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Re: [Xen-devel] [RFC] Overview of work required to implement mem_access for PV guests

  • To: Andres Lagar-Cavilla <andreslc@xxxxxxxxxxxxxx>, Tim Deegan <tim@xxxxxxx>
  • From: "Aravindh Puthiyaparambil (aravindp)" <aravindp@xxxxxxxxx>
  • Date: Tue, 26 Nov 2013 19:46:48 +0000
  • Accept-language: en-US
  • Cc: "xen-devel@xxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxx>
  • Delivery-date: Tue, 26 Nov 2013 19:47:08 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xen.org>
  • Thread-index: AQHO6tcmYCp93KSkS7KT3UtfTSiTNpo36gKg
  • Thread-topic: [RFC] Overview of work required to implement mem_access for PV guests

>>>> The mem_access APIs only work with HVM guests that run on Intel
>>> hardware with EPT support. This effort is to enable it for PV guests
>>> that run with shadow page tables. To facilitate this, the following will be
>>>> 1. A magic page will be created for the mem_access (mem_event) ring
>>>> buffer during the PV domain creation.
>>> As Andrew pointed out, you might have to be careful about this -- if
>>> the page is owned by the domain itself, and it can find out (or
>>> guess) its MFN, it can map and write to it.  You might need to allocate an
>anonymous page for this?
>> Do you mean allocate an anonymous page in dom0 and use that? Won't we
>run in to the problem Andres was mentioning a while back?
>> http://xen.markmail.org/thread/kbrz7vo3oyrvgsnc
>> Or were you meaning something else?
>> I was planning on doing exactly what we do in the mem_access listener for
>HVM guests. The magic page is mapped in and then removed from physmap
>of the guest.
>> http://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=tools/tests/xen-acce
>> ss/xen-
>> 3
>Once the page is removed from the physmap, an hvm guest has no way of
>indexing that page and thus mapping it -- even though it's a page that belongs
>to it, and that it's threaded on its list of pages owned.
>WIth PV, you have an additional means of indexing, which is the raw MFN.
>The PV guest will be able to get at the page because it owns it, if it knows 
>MFN. No PFN/GFN required. This is how, for example, things like the grant
>table are mapped in classic PV domains.
>I don't know how realistic is the concern about the domain guessing the MFN
>for the page. But if it can, and it maps it and mucks with the ring, the thing 
>evaluate is: can the guest throw dom0/host into a tailspin? The answer is 
>"no", because guests can't reasonably do this with other rings they have
>access to, like PV driver backends. But a flaw on the consumer side of mem
>events could yield a vector for DoS.
>If, instead, the page is a xen-owned page (alloc_xenheap_pages), then there
>is no way for the PV domain to map it.

Thanks so much for the explanation. I will use to alloc_xenheap_pages. 

>>> From my reading of xc_domain_decrease_reservation_exact(), I think it
>will also work for PV guests. Or am I missing something here?
>>>> 2. Most of the mem_event / mem_access functions and variable name
>>>> are HVM specific. Given that I am enabling it for PV; I will change
>>>> the names to something more generic. This also holds for the
>>>> mem_access hypercalls, which fall under HVM ops and do_hvm_op(). My
>>>> plan is to make them a memory op or a domctl.
>>> Sure.
>>>> 3. A new shadow option will be added called PG_mem_access. This mode
>>>> is basic shadow mode with the addition of a table that will track
>>>> the access permissions of each page in the guest.
>>>> mem_access_tracker[gfmn] = access_type If there is a place where I
>>>> can stash this in an existing structure, please point me at it.
>>> My suggestion was that you should make another implementation of the
>>> p2m.h interface, which is already called in all the right places.
>>> You might want to borrow the tree-building code from the existing
>>> p2m-pt.c, though there's no reason why your table should be
>>> structured as a pagetable.  The important detail is that you should
>>> be using memory from the shadow pool to hold this datastructure.
>> OK, I will go down the path. I agree that my table needn't be structured as a
>pagetable. The other thing I was thinking about is stashing the access
>information in the per mfn page_info structures. Or is that memory overhead
>too much of an overkill?
>Well, the page/MFN could conceivably be mapped by many domains. There
>are ample bits to play with in the type flag, for example. But as long as you
>don't care about mem_event on pages shared across two or more PV
>domains, then that should be fine. I wouldn't blame you if you didn't care :)

Yup, I don't care :-)

>OTOH, all you need is a byte per pfn, and the great thing is that in PV 
>the physmap is bounded and continuous. Unlike HVM and its PCI holes, etc,
>which demand the sparse tree structure. So you can allocate an easily
>indexable array, notwithstanding super page concerns (I think/hope).

I did not realize that the physmap is bounded and contiguous. I will go with an 
indexable array. 


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