Re: [Xen-devel] [RFC] New shadow paging code

[Tim Deegan <Tim.Deegan@xxxxxxxxxxxxx>]

At 10:32 -0400 on 17 Jul (1153132328), Mike D. Day wrote:
> Can you explain how this works? It is not obvious to me from the patch. 
> In shadow2.c there is in fact a whole string of #ifs. What mechanism 
> causes the file to be multiply compiled? Or do I misunderstand how this 
> works?
 
It's handled by the makefile runes that go into in xen/arch/x86/Makefile.
For each type of host, a number of shadow2_gX_on_sY.o files are 
required, and the rule for shadow2_%.o compiles shadow2.c with the
appropriate definitions for GUEST_PAGING_LEVELS and SHADOW_PAGING_LEVELS.

> Also, can you comment on the differences and similarities with the 
> shadow code contributed by Ben Thomas?

There are some similarities: for example both patches allocate a pool of
shadow memory for each domain, rather than allocating from domheap on
the fly.  Also, we're currently working on incorporating the idea of
storing guest present/writable bits in the shadow pte for fast-path of
"genuine" faults.

The major difference in structure is that the XI patch keeps the
"out-of-sync" algorithm of the old shadow code, letting guests write to
their pagetables and re-syncing the shadow on a fault.  This works well
for linux's fork()/exit() behaviour, but for Windows, which mostly does
demand paging, it makes the cycle of fault-install-retry rather
expensive.  Instead, the shadow2 patch emulates all writes to
pagetables, without ever letting the guest write to a shadowed page
directly.

XI also has a reverse-mapping mechanism for recording where all the
pagetable entries are that reference each page; this doubles the memory
cost of shadow mode, and adds some book-keeping, but avoids having to
search through all the shadows when revoking write access to a guest
page.  We believe it's possible to use heuristics to speed up the
remove-all-writable-mappings process, without the memory overhead.  For
example, just looking in the kernel's linear map works in 99.8% of cases
for Windows 2003 Server.

As for features, XI supports only HVM guests on 64-bit Xen; shadow2
supports 32-bit and PAE Xen as well, and PV guests.  As I understand it,
there is no plan to make XI support any other modes.  XI supports
shadowing superpages directly with superpages, which shadow2 does not
(yet).

Cheers,

Tim.

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