Re: [Xen-devel] [v7][RFC][PATCH 01/13] xen: RMRR fix

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 27.10.14 at 03:00, <tiejun.chen@xxxxxxxxx> wrote:
> n 2014/10/24 18:52, Jan Beulich wrote:
>>>>> On 24.10.14 at 09:34, <tiejun.chen@xxxxxxxxx> wrote:
>>> 5. Before we take real device assignment, any access to RMRR may issue
>>> ept_handle_violation because of p2m_access_n. Then we just call
>>> update_guest_eip() to return.
>>
>> I.e. ignore such accesses? Why?
> 
> Yeah. This illegal access isn't allowed but its enough to ignore that 
> without further protection or punishment.
> 
> Or what procedure should be concerned here based on your opinion?

If the access is illegal, inject a fault to the guest or kill it, unless you
can explain why ignoring such an access is correct/necessary (e.g.
I could see this being the equivalent of an access to a memory region
the address of which is not being decoded by any component in a
physical system).

>>> Now in our case we add a rule:
>>>   - if p2m_access_n is set we also set this mapping.
>>
>> Does that not conflict with eventual use mem-access makes of this
>> type?
>>
> 
> In our case, we always initialize these RMRR ranges with p2m_access_n to 
> make sure we can intercept any illegal access to these range until we 
> can reset them with p2m_access_rw via set_identity_p2m_entry(d, 
> base_pfn, p2m_access_rw).

This restates what the patch does but doesn't answer the question.

Jan


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