Re: [PATCH 00/11] Alternate p2m: support multiple copies of host p2m

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 09/01/2015 21:26, Ed White wrote:
> This set of patches adds support to hvm domains for EPTP switching by creating
> multiple copies of the host p2m (currently limited to 10 copies).
>
> The primary use of this capability is expected to be in scenarios where access
> to memory needs to be monitored and/or restricted below the level at which the
> guest OS page tables operate. Two examples that were discussed at the 2014 Xen
> developer summit are:

Given the nature of VMFUNC, I presume this series is looking to add
support for in-guest entities to be able to monitor/tweak/swap the EPT
tables under the feet of operating system around it?

>
>     VM introspection: 
>         http://www.slideshare.net/xen_com_mgr/
>         zero-footprint-guest-memory-introspection-from-xen
>
>     Secure inter-VM communication:
>         http://www.slideshare.net/xen_com_mgr/nakajima-nvf
>
> Each p2m copy is populated lazily on EPT violations, and only contains entries for
> ram p2m types. Permissions for pages in alternate p2m's can be changed in a similar
> way to the existing memory access interface, and gfn->mfn mappings can be changed.
>
> All this is done through extra HVMOP types.
>
> The cross-domain HVMOP code has been compile-tested only. Also, the cross-domain
> code is hypervisor-only, the toolstack has not been modified.
>
> The intra-domain code has been tested. Violation notifications can only be received
> for pages that have been modified (access permissions and/or gfn->mfn mapping) 
> intra-domain, and only on VCPU's that have enabled notification.
>
> VMFUNC and #VE will both be emulated on hardware without native support.
>
> This code is not compatible with nested hvm functionality and will refuse to work
> with nested hvm active. It is also not compatible with migration. It should be
> considered experimental.

What about shared ept, pci passthrough, ballooning, PoD or any other
mechanisms which involve playing games with the EPT tables behind the
back of the guest?

It appears that this feature only makes sense for a plain, RAM-only VM
with no bells or whistles.

~Andrew