Re: [PATCH v7 00/14] KVM: mm: fd-based approach for supporting KVM guest private memory

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 




This is the v7 of this series which tries to implement the fd-based KVM
guest private memory. The patches are based on latest kvm/queue branch
commit:

   b9b71f43683a (kvm/queue) KVM: x86/mmu: Buffer nested MMU
split_desc_cache only by default capacity

Introduction
------------
In general this patch series introduce fd-based memslot which provides
guest memory through memory file descriptor fd[offset,size] instead of
hva/size. The fd can be created from a supported memory filesystem
like tmpfs/hugetlbfs etc. which we refer as memory backing store. KVM

Thinking a bit, As host side fd on tmpfs or shmem will store memory on host page cache instead of mapping pages into userspace address space. Can we hit double (un-coordinated) page cache problem with this when guest page cache is also used?

Thanks,
Pankaj

and the the memory backing store exchange callbacks when such memslot
gets created. At runtime KVM will call into callbacks provided by the
backing store to get the pfn with the fd+offset. Memory backing store
will also call into KVM callbacks when userspace punch hole on the fd
to notify KVM to unmap secondary MMU page table entries.

Comparing to existing hva-based memslot, this new type of memslot allows
guest memory unmapped from host userspace like QEMU and even the kernel
itself, therefore reduce attack surface and prevent bugs.

Based on this fd-based memslot, we can build guest private memory that
is going to be used in confidential computing environments such as Intel
TDX and AMD SEV. When supported, the memory backing store can provide
more enforcement on the fd and KVM can use a single memslot to hold both
the private and shared part of the guest memory.

mm extension
---------------------
Introduces new MFD_INACCESSIBLE flag for memfd_create(), the file
created with these flags cannot read(), write() or mmap() etc via normal
MMU operations. The file content can only be used with the newly
introduced memfile_notifier extension.

The memfile_notifier extension provides two sets of callbacks for KVM to
interact with the memory backing store:
   - memfile_notifier_ops: callbacks for memory backing store to notify
     KVM when memory gets invalidated.
   - backing store callbacks: callbacks for KVM to call into memory
     backing store to request memory pages for guest private memory.

The memfile_notifier extension also provides APIs for memory backing
store to register/unregister itself and to trigger the notifier when the
bookmarked memory gets invalidated.

The patchset also introduces a new memfd seal F_SEAL_AUTO_ALLOCATE to
prevent double allocation caused by unintentional guest when we only
have a single side of the shared/private memfds effective.

memslot extension
-----------------
Add the private fd and the fd offset to existing 'shared' memslot so
that both private/shared guest memory can live in one single memslot.
A page in the memslot is either private or shared. Whether a guest page
is private or shared is maintained through reusing existing SEV ioctls
KVM_MEMORY_ENCRYPT_{UN,}REG_REGION.

Test
----
To test the new functionalities of this patch TDX patchset is needed.
Since TDX patchset has not been merged so I did two kinds of test:

-  Regresion test on kvm/queue (this patchset)
    Most new code are not covered. Code also in below repo:
    https://github.com/chao-p/linux/tree/privmem-v7

-  New Funational test on latest TDX code
    The patch is rebased to latest TDX code and tested the new
    funcationalities. See below repos:
    Linux: https://github.com/chao-p/linux/tree/privmem-v7-tdx
    QEMU: https://github.com/chao-p/qemu/tree/privmem-v7

An example QEMU command line for TDX test:
-object tdx-guest,id=tdx,debug=off,sept-ve-disable=off \
-machine confidential-guest-support=tdx \
-object memory-backend-memfd-private,id=ram1,size=${mem} \
-machine memory-backend=ram1

Changelog
----------
v7:
   - Move the private/shared info from backing store to KVM.
   - Introduce F_SEAL_AUTO_ALLOCATE to avoid double allocation.
   - Rework on the sync mechanism between zap/page fault paths.
   - Addressed other comments in v6.
v6:
   - Re-organzied patch for both mm/KVM parts.
   - Added flags for memfile_notifier so its consumers can state their
     features and memory backing store can check against these flags.
   - Put a backing store reference in the memfile_notifier and move pfn_ops
     into backing store.
   - Only support boot time backing store register.
   - Overall KVM part improvement suggested by Sean and some others.
v5:
   - Removed userspace visible F_SEAL_INACCESSIBLE, instead using an
     in-kernel flag (SHM_F_INACCESSIBLE for shmem). Private fd can only
     be created by MFD_INACCESSIBLE.
   - Introduced new APIs for backing store to register itself to
     memfile_notifier instead of direct function call.
   - Added the accounting and restriction for MFD_INACCESSIBLE memory.
   - Added KVM API doc for new memslot extensions and man page for the new
     MFD_INACCESSIBLE flag.
   - Removed the overlap check for mapping the same file+offset into
     multiple gfns due to perf consideration, warned in document.
   - Addressed other comments in v4.
v4:
   - Decoupled the callbacks between KVM/mm from memfd and use new
     name 'memfile_notifier'.
   - Supported register multiple memslots to the same backing store.
   - Added per-memslot pfn_ops instead of per-system.
   - Reworked the invalidation part.
   - Improved new KVM uAPIs (private memslot extension and memory
     error) per Sean's suggestions.
   - Addressed many other minor fixes for comments from v3.
v3:
   - Added locking protection when calling
     invalidate_page_range/fallocate callbacks.
   - Changed memslot structure to keep use useraddr for shared memory.
   - Re-organized F_SEAL_INACCESSIBLE and MEMFD_OPS.
   - Added MFD_INACCESSIBLE flag to force F_SEAL_INACCESSIBLE.
   - Commit message improvement.
   - Many small fixes for comments from the last version.

