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[Xen-devel] [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 done:

1. A magic page will be created for the mem_access (mem_event) ring buffer 
during the PV domain creation.

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 

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.
This will be enabled using xc_shadow_control() before attempting to enable 
mem_access on a PV guest.

4. xc_mem_access_enable/disable(): Change the flow to allow mem_access for PV 
guests running with PG_mem_access shadow mode.

5. xc_domain_set_access_required(): No change required

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? Please 

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.

7. xc_(hvm)_get_mem_access(): This will return the access type for gmfn from 
the mem_access_tracker table.

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

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.  

Please give feedback on the above.


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