Re: [Xen-devel] Question about mapping hvm‘s memory to dom0

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Mon, Jul 1, 2013 at 3:32 AM, Kai Luo <kluo@xxxxxxxxxx> wrote:
> Hi:
>         I am trying to make some source code modifications to map the hvm
> memory to dom0,I use the xc_map_foreign_bulk to do this,It works and I
> export the hvm's memory to a file. My question is,after mapping the memory I
> use xc_domain_destroy to shutdown the running hvm,theoretically the mapped
> memory will be scrubbed because xen will scrubbed the hvm's memory before
> they are freed to xen hyperviser,however I export the mapped memory after
> destroying hvm,finding the content in mapped memory nearly same as formly
> exported,they are not scrubbed at all.That confused me.Can you give me any
> suggestion?

Just let me clarify a bit: You're saying that you 1) map the guest
memory, 2) destroy the domain, 3) read the already-mapped memory
again?

If you keep the guest memory mapped by dom0 after you destroy the
domain, then the memory still has reference counts and isn't freed,
and thus won't be scrubbed.  (In fact, if you do "xl list", I bet
you'll find a "zombie domain" still left over which cant' shut down
because its pages aren't freed yet.)  When you unmap the pages in
dom0, they will then be freed, and since the domain is in the
"is_dying" state, they will be scrubbed.

The logic can be found in xen/common/page_alloc.c:free_domheap_pages().

 -George

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