Linux Kernel Selftest development
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@ 2026-05-29 14:32 George Guo
  2026-05-29 14:32 ` [PATCH v2 1/4] LoongArch: kexec: add KHO support for FDT-based systems George Guo
                   ` (4 more replies)
  0 siblings, 5 replies; 7+ messages in thread
From: George Guo @ 2026-05-29 14:32 UTC (permalink / raw)
  To: Huacai Chen, Mike Rapoport, Pasha Tatashin, Pratyush Yadav,
	Shuah Khan
  Cc: WANG Xuerui, Alexander Graf, loongarch, linux-kernel, kexec,
	linux-mm, linux-kselftest, George Guo

From: George Guo <guodongtai@kylinos.cn>

WARNING: Prefer a maximum 75 chars per line (possible unwrapped commit description?)
#26: 
containing only /chosen with the two KHO properties.  Since DEVICE_TREE_GUID

ERROR: Invalid commit separator - some tools may have problems applying this
#34: 
-------------------------------

total: 2 errors, 1 warnings, 0 lines checked
>
Date: Fri, 29 May 2026 21:54:01 +0800
Subject: [PATCH v2 0/4] LoongArch: add KHO support and selftests

This series adds Kexec Handover (KHO) support for LoongArch and extends
the KHO selftest infrastructure to run on LoongArch under QEMU.

KHO passes metadata (the KHO state FDT and scratch area addresses) to the
second kernel via the FDT /chosen node, using the linux,kho-fdt and
linux,kho-scratch properties that drivers/of/kexec.c:kho_add_chosen()
writes and drivers/of/fdt.c:early_init_dt_check_kho() reads.

KHO support (patches 1-2):

Patch 1 adds KHO support for FDT-based systems (initial_boot_params !=
NULL, e.g. QEMU virt without OVMF).  kho_load_fdt() copies the running
kernel's FDT, appends linux,kho-fdt and linux,kho-scratch to /chosen,
and loads the result as a kexec segment.  machine_kexec() updates the
DEVICE_TREE_GUID entry in the EFI config table to point to this segment
so the second kernel's fdt_setup() can find and parse it.

Patch 2 adds KHO support for ACPI-only systems (initial_boot_params ==
NULL, e.g. LoongArch servers with UEFI or QEMU with OVMF).  Because no
system FDT is available, kho_load_fdt() builds a minimal FDT from
scratch containing only /chosen with the two KHO properties.  Since
DEVICE_TREE_GUID is absent from the EFI config table on ACPI-only
systems, a new extended config table is built with the entry appended
and loaded as a kexec segment; machine_kexec() switches st->tables to
point to it before jumping.  The second kernel's fdt_setup() calls
efi_fdt_pointer() to detect the KHO FDT and passes it to
early_init_dt_check_kho().

Selftest support (patches 3-4):

Patch 3 adds loongarch.conf and extends vmtest.sh to recognise loongarch64
as a build target.  The LoongArch virt machine is FDT-only (no ACPI), so
'earlycon' must appear on the kernel cmdline or the console UART is never
discovered.  PS/2 input devices are also disabled since QEMU's LoongArch
virt machine has no i8042 controller; the fallback port probe hits a page
fault and panics before reaching userspace.

Patch 4 handles QEMU not exiting after kexec on LoongArch.  QEMU provides
no EFI runtime services, so machine_restart() falls through to an infinite
idle loop.  QEMU_NEEDS_KILL=1 in loongarch.conf signals vmtest.sh to run
QEMU in the background, poll the serial output for the test verdict, and
kill QEMU once it appears, so the test completes unattended.

