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Re: [Xen-devel] [PATCH 05/15] xen: p2m: new 'p2m_epc' type for EPC mapping

On 7/13/2017 12:21 AM, George Dunlap wrote:

On Jul 12, 2017, at 1:01 PM, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:

On 09/07/17 10:12, Kai Huang wrote:
A new 'p2m_epc' type is added for EPC mapping type. Two wrapper functions
set_epc_p2m_entry and clear_epc_p2m_entry are also added for further use.

Other groups in Intel have been looking to reduce the number of p2m types we 
have, so we can use more hardware defined bits in the EPT pagetable entries.

If we need a new type then we will certainly add one, but it is not clear why 
this type is needed.

Does the hypervisor need to know which pages of a domain’s p2m 1) have valid config 
set up, but 2) aren’t accessible to itself or any other domain?

Hi Andrew, George,

Actually I haven't thought this thoroughly, but my first glance is there's no existing p2m_type that can be reasonably used for EPC. Probably p2m_ram_rw or p2m_mmio_direct are two potential candidates. For EPC, for *static partitioning* Xen hypervisor just needs to setup mappings and then leave it until guest is destroyed. But for p2m_ram_rw and p2m_mmio_direct there are additional logic when Xen learns about the two types. To me adding 'p2m_epc' is more straightforward and safe. Maybe we can change to a more generic name such as 'p2m_ram_encrypted'? But again I am not sure other encryption technology can also be applied to EPC.

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