Hi Chao,

Thank you for your reply.

On Mon, Aug 29, 2022 at 4:23 PM Chao Peng [off-list ref] wrote:

On Fri, Aug 26, 2022 at 04:19:25PM +0100, Fuad Tabba wrote:

quoted

Hi,

On Wed, Jul 6, 2022 at 9:24 AM Chao Peng [off-list ref] wrote:

quoted

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
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.

I'm on the Android pKVM team at Google, and we've been looking into
how this approach fits with what we've been doing with pkvm/arm64.
I've had a go at porting your patches, along with some fixes and
additions so it would go on top of our latest pkvm patch series [1] to
see how well this proposal fits with what we’re doing. You can find
the ported code at this link [2].

In general, an fd-based approach fits very well with pKVM for the
reasons you mention. It means that we don't necessarily need to map
the guest memory, and with the new extensions it allows the host
kernel to control whether to restrict migration and swapping.

Good to hear that.

quoted

For pKVM, we would also need the guest private memory not to be
GUP’able by the kernel so that userspace can’t trick the kernel into
accessing guest private memory in a context where it isn’t prepared to
handle the fault injected by the hypervisor. We’re looking at whether
we could use memfd_secret to achieve this, or maybe whether extending
your work might solve the problem.

This is interesting and can be a valuable addition to this series.

I'll keep you posted as it goes. I think with the work that you've
already put in, it wouldn't require that much more.

quoted

However, during the porting effort, the main issue we've encountered
is that many of the details of this approach seem to be targeted at
TDX/SEV and don’t readily align with the design of pKVM. My knowledge
on TDX is very rudimentary, so please bear with me if I get things
wrong.

No doubt this series is initially designed for confidential computing
usages, but pKVM can definitely extend it if it finds useful.

quoted

The idea of the memslot having two references to the backing memory,
the (new) private_fd (a file descriptor) as well as the userspace_addr
(a memory address), with the meaning changing depending on whether the
memory is private or shared. Both can potentially be live at the same
time, but only one is used by the guest depending on whether the
memory is shared or private. For pKVM, the memory region is the same,
and whether the underlying physical page is shared or private is
determined by the hypervisor based on the initial configuration of the
VM and also in response to hypercalls from the guest.

For confidential computing usages, this is actually the same. The shared
or private is determined by initial configuration or guest hypercalls.

quoted

So at least from
our side, having a private_fd isn't the best fit, but rather just
having an fd instead of a userspace_addr.

Let me understand this a bit: pKVM basically wants to maintain the
shared and private memory in only one fd, and not use userspace_addr at
all, right? Any blocking for pKVM to use private_fd + userspace_addr
instead?

quoted

Moreover, something which was discussed here before [3], is the
ability to share in-place. For pKVM/arm64, the conversion between
shared and private involves only changes to the stage-2 page tables,
which are controlled by the hypervisor. Android supports this in-place
conversion already, and I think that the cost of copying for many
use-cases that would involve large amounts of data would be big. We
will measure the relative costs in due course, but in the meantime
we’re nervous about adopting a new user ABI which doesn’t appear to
cater for in-place conversion; having just the fd would simplify that
somewhat

I understand there is difficulty to achieve that with the current
private_fd + userspace_addr (they basically in two separate fds), but is
it possible for pKVM to extend this? Brainstorming for example, pKVM can
ignore userspace_addr and only use private_fd to cover both shared and
private memory, or pKVM introduce new KVM memslot flag?

It's not that there's anything blocking pKVM from doing that. It's
that the disconnect of using a memory address for the shared memory,
and a file descriptor for the private memory doesn't really make sense
for pKVM. I see how it makes sense for TDX and the Intel-specific
implementation. It just seems that this is baking in an
implementation-specific aspect as a part of the KVM general api, and
the worry is that this might have some unintended consequences in the
future.

quoted

In the memfd approach, what is the plan for being able to initialize
guest private memory from the host? In my port of this patch series,
I've added an fcntl() command that allows setting INACCESSIBLE after
the memfd has been created. So the memory can be mapped, initialized,
then unmapped. Of course there is no way to enforce that the memory is
unmapped from userspace before being used as private memory, but the
hypervisor will take care of the stage-2 mapping and so a user access
to the private memory would result in a SEGV regardless of the flag

There is discussion on removing MFD_INACCESSIBLE and delaying the
alignment of the flag to the KVM/backing store binding time
(https://lkml.kernel.org/lkml/20220824094149.GA1383966@chaop.bj.intel.com/).

Creating new API like what you are playing with fcntl() also works if it
turns out the MFD_INACCESSIBLE has to be set at the memfd_create time.

That makes sense.

quoted

Now, moving on to implementation-specific issues in this patch series
that I have encountered:

- There are a couple of small issues in porting the patches, some of
which have been mentioned already by others. I will point out the rest
in direct replies to these patches.

Thanks.

quoted

- MEMFILE_F_UNRECLAIMABLE and MEMFILE_F_UNMOVABLE are never set in
this patch series. MFD_INACCESSIBLE only sets
MEMFILE_F_USER_INACCESSIBLE. Is this intentional?

It gets set in kvm_private_mem_register() of patch 13, basically those
flags are expected to be set by architecture code.

quoted

- Nothing in this patch series enforces that MFD_INACCESSIBLE or that
any of the MEMFILE_F_* flags are set for the file descriptor to be
used as a private_fd. Is this also intentional?

With KVM_MEM_PRIVATE memslot flag, the MEMFILE_F_* are enforced by the
architecture code.

Right. I was expecting them to be in the mem_fd, but I see now how
they are being set and enforced in patch 13. This makes a lot of sense
now. Thanks!

quoted

Most of us working on pKVM will be at KVM forum Dublin in September,
so it would be great if we could have a chat (and/or beer!) face to
face sometime during the conference to help us figure out an
upstreamable solution for Android

I would like to, but currently I have no travel plan due to COVID-19 :(
We can have more online discussions anyway.

Of course! We'll continue this online, and hopefully we will get a
chance to meet in person soon.

Cheers,
/fuad

Thanks,
Chao

quoted

Cheers,
/fuad

[1] https://lore.kernel.org/all/20220630135747.26983-1-will@kernel.org/ (local)
[2] https://android-kvm.googlesource.com/linux/+/refs/heads/tabba/fdmem
[3] https://lore.kernel.org/all/YkcTTY4YjQs5BRhE@google.com/ (local)

quoted

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@google.com/
[2] Updated proposal and RFC patch v1:
https://lkml.kernel.org/linux-fsdevel/20211111141352.26311-1-chao.p.peng@linux.intel.com/
[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

--
2.25.1