George Guo (4):
  LoongArch: kexec: add KHO support for FDT-based systems
  LoongArch: kexec: add KHO support for ACPI-only systems
  selftests/kho: add LoongArch vmtest support
  selftests/kho: handle QEMU not exiting after kexec on LoongArch

 arch/loongarch/Kconfig                     |   3 +
 arch/loongarch/include/asm/kexec.h         |   7 +
 arch/loongarch/kernel/machine_kexec.c      |  38 +++
 arch/loongarch/kernel/machine_kexec_file.c | 256 +++++++++++++++++++++
 arch/loongarch/kernel/setup.c              |  21 +-
 tools/testing/selftests/kho/loongarch.conf |  13 ++
 tools/testing/selftests/kho/vmtest.sh      |  35 ++-
 7 files changed, 365 insertions(+), 8 deletions(-)
 create mode 100644 tools/testing/selftests/kho/loongarch.conf

-- 
2.25.1


^ permalink raw reply	[flat|nested] 7+ messages in thread
* (no subject)
@ 2021-06-06 19:19 Davidlohr Bueso
  2021-06-07 16:02 ` André Almeida
  0 siblings, 1 reply; 7+ messages in thread
From: Davidlohr Bueso @ 2021-06-06 19:19 UTC (permalink / raw)
  To: Andr� Almeida
  Cc: Thomas Gleixner, Ingo Molnar, Peter Zijlstra, Darren Hart,
	linux-kernel, Steven Rostedt, Sebastian Andrzej Siewior, kernel,
	krisman, pgriffais, z.figura12, joel, malteskarupke, linux-api,
	fweimer, libc-alpha, linux-kselftest, shuah, acme, corbet,
	Peter Oskolkov, Andrey Semashev, mtk.manpages

Bcc:
Subject: Re: [PATCH v4 07/15] docs: locking: futex2: Add documentation
Reply-To:
In-Reply-To: <20210603195924.361327-8-andrealmeid@collabora.com>

On Thu, 03 Jun 2021, Andr� Almeida wrote:

>Add a new documentation file specifying both userspace API and internal
>implementation details of futex2 syscalls.

I think equally important would be to provide a manpage for each new
syscall you are introducing, and keep mkt in the loop as in the past he
extensively documented and improved futex manpages, and overall has a
lot of experience with dealing with kernel interfaces.

