[PATCH v6 00/24] Tracefs support for pKVM

Vincent Donnefort
2025-08-21T08:14:20Z

The growing set of features supported by the hypervisor in protected
mode necessitates debugging and profiling tools. Tracefs is the
ideal candidate for this task:

* It is simple to use and to script.

* It is supported by various tools, from the trace-cmd CLI to the
Android web-based perfetto.

* The ring-buffer, where are stored trace events consists of linked
pages, making it an ideal structure for sharing between kernel and
hypervisor.

This series first introduces a new generic way of creating remote events and
remote buffers. Then it adds support to the pKVM hypervisor.

1. ring-buffer
--------------

To setup the per-cpu ring-buffers, a new interface is created:

ring_buffer_remote: Describes what the kernel needs to know about the
remote writer, that is, the set of pages forming the
ring-buffer and a callback for the reader/head
swapping (enables consuming read)

ring_buffer_remote(): Creates a read-only ring-buffer from a
ring_buffer_remote.

To keep the internals of `struct ring_buffer` in sync with the remote,
the meta-page is used. It was originally introduced to enable user-space
mapping of the ring-buffer [1]. In this case, the kernel is not the
producer anymore but the reader. The function to read that meta-page is:

ring_buffer_poll_remote():
Update `struct ring_buffer` based on the remote
meta-page. Wake-up readers if necessary.

The kernel has to poll the meta-page to be notified of newly written
events.

2. Tracefs
----------

This series introduce a new trace_remote that does the link between
tracefs and the remote ring-buffer.

The interface is found in the remotes/ directory at the root of the
tracefs mount point. Each remote is like an instance and you'll find
there a subset of the regular Tracefs user-space interface:

remotes/test/
buffer_size_kb
trace_clock
trace_pipe
trace
per_cpu/
cpuX/
trace
trace_pipe
events/

test/
selftest/
enable
id

Behind the scenes, kernel/trace/trace_remote.c creates this tracefs
hierarchy without relying on kernel/trace/trace.c. This is due to
fundamental differences:

* Remote tracing doesn't support trace_array's system-specific
features (snapshots, tracers, etc.).

* Logged event formats differ (e.g., no PID for remote events).

* Buffer operations require specific remote interactions.

3. Simple Ring-Buffer
---------------------

As the current ring-buffer.c implementation has too many dependencies to
be used directly by the pKVM hypervisor. A new simple implementation is
created and can be found in kernel/trace/simple-ring-buffer.c.

This implementation is write-only and is used by both the pKVM
hypervisor and a trace_remote test module.

4. Events
---------

A new REMOTE_EVENT() macro is added to simplify the creation of events
on the kernel side. As remote tracing buffer are read only, only the
event structure and a way of printing must be declared. The prototype of
the macro is very similar to the well-known TRACE_EVENT()

REMOTE_EVENT(my_event, id,
RE_STRUCT(
re_field(u64, foobar)
),
RE_PRINTK("foobar=%lld", __entry->foobar)
)
)

5. pKVM
-------

The pKVM support simply creates a "hypervisor" trace_remote on the
kernel side and inherits from simple-ring-buffer.c on the hypervisor
side.

A new event macro is created HYP_EVENT() that is under the hood re-using
REMOTE_EVENT() (defined in the previous paragaph) as well as generate
hypervisor specific struct and trace_<event>() functions.

5. Limitations:
---------------

Non-consuming reading of the buffer isn't supported (i.e. cat trace ->
-EPERM) due to current the lack of support in the ring-buffer meta-page.

[1] https://tracingsummit.org/ts/2022/hypervisortracing/
[2] https://lore.kernel.org/all/20240510140435.3550353-1-vdonnefort@google.com/

Changes since v5 (https://lore.kernel.org/all/20250516134031.661124-1-vdonnefort@google.com/)

- Add tishift lib to the hyp (Aneesh)
- Rebase on 6.17-rc2

Changes since v4 (https://lore.kernel.org/all/20250506164820.515876-1-vdonnefort@google.com/)

- Extend meta-page with pages_touched and pages_lost
- Create ring_buffer_types.h
- Fix simple_ring_buffer build for 32-bits arch and x86
- Try unload buffer on reset (+ test)
- Minor renaming and comments

Changes since v3 (https://lore.kernel.org/all/20250224121353.98697-1-vdonnefort@google.com/)

- Move tracefs support from kvm/hyp_trace.c into a generic trace_remote.c.
- Move ring-buffer implementation from nvhe/trace.c into a generic
simple-ring-buffer.c
- Rebase on 6.15-rc4.

