[Xen-devel] [PATCH v6 0/9] VM forking

[Tamas K Lengyel <tamas.lengyel@xxxxxxxxx>]

The following series implements VM forking for Intel HVM guests to allow for
the fast creation of identical VMs without the assosciated high startup costs
of booting or restoring the VM from a savefile.

JIRA issue: https://xenproject.atlassian.net/browse/XEN-89

The fork operation is implemented as part of the "xl fork-vm" command:
    xl fork-vm -C <config_file_for_fork> -Q <qemu_save_file> <parent_domid>
    
By default a fully functional fork is created. The user is in charge however to
create the appropriate config file for the fork and to generate the QEMU save
file before the fork-vm call is made. The config file needs to give the
fork a new name at minimum but other settings may also require changes.

The interface also allows to split the forking into two steps:
    xl fork-vm --launch-dm no \
               -p <parent_domid>
    xl fork-vm --launch-dm late \
               -C <config_file_for_fork> \
               -Q <qemu_save_file> \
               <fork_domid>

The split creation model is useful when the VM needs to be created as fast as
possible. The forked VM can be unpaused without the device model being launched
to be monitored and accessed via VMI. Note however that without its device
model running (depending on what is executing in the VM) it is bound to
misbehave or even crash when its trying to access devices that would be
emulated by QEMU. We anticipate that for certain use-cases this would be an
acceptable situation, in case for example when fuzzing is performed of code
segments that don't access such devices.

Launching the device model requires the QEMU Xen savefile to be generated
manually from the parent VM. This can be accomplished simply by connecting to
its QMP socket and issuing the "xen-save-devices-state" command. For example
using the standard tool socat these commands can be used to generate the file:
    socat - UNIX-CONNECT:/var/run/xen/qmp-libxl-<parent_domid>
    { "execute": "qmp_capabilities" }
    { "execute": "xen-save-devices-state", \
        "arguments": { "filename": "/path/to/save/qemu_state", \
                        "live": false} }

At runtime the forked VM starts running with an empty p2m which gets lazily
populated when the VM generates EPT faults, similar to how altp2m views are
populated. If the memory access is a read-only access, the p2m entry is
populated with a memory shared entry with its parent. For write memory accesses
or in case memory sharing wasn't possible (for example in case a reference is
held by a third party), a new page is allocated and the page contents are
copied over from the parent VM. Forks can be further forked if needed, thus
allowing for further memory savings.

A VM fork reset hypercall is also added that allows the fork to be reset to the
state it was just after a fork, also accessible via xl:
    xl fork-vm --fork-reset -p <fork_domid>

This is an optimization for cases where the forks are very short-lived and run
without a device model, so resetting saves some time compared to creating a
brand new fork provided the fork has not aquired a lot of memory. If the fork
has a lot of memory deduplicated it is likely going to be faster to create a
new fork from scratch and asynchronously destroying the old one.

The series has been tested with both Linux and Windows VMs and functions as
expected. VM forking time has been measured to be 0.0007s, device model launch
to be around 1s depending largely on the number of devices being emulated. Fork
resets have been measured to be 0.0001s under the optimal circumstances.

New in v6: minor fixes and improvements only + rebasing on staging

Patch 1 is a mem_sharing bugfix

Patches 2-3 implement changes to existing internal Xen APIs to make VM forking
possible.

Patches 4-5 are simple code-cleanups to mem_sharing

Patch 6 adds the hypervisor-side code implementing VM forking.

Patch 7 is integration of mem_access with forked VMs.

Patch 8 implements the VM fork reset operation hypervisor side bits.

Patch 9 adds the toolstack-side code implementing VM forking and reset.

Tamas K Lengyel (9):
  x86/hvm: introduce hvm_copy_context_and_params
  xen/x86: Make hap_get_allocation accessible
  x86/p2m: Allow p2m_get_page_from_gfn to return shared entries
  x86/mem_sharing: Replace MEM_SHARING_DEBUG with gdprintk
  x86/mem_sharing: use default_access in add_to_physmap
  xen/mem_sharing: VM forking
  xen/mem_access: Use __get_gfn_type_access in set_mem_access
  x86/mem_sharing: reset a fork
  xen/tools: VM forking toolstack side

 docs/man/xl.1.pod.in              |  36 +++
 tools/libxc/include/xenctrl.h     |  13 +
 tools/libxc/xc_memshr.c           |  22 ++
 tools/libxl/libxl.h               |   7 +
 tools/libxl/libxl_create.c        | 237 ++++++++++++------
 tools/libxl/libxl_dm.c            |   2 +-
 tools/libxl/libxl_dom.c           |  43 +++-
 tools/libxl/libxl_internal.h      |   1 +
 tools/libxl/libxl_types.idl       |   1 +
 tools/xl/xl.h                     |   5 +
 tools/xl/xl_cmdtable.c            |  12 +
 tools/xl/xl_saverestore.c         |  95 ++++++++
 tools/xl/xl_vmcontrol.c           |   8 +
 xen/arch/x86/domain.c             |  11 +
 xen/arch/x86/hvm/hvm.c            | 238 +++++++++++-------
 xen/arch/x86/mm/hap/hap.c         |   3 +-
 xen/arch/x86/mm/mem_access.c      |   5 +-
 xen/arch/x86/mm/mem_sharing.c     | 385 ++++++++++++++++++++++++++----
 xen/arch/x86/mm/p2m.c             |  25 +-
 xen/include/asm-x86/hap.h         |   1 +
 xen/include/asm-x86/hvm/hvm.h     |   2 +
 xen/include/asm-x86/mem_sharing.h |  20 +-
 xen/include/public/memory.h       |   6 +
 xen/include/xen/sched.h           |   2 +
 24 files changed, 947 insertions(+), 233 deletions(-)

-- 
2.20.1


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