On Mon, Jun 14, 2010, Avi Kivity wrote about "Re: [PATCH 5/24] Introduce vmcs12: a VMCS structure for L1":
> >+struct __attribute__ ((__packed__)) vmcs12 {
> >+ /* According to the Intel spec, a VMCS region must start with the
> >+ * following two fields. Then follow implementation-specific data.
> >+ */
> >+ u32 revision_id;
> >+ u32 abort;
> >+};
> >
>
> Note that this structure becomes an ABI, it cannot change except in a
> backward compatible way due to the need for live migration. So I'd like
> a documentation patch that adds a description of the content to
> Documentation/kvm/. It can be as simple as listing the structure
> definition.
I agree that if struct vmcs12 is changed, this will cause problems for live
migration, but why does this mean that the struct's fields or layout an ABI
worth documenting?
After all, isn't the idea of VMCS that its internal content and layout
is opaque for the L1 guest - he can only read/write it with VMREAD/VMWRITE,
and those two instructions are the ABI (which is of course documented in the
Intel spec) - not the content of the vmcs12 structure. Even if the guest knew
the exact layout of this structure, he's not supposed to use it.
By the way, we have not actually checked that live migration is working
as expected with nested virtualization running. I expect there to be more
pitfalls and bugs even before we consider migration between two different
versions. We would indeed like to allow live migration of different kinds
(of L1 with all its L2 guests; Of all L2 guests of a L1; Of a single L2
guest), but we're trying to finish the more basic functionality first.
Thanks,
Nadav.
--
Nadav Har'El | Wednesday, Jun 16 2010, 4 Tammuz 5770
nyh@xxxxxxxxxxxxxxxxxxx |-----------------------------------------
Phone +972-523-790466, ICQ 13349191 |It is error alone which needs the support
http://nadav.harel.org.il |of government. Truth can stand by itself.
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