[Xen-devel] [RFC 0/3] xen/arm: Support XENMEM_exchange

[Julien Grall <julien.grall@xxxxxxxxxx>]

Hi all,

Currently XENMEM_exchange is not supported on ARM because the steal_page is
left unimplemented.

However, even if steal_page is implemented, the hypercall can't work for ARM
because:
    - Direct mapped domain is not supported
    - ARM doesn't have a M2P and therefore usage of mfn_to_gmfn is
    invalid

I'm not sure how to fix properly the latter hence the RFC. mfn_to_gmfn
is used in few places in the tree, however the implementation for ARM is
completely wrong (see xen/include/asm-arm/mm.h):

 #define mfn_to_gmfn(_d, mfn)  (mfn)

All the guest but DOM0, which is direct mapped, don't have mfn == gfn.

I think implementing a M2P for ARM is not option because it will only be used
in few places and we don't have much virtual address space left for ARM32.
So I would rather drop the mfn_to_gmfn call from common code.

I've looked at reworking XENMEM_exchange to avoid mfn_to_gmfn. My idea would
be to allocate a temporary array to store the GFN between the two loops.
However, the array would be quite large (the max default is 18 on ARM),
so I don't know if it's acceptable.

Anyone have a better solution?

Regards,

Cc: Ian Campbell <ian.campbell@xxxxxxxxxx>
Cc: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
Cc: Keir Fraser <keir@xxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Julien Grall (3):
  xen/common: memory: Introduce check_range_domain_direct_mapped
  xen/common: memory: Add support for direct mapped domain in
    XEMEM_exchange
  xen/common: memory: Move steal_page in common code

 xen/arch/arm/mm.c        |   6 --
 xen/arch/x86/mm.c        |  51 -------------
 xen/common/memory.c      | 195 +++++++++++++++++++++++++++++++----------------
 xen/common/page_alloc.c  |  51 +++++++++++++
 xen/include/asm-arm/mm.h |   4 +-
 xen/include/asm-x86/mm.h |   2 -
 xen/include/xen/mm.h     |   3 +
 7 files changed, 186 insertions(+), 126 deletions(-)

-- 
2.1.4


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