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

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

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.

Either way this patch is wrong and is papering over an actual issue.


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