Hi,
(Here's the motivation for this series, which I realized was missing from
the earlier revisions of this series)
Up till now, guest_memfd supports the entire inode worth of memory being
used as all-shared, or all-private. CoCo VMs may request guest memory to be
converted between private and shared states, and the only way to support
that currently would be to have the userspace VMM provide two sources of
backing memory from completely different areas of physical memory.
pKVM has a use case for in-place sharing: the guest and host may be
cooperating on given data, and pKVM doesn't protect data through
encryption, so copying that given data between different areas of physical
memory as part of conversions would be unnecessary work.
This series also serves as a foundation for guest_memfd huge page
support. Now, guest_memfd only supports PAGE_SIZE pages, so if two sources
of backing memory are used, the userspace VMM could maintain a steady total
memory utilized by punching out the pages that are not used. When huge
pages are available in guest_memfd, even if the backing memory source
supports hole punching within a huge page, punching out pages to maintain
the total memory utilized by a VM would be introducing lots of
fragmentation.
In-place conversion avoids fragmentation by allowing the same physical
memory to be used for both shared and private memory, with guest_memfd
tracks the shared/private status of all the pages at a per-page
granularity.
The central principle, which guest_memfd continues to uphold, is that any
guest-private page will not be mappable to host userspace. All pages will
be mmap()-able in host userspace, but accesses to guest-private pages (as
tracked by guest_memfd) will result in a SIGBUS.
This series introduces a guest_memfd ioctl (not kvm, vm or vcpu, but
guest_memfd ioctl) that allows userspace to set memory
attributes (shared/private) directly through the guest_memfd. This is the
appropriate interface because shared/private-ness is a property of memory
and hence the request should be sent directly to the memory provider -
guest_memfd.
I'm intending RFC (v3) as a basis for discussion of flags/content
modes (name TBD) to allow userspace to request guarantees on how the memory
contents will look like after setting memory attributes. The last 6 patches
implement content mode support. These patches will be reordered, and some
of them could be absorbed into earlier patches, in later revisions.
Here are the discussion points I can think of (please add on):
1. (Might hopefully resolve soon?) Should ZERO be supported on shared to
private conversions? Discussion is at [6].
2. Do we need a CAP for userspace to query the flags/modes supported?
It seems like there won't be anything dynamic about the flags/modes
supported.
The userspace code can check what platform it is running on, and then
decide ZERO or PRESERVE based on the platform:
If the VM is running on TDX, it would want to specify ZERO all the
time. If the VM were running on pKVM it would want to specify PRESERVE
if it wants to enable in-place sharing, and ZERO if it wants to zero the
memory.
If someday TDX supports PRESERVE, then there's room for discovery of
which algorithm to choose when running the guest. Perhaps that's when
the CAP should be introduced?
3. What do people think of the structure of how various content modes are
checked for support or applied? I used overridable weak functions for
architectures that haven't defined support, and defined overrides for
x86 to show how I think it would work. For CoCo platforms, I only
implemented TDX for illustration purposes and might need help with the
other platforms. Should I have used kvm_x86_ops? I tried and found
myself defining lots of boilerplate.
4. enum for ZERO and PRESERVE?
Pros:
* No way to define both ZERO and PRESERVE (make impossible states
unrepresentable)
* e.g. enum kvm_device_type in __u32 type in struct
kvm_create_device
* But maybe someday some modes can be used together?
* Content modes is a defined axis/aspect of setting memory attributes,
having a separate field avoids having different axes of configuration
in one field. e.g. MAP_HUGETLB for mmap() is on a different axis from
MAP_PRIVATE for example
* I just used flags for this RFC since it's the most common
approach.
TODOs:
+ Let architectures override content mode handlers on a per-inode basis
since per-folio overrides means even no-ops, like ZERO on TDX, would
require iterating all the folios.
Also, in RFC v3, TEST_EXPECT_SIGBUS() is updated to assert that the default
signal handler is installed, so that developers get a clear, explicit
failure if/when something goes wrong.
This series is based on kvm/next, and here's the tree for your convenience:
https://github.com/googleprodkernel/linux-cc/commits/guest_memfd-inplace-conversion-v3
Older series:
+ RFCv2 is at [5]
+ RFCv1 is at [4]
+ Previous versions of this feature, part of other series, are available at
[1][2][3].
