Re: [Xen-devel] Question about mapping hvmâs memory to dom0

[Kai Luo <kluo@xxxxxxxxxx>]

I know every frame has a struct to help xen manage memory,in addition to normal 
mapping,when the frame refcount increases for hvm domain?page sharing?Or 
something else?
----- Original Message -----
From: "Dietmar Hahn" <dietmar.hahn@xxxxxxxxxxxxxx>
To: xen-devel@xxxxxxxxxxxxx
Cc: "George Dunlap" <George.Dunlap@xxxxxxxxxxxxx>, "Kai Luo" <kluo@xxxxxxxxxx>, 
xen-devel@xxxxxxxxxxxxxxxxxxx
Sent: Monday, July 1, 2013 6:19:10 PM
Subject: Re: [Xen-devel] Question about mapping hvmâs memory to dom0

Am Montag 01 Juli 2013, 10:52:45 schrieb George Dunlap:
> 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.)

Not sure if 'xl list' shows the dying domain. But you can see this via the
hypervisor console when pressing 'q'.

Dietmar.

> 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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