Thanks,
Davidlohr

>
>Signed-off-by: André Almeida <andrealmeid@collabora.com>
>---
> Documentation/locking/futex2.rst | 198 +++++++++++++++++++++++++++++++
> Documentation/locking/index.rst  |   1 +
> 2 files changed, 199 insertions(+)
> create mode 100644 Documentation/locking/futex2.rst
>
>diff --git a/Documentation/locking/futex2.rst b/Documentation/locking/futex2.rst
>new file mode 100644
>index 000000000000..2f74d7c97a55
>--- /dev/null
>+++ b/Documentation/locking/futex2.rst
>@@ -0,0 +1,198 @@
>+.. SPDX-License-Identifier: GPL-2.0
>+
>+======
>+futex2
>+======
>+
>+:Author: André Almeida <andrealmeid@collabora.com>
>+
>+futex, or fast user mutex, is a set of syscalls to allow userspace to create
>+performant synchronization mechanisms, such as mutexes, semaphores and
>+conditional variables in userspace. C standard libraries, like glibc, uses it
>+as a means to implement more high level interfaces like pthreads.
>+
>+The interface
>+=============
>+
>+uAPI functions
>+--------------
>+
>+.. kernel-doc:: kernel/futex2.c
>+   :identifiers: sys_futex_wait sys_futex_wake sys_futex_waitv sys_futex_requeue
>+
>+uAPI structures
>+---------------
>+
>+.. kernel-doc:: include/uapi/linux/futex.h
>+
>+The ``flag`` argument
>+---------------------
>+
>+The flag is used to specify the size of the futex word
>+(FUTEX_[8, 16, 32, 64]). It's mandatory to define one, since there's no
>+default size.
>+
>+By default, the timeout uses a monotonic clock, but can be used as a realtime
>+one by using the FUTEX_REALTIME_CLOCK flag.
>+
>+By default, futexes are of the private type, that means that this user address
>+will be accessed by threads that share the same memory region. This allows for
>+some internal optimizations, so they are faster. However, if the address needs
>+to be shared with different processes (like using ``mmap()`` or ``shm()``), they
>+need to be defined as shared and the flag FUTEX_SHARED_FLAG is used to set that.
>+
>+By default, the operation has no NUMA-awareness, meaning that the user can't
>+choose the memory node where the kernel side futex data will be stored. The
>+user can choose the node where it wants to operate by setting the
>+FUTEX_NUMA_FLAG and using the following structure (where X can be 8, 16, 32 or
>+64)::
>+
>+ struct futexX_numa {
>+         __uX value;
>+         __sX hint;
>+ };
>+
>+This structure should be passed at the ``void *uaddr`` of futex functions. The
>+address of the structure will be used to be waited on/waken on, and the
>+``value`` will be compared to ``val`` as usual. The ``hint`` member is used to
>+define which node the futex will use. When waiting, the futex will be
>+registered on a kernel-side table stored on that node; when waking, the futex
>+will be searched for on that given table. That means that there's no redundancy
>+between tables, and the wrong ``hint`` value will lead to undesired behavior.
>+Userspace is responsible for dealing with node migrations issues that may
>+occur. ``hint`` can range from [0, MAX_NUMA_NODES), for specifying a node, or
>+-1, to use the same node the current process is using.
>+
>+When not using FUTEX_NUMA_FLAG on a NUMA system, the futex will be stored on a
>+global table on allocated on the first node.
>+
>+The ``timo`` argument
>+---------------------
>+
>+As per the Y2038 work done in the kernel, new interfaces shouldn't add timeout
>+options known to be buggy. Given that, ``timo`` should be a 64-bit timeout at
>+all platforms, using an absolute timeout value.
>+
>+Implementation
>+==============
>+
>+The internal implementation follows a similar design to the original futex.
>+Given that we want to replicate the same external behavior of current futex,
>+this should be somewhat expected.
>+
>+Waiting
>+-------
>+
>+For the wait operations, they are all treated as if you want to wait on N
>+futexes, so the path for futex_wait and futex_waitv is the basically the same.
>+For both syscalls, the first step is to prepare an internal list for the list
>+of futexes to wait for (using struct futexv_head). For futex_wait() calls, this
>+list will have a single object.
>+
>+We have a hash table, where waiters register themselves before sleeping. Then
>+the wake function checks this table looking for waiters at uaddr.  The hash
>+bucket to be used is determined by a struct futex_key, that stores information
>+to uniquely identify an address from a given process. Given the huge address
>+space, there'll be hash collisions, so we store information to be later used on
>+collision treatment.
>+
>+First, for every futex we want to wait on, we check if (``*uaddr == val``).
>+This check is done holding the bucket lock, so we are correctly serialized with
>+any futex_wake() calls. If any waiter fails the check above, we dequeue all
>+futexes. The check (``*uaddr == val``) can fail for two reasons:
>+
>+- The values are different, and we return -EAGAIN. However, if while
>+  dequeueing we found that some futexes were awakened, we prioritize this
>+  and return success.
>+
>+- When trying to access the user address, we do so with page faults
>+  disabled because we are holding a bucket's spin lock (and can't sleep
>+  while holding a spin lock). If there's an error, it might be a page
>+  fault, or an invalid address. We release the lock, dequeue everyone
>+  (because it's illegal to sleep while there are futexes enqueued, we
>+  could lose wakeups) and try again with page fault enabled. If we
>+  succeed, this means that the address is valid, but we need to do
