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

[Tim Deegan <tim@xxxxxxx>]

Hi,

At 07:49 +0000 on 25 Nov (1385362167), Aravindh Puthiyaparambil (aravindp) 
wrote:
> 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 done:
> 
> 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?

> 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.

> 6. xc_(hvm)_set_mem_access(): This API has two modes, one if the
> start pfn/gmfn is ~0ull, it takes it as a request to set default
> access. Here we will call shadow_blow_tables() after recording the
> default access type for the domain. In the mode where it is setting
> mem_access type for individual gmfns, we will call a function that
> will drop the shadow for that individual gmfn. I am not sure which
> function to call. Will sh_remove_all_mappings(gmfn) do the trick?

Yes, sh_remove_all_mappings() is the one you want.

> The other issue here is that in the HVM case we could use
> xc_hvm_set_mem_access(gfn, nr) and the permissions for the range gfn
> to gfn+nr would be set. This won't be possible in the PV case as we
> are actually dealing with mfns and mfn to mfn+nr need not belong to
> the same guest. But given that setting *all* page access permissions
> are done implicitly when setting default access, I think we can live
> with setting page permissions one at a time as they are faulted in.

Seems OK to me.

> 8. In sh_page_fault() perform access checks similar to
> ept_handle_violation() / hvm_hap_nested_page_fault().

Yep.

> 9. Hook in to _sh_propagate() and set up the L1 entries based on
> access permissions. This will be similar to
> ept_p2m_type_to_flags(). I think I might also have to hook in to the
> code that emulates page table writes to ensure access permissions
> are honored there too.

I guess you might; again, the p2m interface will help here, and
probably the exisitng tidy-up code in emulate_gva_to_mfn will be the
place to hook.

Cheers,

Tim.

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