Changes since v2 (https://lore.kernel.org/all/20250108114536.627715-1-vdonnefort@google.com/)

- Fix ring-buffer remote reset
- Fix fast-forward in rb_page_desc()
- Refactor nvhe/trace.c
- struct hyp_buffer_page more compact
- Add a struct_len to trace_page_desc
- Extend reset testing
- Rebase on 6.14-rc3

Changes since v1 (https://lore.kernel.org/all/20240911093029.3279154-1-vdonnefort@google.com/)

- Add 128-bits mult fallback in the unlikely event of an overflow. (John)
- Fix ELF section sort.
- __always_inline trace_* event macros.
- Fix events/<event>/enable permissions.
- Rename ring-buffer "writer" to "remote".
- Rename CONFIG_PROTECTED_NVHE_TESTING to PKVM_SELFTEST to align with
Quentin's upcoming selftest
- Rebase on 6.13-rc3.

Changes since RFC (https://lore.kernel.org/all/20240805173234.3542917-1-vdonnefort@google.com/)

- hypervisor trace clock:
- mult/shift computed in hyp_trace.c. (John)
- Update clock when it deviates from kernel boot clock. (John)
- Add trace_clock file.
- Separate patch for better readability.
- Add a proper reset interface which does not need to teardown the
tracing buffers. (Steven)
- Return -EPERM on trace access. (Steven)
- Add per-cpu trace file.
- Automatically teardown and free the tracing buffer when it is empty,
without readers and not currently tracing.
- Show in buffer_size_kb if the buffer is loaded in the hypervisor or
not.
- Extend tests to cover reset and unload.
- CC timekeeping folks on relevant patches (Marc)

Vincent Donnefort (24):
ring-buffer: Add page statistics to the meta-page
ring-buffer: Introduce ring-buffer remotes
tracing: Introduce trace remotes
tracing: Add reset to trace remotes
tracing: Add init callback to trace remotes
tracing: Add events to trace remotes
tracing: Add events/ root files to trace remotes
tracing: Add helpers to create trace remote events
ring-buffer: Export buffer_data_page and macros
tracing: Introduce simple_ring_buffer
tracing: Add a trace remote module for testing
tracing: selftests: Add trace remote tests
tracing: load/unload page callbacks for simple_ring_buffer
tracing: Check for undefined symbols in simple_ring_buffer
KVM: arm64: Support unaligned fixmap in the pKVM hyp
KVM: arm64: Add clock support for the pKVM hyp
KVM: arm64: Add tracing capability for the pKVM hyp
KVM: arm64: Add trace remote for the pKVM hyp
KVM: arm64: Sync boot clock with the pKVM hyp
KVM: arm64: Add trace reset to the pKVM hyp
KVM: arm64: Add event support to the pKVM hyp and trace remote
KVM: arm64: Add hyp_enter/hyp_exit events to pKVM hyp
KVM: arm64: Add selftest event support to pKVM hyp
tracing: selftests: Add pKVM trace remote tests