>+  all the work again. For serialization reasons, we need to have the
>+  spin lock when getting the user value. Additionally, for shared
>+  futexes, we also need to recalculate the hash, since the underlying
>+  mapping mechanisms could have changed when dealing with page fault.
>+  If, even with page fault enabled, we can't access the address, it
>+  means it's an invalid user address, and we return -EFAULT. For this
>+  case, we prioritize the error, even if some futexes were awaken.
>+
>+If the check is OK, they are enqueued on a linked list in our bucket, and
>+proceed to the next one. If all waiters succeed, we put the thread to sleep
>+until a futex_wake() call, timeout expires or we get a signal. After waking up,
>+we dequeue everyone, and check if some futex was awakened. This dequeue is done
>+by iteratively walking at each element of struct futex_head list.
>+
>+All enqueuing/dequeuing operations requires to hold the bucket lock, to avoid
>+racing while modifying the list.
>+
>+Waking
>+------
>+
>+We get the bucket that's storing the waiters at uaddr, and wake the required
>+number of waiters, checking for hash collision.
>+
>+There's an optimization that makes futex_wake() not take the bucket lock if
>+there's no one to be woken on that bucket. It checks an atomic counter that each
>+bucket has, if it says 0, then the syscall exits. In order for this to work, the
>+waiter thread increases it before taking the lock, so the wake thread will
>+correctly see that there's someone waiting and will continue the path to take
>+the bucket lock. To get the correct serialization, the waiter issues a memory
>+barrier after increasing the bucket counter and the waker issues a memory
>+barrier before checking it.
>+
>+Requeuing
>+---------
>+
>+The requeue path first checks for each struct futex_requeue and their flags.
>+Then, it will compare the expected value with the one at uaddr1::uaddr.
>+Following the same serialization explained at Waking_, we increase the atomic
>+counter for the bucket of uaddr2 before taking the lock. We need to have both
>+buckets locks at same time so we don't race with other futex operation. To
>+ensure the locks are taken in the same order for all threads (and thus avoiding
>+deadlocks), every requeue operation takes the "smaller" bucket first, when
>+comparing both addresses.
>+
>+If the compare with user value succeeds, we proceed by waking ``nr_wake``
>+futexes, and then requeuing ``nr_requeue`` from bucket of uaddr1 to the uaddr2.
>+This consists in a simple list deletion/addition and replacing the old futex key
>+with the new one.
>+
>+Futex keys
>+----------
>+
>+There are two types of futexes: private and shared ones. The private are futexes
>+meant to be used by threads that share the same memory space, are easier to be
>+uniquely identified and thus can have some performance optimization. The
>+elements for identifying one are: the start address of the page where the
>+address is, the address offset within the page and the current->mm pointer.
>+
>+Now, for uniquely identifying a shared futex:
>+
>+- If the page containing the user address is an anonymous page, we can
>+  just use the same data used for private futexes (the start address of
>+  the page, the address offset within the page and the current->mm
>+  pointer); that will be enough for uniquely identifying such futex. We
>+  also set one bit at the key to differentiate if a private futex is
>+  used on the same address (mixing shared and private calls does not
>+  work).
>+
>+- If the page is file-backed, current->mm maybe isn't the same one for
>+  every user of this futex, so we need to use other data: the
>+  page->index, a UUID for the struct inode and the offset within the
>+  page.
>+
>+Note that members of futex_key don't have any particular meaning after they
>+are part of the struct - they are just bytes to identify a futex.  Given that,
>+we don't need to use a particular name or type that matches the original data,
>+we only need to care about the bitsize of each component and make both private
>+and shared fit in the same memory space.
>+
>+Source code documentation
>+=========================
>+
>+.. kernel-doc:: kernel/futex2.c
>+   :no-identifiers: sys_futex_wait sys_futex_wake sys_futex_waitv sys_futex_requeue
>diff --git a/Documentation/locking/index.rst b/Documentation/locking/index.rst
>index 7003bd5aeff4..9bf03c7fa1ec 100644
>--- a/Documentation/locking/index.rst
>+++ b/Documentation/locking/index.rst
>@@ -24,6 +24,7 @@ locking
>     percpu-rw-semaphore
>     robust-futexes
>     robust-futex-ABI
>+    futex2
>
> .. only::  subproject and html
>
>--
>2.31.1
>

^ permalink raw reply	[flat|nested] 7+ messages in thread
end of thread, other threads:[~2026-05-31  9:28 UTC | newest]

Thread overview: 7+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2026-05-29 14:32 George Guo
2026-05-29 14:32 ` [PATCH v2 1/4] LoongArch: kexec: add KHO support for FDT-based systems George Guo
2026-05-29 14:32 ` [PATCH v2 2/4] LoongArch: kexec: add KHO support for ACPI-only systems George Guo
2026-05-29 14:32 ` [PATCH v2 3/4] selftests/kho: add LoongArch vmtest support George Guo
2026-05-29 14:32 ` [PATCH v2 4/4] selftests/kho: handle QEMU not exiting after kexec on LoongArch George Guo
2026-05-31  9:28 ` Mike Rapoport
  -- strict thread matches above, loose matches on Subject: below --
2021-06-06 19:19 Davidlohr Bueso
2021-06-07 16:02 ` André Almeida

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