net.
> 
>> Since PVH guests are very similar to HVM guests in terms of memory
>> management, start the guest with the maximum memory assigned and let
>> it balloon down.
> 
> Both before and after this patch an HVM guest would be launched with
> target_memkb though, not max_memkb (presumably relying on PoD), so the
> comparison made in the commit log doesn't tally to me given that you are
> making PVH (and only PVH) use max_memkb.
> 
> This patch seems to make it impossible to boot a PVH guest
> pre-ballooned. It only appears to "work" because I presume you actually
> have enough RAM to satisfy maxmem for a short time, but that defeats the
> purpose.
> 
> Either a PVH guest is similar enough to an HVM guest in this area to
> make use of PoD for early ballooning *or* it is similar enough to a PV
> guest that it can use the PV kernel entry point to get in early enough
> to initialise the balloon driver (via the XEN_EXTRA_MEM_MAX_REGIONS
> stuff, I presume) before the kernels normal init sequence can start
> mucking with that memory.

A decision on which needs to be made and /documented/.  If the PV-like
approach is taken, I won't be accepting any Linux patches without such
documentation.

I now regret accepting the PVH support in Linux without a clear
specification of what PVH actually is.

David