arch/arm64/include/asm/kvm_asm.h | 8 +
arch/arm64/include/asm/kvm_define_hypevents.h | 21 +
arch/arm64/include/asm/kvm_hyp.h | 1 -
arch/arm64/include/asm/kvm_hypevents.h | 41 +
arch/arm64/include/asm/kvm_hyptrace.h | 26 +
arch/arm64/kernel/image-vars.h | 4 +
arch/arm64/kernel/vmlinux.lds.S | 18 +
arch/arm64/kvm/Kconfig | 18 +
arch/arm64/kvm/Makefile | 2 +
arch/arm64/kvm/arm.c | 4 +
arch/arm64/kvm/hyp/include/nvhe/arm-smccc.h | 13 +
arch/arm64/kvm/hyp/include/nvhe/clock.h | 16 +
.../kvm/hyp/include/nvhe/define_events.h | 21 +
arch/arm64/kvm/hyp/include/nvhe/trace.h | 60 ++
arch/arm64/kvm/hyp/nvhe/Makefile | 3 +-
arch/arm64/kvm/hyp/nvhe/clock.c | 65 ++
arch/arm64/kvm/hyp/nvhe/events.c | 36 +
arch/arm64/kvm/hyp/nvhe/ffa.c | 2 +-
arch/arm64/kvm/hyp/nvhe/hyp-main.c | 86 ++
arch/arm64/kvm/hyp/nvhe/hyp.lds.S | 6 +
arch/arm64/kvm/hyp/nvhe/mm.c | 2 +-
arch/arm64/kvm/hyp/nvhe/psci-relay.c | 5 +-
arch/arm64/kvm/hyp/nvhe/switch.c | 5 +-
arch/arm64/kvm/hyp/nvhe/trace.c | 286 ++++++
arch/arm64/kvm/hyp_trace.c | 385 ++++++++
arch/arm64/kvm/hyp_trace.h | 11 +
include/linux/ring_buffer.h | 58 ++
include/linux/ring_buffer_types.h | 41 +
include/linux/simple_ring_buffer.h | 58 ++
include/linux/trace_remote.h | 27 +
include/linux/trace_remote_event.h | 33 +
include/trace/define_remote_events.h | 73 ++
include/uapi/linux/trace_mmap.h | 8 +-
kernel/trace/Kconfig | 14 +
kernel/trace/Makefile | 20 +
kernel/trace/remote_test.c | 263 +++++
kernel/trace/remote_test_events.h | 10 +
kernel/trace/ring_buffer.c | 259 ++++-
kernel/trace/simple_ring_buffer.c | 413 ++++++++
kernel/trace/trace.c | 2 +-
kernel/trace/trace.h | 6 +
kernel/trace/trace_remote.c | 923 ++++++++++++++++++
.../ftrace/test.d/remotes/buffer_size.tc | 24 +
.../selftests/ftrace/test.d/remotes/functions | 33 +
.../ftrace/test.d/remotes/pkvm/buffer_size.tc | 10 +
.../ftrace/test.d/remotes/pkvm/reset.tc | 10 +
.../ftrace/test.d/remotes/pkvm/trace_pipe.tc | 10 +
.../ftrace/test.d/remotes/pkvm/unloading.tc | 10 +
.../selftests/ftrace/test.d/remotes/reset.tc | 105 ++
.../ftrace/test.d/remotes/trace_pipe.tc | 57 ++
.../ftrace/test.d/remotes/unloading.tc | 40 +
51 files changed, 3598 insertions(+), 54 deletions(-)
create mode 100644 arch/arm64/include/asm/kvm_define_hypevents.h
create mode 100644 arch/arm64/include/asm/kvm_hypevents.h
create mode 100644 arch/arm64/include/asm/kvm_hyptrace.h
create mode 100644 arch/arm64/kvm/hyp/include/nvhe/arm-smccc.h
create mode 100644 arch/arm64/kvm/hyp/include/nvhe/clock.h
create mode 100644 arch/arm64/kvm/hyp/include/nvhe/define_events.h
create mode 100644 arch/arm64/kvm/hyp/include/nvhe/trace.h
create mode 100644 arch/arm64/kvm/hyp/nvhe/clock.c
create mode 100644 arch/arm64/kvm/hyp/nvhe/events.c
create mode 100644 arch/arm64/kvm/hyp/nvhe/trace.c
create mode 100644 arch/arm64/kvm/hyp_trace.c
create mode 100644 arch/arm64/kvm/hyp_trace.h
create mode 100644 include/linux/ring_buffer_types.h
create mode 100644 include/linux/simple_ring_buffer.h
create mode 100644 include/linux/trace_remote.h
create mode 100644 include/linux/trace_remote_event.h
create mode 100644 include/trace/define_remote_events.h
create mode 100644 kernel/trace/remote_test.c
create mode 100644 kernel/trace/remote_test_events.h
create mode 100644 kernel/trace/simple_ring_buffer.c
create mode 100644 kernel/trace/trace_remote.c
create mode 100644 tools/testing/selftests/ftrace/test.d/remotes/buffer_size.tc
create mode 100644 tools/testing/selftests/ftrace/test.d/remotes/functions
create mode 100644 tools/testing/selftests/ftrace/test.d/remotes/pkvm/buffer_size.tc
create mode 100644 tools/testing/selftests/ftrace/test.d/remotes/pkvm/reset.tc
create mode 100644 tools/testing/selftests/ftrace/test.d/remotes/pkvm/trace_pipe.tc
create mode 100644 tools/testing/selftests/ftrace/test.d/remotes/pkvm/unloading.tc
create mode 100644 tools/testing/selftests/ftrace/test.d/remotes/reset.tc
create mode 100644 tools/testing/selftests/ftrace/test.d/remotes/trace_pipe.tc
create mode 100644 tools/testing/selftests/ftrace/test.d/remotes/unloading.tc

base-commit: c17b750b3ad9f45f2b6f7e6f7f4679844244f0b9
--
2.51.0.rc2.233.g662b1ed5c5-goog