[1]
https://lore.kernel.org/all/bd163de3118b626d1005aa88e71ef2fb72f0be0f.1726009989.git.ackerley...@google.com/
[2] https://lore.kernel.org/all/[email protected]/
[3]
https://lore.kernel.org/all/b784326e9ccae6a08388f1bf39db70a2204bdc51.1747264138.git.ackerley...@google.com/
[4] https://lore.kernel.org/all/[email protected]/T/
[5] https://lore.kernel.org/all/[email protected]/T/
[6]
https://lore.kernel.org/all/caevnrgfuc+9xcon9yo5nthhrvbccwahpwp9nnm2fvx7qqrc...@mail.gmail.com/
Signed-off-by: Ackerley Tng <[email protected]>
---
Ackerley Tng (25):
KVM: guest_memfd: Update kvm_gmem_populate() to use gmem attributes
KVM: Introduce KVM_SET_MEMORY_ATTRIBUTES2
KVM: guest_memfd: Add support for KVM_SET_MEMORY_ATTRIBUTES2
KVM: guest_memfd: Handle lru_add fbatch refcounts during conversion
safety check
KVM: selftests: Update framework to use KVM_SET_MEMORY_ATTRIBUTES2
KVM: selftests: Test using guest_memfd for guest private memory
KVM: selftests: Test basic single-page conversion flow
KVM: selftests: Test conversion flow when INIT_SHARED
KVM: selftests: Test indexing in guest_memfd
KVM: selftests: Test conversion before allocation
KVM: selftests: Convert with allocated folios in different layouts
KVM: selftests: Test precision of conversion
KVM: selftests: Test that truncation does not change shared/private status
KVM: selftests: Test conversion with elevated page refcount
KVM: selftests: Reset shared memory after hole-punching
KVM: selftests: Provide function to look up guest_memfd details from gpa
KVM: selftests: Make TEST_EXPECT_SIGBUS thread-safe
KVM: selftests: Update private_mem_conversions_test to mmap() guest_memfd
KVM: selftests: Add script to exercise private_mem_conversions_test
KVM: guest_memfd: Introduce default handlers for content modes
KVM: guest_memfd: Apply content modes while setting memory attributes
KVM: x86: Add support for applying content modes
KVM: x86: Support content mode ZERO for TDX
KVM: selftests: Allow flags to be specified in set_memory_attributes
functions
KVM: selftests: Update tests to use flag-enabled library functions
Sean Christopherson (18):
KVM: guest_memfd: Introduce per-gmem attributes, use to guard user
mappings
KVM: Rename KVM_GENERIC_MEMORY_ATTRIBUTES to KVM_VM_MEMORY_ATTRIBUTES
KVM: Enumerate support for PRIVATE memory iff kvm_arch_has_private_mem is
defined
KVM: Stub in ability to disable per-VM memory attribute tracking
KVM: guest_memfd: Wire up kvm_get_memory_attributes() to per-gmem
attributes
KVM: guest_memfd: Enable INIT_SHARED on guest_memfd for x86 Coco VMs
KVM: Move KVM_VM_MEMORY_ATTRIBUTES config definition to x86
KVM: Let userspace disable per-VM mem attributes, enable per-gmem
attributes
KVM: selftests: Create gmem fd before "regular" fd when adding memslot
KVM: selftests: Rename guest_memfd{,_offset} to gmem_{fd,offset}
KVM: selftests: Add support for mmap() on guest_memfd in core library
KVM: selftests: Add selftests global for guest memory attributes
capability
KVM: selftests: Add helpers for calling ioctls on guest_memfd
KVM: selftests: Test that shared/private status is consistent across
processes
KVM: selftests: Provide common function to set memory attributes
KVM: selftests: Check fd/flags provided to mmap() when setting up memslot
KVM: selftests: Update pre-fault test to work with per-guest_memfd
attributes
KVM: selftests: Update private memory exits test work with per-gmem
attributes
Documentation/virt/kvm/api.rst | 112 ++++-
arch/x86/include/asm/kvm_host.h | 2 +-
arch/x86/kvm/Kconfig | 15 +-
arch/x86/kvm/mmu/mmu.c | 4 +-
arch/x86/kvm/x86.c | 86 +++-
include/linux/kvm_host.h | 62 ++-
include/trace/events/kvm.h | 4 +-
include/uapi/linux/kvm.h | 21 +
tools/testing/selftests/kvm/.gitignore | 1 +
tools/testing/selftests/kvm/Makefile.kvm | 1 +
.../selftests/kvm/guest_memfd_conversions_test.c | 496 +++++++++++++++++++++
tools/testing/selftests/kvm/guest_memfd_test.c | 57 ++-
tools/testing/selftests/kvm/include/kvm_util.h | 136 +++++-
tools/testing/selftests/kvm/include/test_util.h | 32 +-
tools/testing/selftests/kvm/lib/kvm_util.c | 130 +++---
tools/testing/selftests/kvm/lib/test_util.c | 7 -
.../testing/selftests/kvm/pre_fault_memory_test.c | 2 +-
.../kvm/x86/private_mem_conversions_test.c | 54 ++-
.../kvm/x86/private_mem_conversions_test.py | 152 +++++++
.../selftests/kvm/x86/private_mem_kvm_exits_test.c | 36 +-
virt/kvm/Kconfig | 3 +-
virt/kvm/guest_memfd.c | 488 +++++++++++++++++++-
virt/kvm/kvm_main.c | 104 ++++-
23 files changed, 1831 insertions(+), 174 deletions(-)
---
base-commit: d2ea4ff1ce50787a98a3900b3fb1636f3620b7cf
change-id: 20260225-gmem-inplace-conversion-bd0dbd39753a
Best regards,
--
Ackerley Tng <[email protected]>