Links to previous discussions
-----------------------------
[1] Original design proposal:
https://lkml.kernel.org/kvm/20210824005248.200037-1-seanjc@xxxxxxxxxx/
[2] Updated proposal and RFC patch v1:
https://lkml.kernel.org/linux-fsdevel/20211111141352.26311-1-chao.p.peng@xxxxxxxxxxxxxxx/
[3] Patch v5: https://lkml.org/lkml/2022/5/19/861

Chao Peng (12):
   mm: Add F_SEAL_AUTO_ALLOCATE seal to memfd
   selftests/memfd: Add tests for F_SEAL_AUTO_ALLOCATE
   mm: Introduce memfile_notifier
   mm/memfd: Introduce MFD_INACCESSIBLE flag
   KVM: Rename KVM_PRIVATE_MEM_SLOTS to KVM_INTERNAL_MEM_SLOTS
   KVM: Use gfn instead of hva for mmu_notifier_retry
   KVM: Rename mmu_notifier_*
   KVM: Extend the memslot to support fd-based private memory
   KVM: Add KVM_EXIT_MEMORY_FAULT exit
   KVM: Register/unregister the guest private memory regions
   KVM: Handle page fault for private memory
   KVM: Enable and expose KVM_MEM_PRIVATE

Kirill A. Shutemov (1):
   mm/shmem: Support memfile_notifier

  Documentation/virt/kvm/api.rst             |  77 +++++-
  arch/arm64/kvm/mmu.c                       |   8 +-
  arch/mips/include/asm/kvm_host.h           |   2 +-
  arch/mips/kvm/mmu.c                        |  10 +-
  arch/powerpc/include/asm/kvm_book3s_64.h   |   2 +-
  arch/powerpc/kvm/book3s_64_mmu_host.c      |   4 +-
  arch/powerpc/kvm/book3s_64_mmu_hv.c        |   4 +-
  arch/powerpc/kvm/book3s_64_mmu_radix.c     |   6 +-
  arch/powerpc/kvm/book3s_hv_nested.c        |   2 +-
  arch/powerpc/kvm/book3s_hv_rm_mmu.c        |   8 +-
  arch/powerpc/kvm/e500_mmu_host.c           |   4 +-
  arch/riscv/kvm/mmu.c                       |   4 +-
  arch/x86/include/asm/kvm_host.h            |   3 +-
  arch/x86/kvm/Kconfig                       |   3 +
  arch/x86/kvm/mmu.h                         |   2 -
  arch/x86/kvm/mmu/mmu.c                     |  74 +++++-
  arch/x86/kvm/mmu/mmu_internal.h            |  18 ++
  arch/x86/kvm/mmu/mmutrace.h                |   1 +
  arch/x86/kvm/mmu/paging_tmpl.h             |   4 +-
  arch/x86/kvm/x86.c                         |   2 +-
  include/linux/kvm_host.h                   | 105 +++++---
  include/linux/memfile_notifier.h           |  91 +++++++
  include/linux/shmem_fs.h                   |   2 +
  include/uapi/linux/fcntl.h                 |   1 +
  include/uapi/linux/kvm.h                   |  37 +++
  include/uapi/linux/memfd.h                 |   1 +
  mm/Kconfig                                 |   4 +
  mm/Makefile                                |   1 +
  mm/memfd.c                                 |  18 +-
  mm/memfile_notifier.c                      | 123 ++++++++++
  mm/shmem.c                                 | 125 +++++++++-
  tools/testing/selftests/memfd/memfd_test.c | 166 +++++++++++++
  virt/kvm/Kconfig                           |   3 +
  virt/kvm/kvm_main.c                        | 272 ++++++++++++++++++---
  virt/kvm/pfncache.c                        |  14 +-
  35 files changed, 1074 insertions(+), 127 deletions(-)
  create mode 100644 include/linux/memfile_notifier.h
  create mode 100644 mm/memfile_notifier.c





[Index of Archives]     [Linux Wireless]     [Linux Kernel]     [ATH6KL]     [Linux Bluetooth]     [Linux Netdev]     [Kernel Newbies]     [Share Photos]     [IDE]     [Security]     [Git]     [Netfilter]     [Bugtraq]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux RAID]     [Linux ATA RAID]     [Samba]     [Device Mapper]

  Powered by Linux