[Xen-devel] non-zero order allocations in shadow code may prevent live migration

["Jan Beulich" <jbeulich@xxxxxxxxxx>]

Tim,

after a lot of walking dead end routes with a customer issue stating that he
can't reliably run live migration I finally concluded that the problem can only 
be
explained by the non-zero order allocations done in shadow code (on x86-64
and x86-32/pae). However, from a PV-domain-live-migration perspective it
would seem to me that these order 2 allocations are entirely pointless; there
are really just 2 cases where non-zero order allocations are needed: a guest
in 32-bit non-PAE mode (can only be PV on a 32-bit non-PAE hypervisor, in
which case no non-zero order allocations are needed at all, or hvm) or
shadow_alloc_p2m_pages(). The latter is neither used for live migration nor
does it really require non-zero order allocations - its sole caller is
shadow_alloc_p2m_page(), which really only ever wants to return single
pages (i.e. allocating more than one page here acts at best as a short cut,
but I think there's really very little win from doing so).

So the bottom line is - sh_set_allocation() really shouldn't need to allocate
non-zero order pages except for hvm domains.

As this implies quite a few changes, before going that route I'd like to
understand whether I'm mistaken with anything here.

Thanks, Jan


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