xen/kexec: use hypercall_create_continuation to protect KEXEC ops

xen/kexec: use hypercall_create_continuation to protect KEXEC ops

When we concurrently try to unload and load crash
images we eventually get:

 Xen call trace:
    [<ffff82d08018b04f>] machine_kexec_add_page+0x3a0/0x3fa
    [<ffff82d08018b184>] machine_kexec_load+0xdb/0x107
    [<ffff82d080116e8d>] kexec.c#kexec_load_slot+0x11/0x42
    [<ffff82d08011724f>] kexec.c#kexec_load+0x119/0x150
    [<ffff82d080117c1e>] kexec.c#do_kexec_op_internal+0xab/0xcf
    [<ffff82d080117c60>] do_kexec_op+0xe/0x1e
    [<ffff82d08025c620>] pv_hypercall+0x20a/0x44a
    [<ffff82d080260116>] cpufreq.c#test_all_events+0/0x30

 Pagetable walk from ffff820040088320:
  L4[0x104] = 00000002979d1063 ffffffffffffffff
  L3[0x001] = 00000002979d0063 ffffffffffffffff
  L2[0x000] = 00000002979c7063 ffffffffffffffff
  L1[0x088] = 80037a91ede97063 ffffffffffffffff

The interesting thing is that the page bits (063) look legit.

The operation on which we blow up is us trying to write
in the L1 and finding that the L2 entry points to some
bizzare MFN. It stinks of a race, and it looks like
the issue is due to no concurrency locks when dealing
with the crash kernel space.

Specifically we concurrently call kimage_alloc_crash_control_page
which iterates over the kexec_crash_area.start -> kexec_crash_area.size
and once found:

  if ( page )
  {
      image->next_crash_page = hole_end;
      clear_domain_page(_mfn(page_to_mfn(page)));
  }

clears. Since the parameters of what MFN to use are provided
by the callers (and the area to search is bounded) the the 'page'
is probably the same. So #1 we concurrently clear the
'control_code_page'.

The next step is us passing this 'control_code_page' to
machine_kexec_add_page. This function requires the MFNs:
page_to_maddr(image->control_code_page).

And this would always return the same virtual address, as
the MFN of the control_code_page is inside of the
kexec_crash_area.start -> kexec_crash_area.size area.

Then machine_kexec_add_page updates the L1 .. which can be done
concurrently and on subsequent calls we mangle it up.

This is all a theory at this time, but testing reveals
that adding the hypercall_create_continuation() at the
kexec hypercall fixes the crash.

NOTE: This patch follows 5c5216 (kexec: clear kexec_image slot
when unloading kexec image) to prevent crashes during
simultaneous load/unloads.

NOTE: Consideration was given to using the existing flag
KEXEC_FLAG_IN_PROGRESS to denote a kexec hypercall in
progress. This, however, overloads the original intent of
the flag which is to denote that we are about-to/have made
the jump to the crash path. The overloading would lead to
failures in existing checks on this flag as the flag would
always be set at the top level in do_kexec_op_internal().
For this reason, the new flag KEXEC_FLAG_HC_IN_PROGRESS
was introduced.

While at it, fixed the #define mismatched spacing

Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
Reviewed-by: Bhavesh Davda <bhavesh.davda@oracle.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Release-acked-by: Julien Grall <julien.grall@arm.com>