Il 03/09/2013 16:22, Gleb Natapov ha scritto:
>> ... This problem seems to be rooted in the same read-only-memslot thing
>> that we recently encountered on an Intel host that lacks EPT: the SVM
>> reference manual also says, under 2.20.3 Permission Checks:
>>
>> When nested paging is enabled, pages accessed by the guest must be
>> marked as present and accessible at the user-level in the host page
>> table---regardless of the current guest CPL. Further, the host mapping
>> must permit writes for the guest to be able to write the page. A
>> failed host access check (for an access that is otherwise legal at the
>> guest level) results in a #VMEXIT(NPF).
>>
>> Note: Host permissions are checked on every reference to a guest
>> physical address---even those caused by guest page table walks.
>> In particular, when attempting to set an "Accessed" or "Dirty"
>> bit while walking the guest tables (which reside in guest
>> physical space), the processor checks whether the corresponding
>> host virtual page is present and user-level writable; if not,
>> the processor raises a #VMEXIT(NPF).
>>
>> Since OVMF has the initial page tables in ROM, the above won't fly.
>>
> Why want it fly? Since guest page tables have "Accessed" and "Dirty"
> bits set already processor should not check page writablility. But Paolo
> says that "host table walks for guest page tables are always treated as
> data writes", Paolo where in the spec is this?
In my copy of the AMD manual (an old one---rev 3.15, November 2009) it
is in 15.24.5 ("Nested Table Walk"). It is also mentioned in the
description of EXITINFO1, in 15.24.6 ("Nested versus Guest Page Faults,
Fault Ordering").
Paolo
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