[PATCH v5 2/7] ptp: vmclock: support device notifications

Chalios, Babis
2026-01-07T13:27:10Z

Add optional support for device notifications in VMClock. When
supported, the hypervisor will send a device notification every time it
updates the seq_count to a new even value.

Moreover, add support for poll() in VMClock as a means to propagate this
notification to user space. poll() will return a POLLIN event to
listeners every time seq_count changes to a value different than the one
last seen (since open() or last read()/pread()). This means that when
poll() returns a POLLIN event, listeners need to use read() to observe
what has changed and update the reader's view of seq_count. In other
words, after a poll() returned, all subsequent calls to poll() will
immediately return with a POLLIN event until the listener calls read().

The device advertises support for the notification mechanism by setting
flag VMCLOCK_FLAG_NOTIFICATION_PRESENT in vmclock_abi flags field. If
the flag is not present the driver won't setup the ACPI notification
handler and poll() will always immediately return POLLHUP.

Signed-off-by: Babis Chalios <bchalios@amazon.es>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
---
drivers/ptp/ptp_vmclock.c | 125 +++++++++++++++++++++++++++++--
include/uapi/linux/vmclock-abi.h | 5 ++
2 files changed, 123 insertions(+), 7 deletions(-)

diff --git a/drivers/ptp/ptp_vmclock.c b/drivers/ptp/ptp_vmclock.c
index b3a83b03d9c1..38b2bacb755e 100644
--- a/drivers/ptp/ptp_vmclock.c
+++ b/drivers/ptp/ptp_vmclock.c
@@ -5,6 +5,9 @@
* Copyright © 2024 Amazon.com, Inc. or its affiliates.
*/

+#include "linux/poll.h"
+#include "linux/types.h"
+#include "linux/wait.h"
#include <linux/acpi.h>
#include <linux/device.h>
#include <linux/err.h>
@@ -39,6 +42,7 @@ struct vmclock_state {
struct resource res;
struct vmclock_abi *clk;
struct miscdevice miscdev;
+ wait_queue_head_t disrupt_wait;
struct ptp_clock_info ptp_clock_info;
struct ptp_clock *ptp_clock;
enum clocksource_ids cs_id, sys_cs_id;
@@ -357,10 +361,15 @@ static struct ptp_clock *vmclock_ptp_register(struct device *dev,
return ptp_clock_register(&st->ptp_clock_info, dev);
}

+struct vmclock_file_state {
+ struct vmclock_state *st;
+ atomic_t seq;
+};
+
static int vmclock_miscdev_mmap(struct file *fp, struct vm_area_struct *vma)
{
- struct vmclock_state *st = container_of(fp->private_data,
- struct vmclock_state, miscdev);
+ struct vmclock_file_state *fst = fp->private_data;
+ struct vmclock_state *st = fst->st;

if ((vma->vm_flags & (VM_READ|VM_WRITE)) != VM_READ)
return -EROFS;
@@ -379,11 +388,11 @@ static int vmclock_miscdev_mmap(struct file *fp, struct vm_area_struct *vma)
static ssize_t vmclock_miscdev_read(struct file *fp, char __user *buf,
size_t count, loff_t *ppos)
{
- struct vmclock_state *st = container_of(fp->private_data,
- struct vmclock_state, miscdev);
ktime_t deadline = ktime_add(ktime_get(), VMCLOCK_MAX_WAIT);
+ struct vmclock_file_state *fst = fp->private_data;
+ struct vmclock_state *st = fst->st;
+ uint32_t seq, old_seq;
size_t max_count;
- uint32_t seq;

if (*ppos >= PAGE_SIZE)
return 0;
@@ -392,6 +401,7 @@ static ssize_t vmclock_miscdev_read(struct file *fp, char __user *buf,
if (count > max_count)
count = max_count;

+ old_seq = atomic_read(&fst->seq);
while (1) {
seq = le32_to_cpu(st->clk->seq_count) & ~1U;
/* Pairs with hypervisor wmb */
@@ -402,8 +412,16 @@ static ssize_t vmclock_miscdev_read(struct file *fp, char __user *buf,

/* Pairs with hypervisor wmb */
virt_rmb();
- if (seq == le32_to_cpu(st->clk->seq_count))
- break;
+ if (seq == le32_to_cpu(st->clk->seq_count)) {
+ /*
+ * Either we updated fst->seq to seq (the latest version we observed)
+ * or someone else did (old_seq == seq), so we can break.
+ */
+ if (atomic_try_cmpxchg(&fst->seq, &old_seq, seq) ||
+ old_seq == seq) {
+ break;
+ }
+ }

if (ktime_after(ktime_get(), deadline))
return -ETIMEDOUT;
@@ -413,10 +431,58 @@ static ssize_t vmclock_miscdev_read(struct file *fp, char __user *buf,
return count;
}

+static __poll_t vmclock_miscdev_poll(struct file *fp, poll_table *wait)
+{
+ struct vmclock_file_state *fst = fp->private_data;
+ struct vmclock_state *st = fst->st;
+ uint32_t seq;
+
+ /*
+ * Hypervisor will not send us any notifications, so fail immediately
+ * to avoid having caller sleeping for ever.
+ */
+ if (!(le64_to_cpu(st->clk->flags) & VMCLOCK_FLAG_NOTIFICATION_PRESENT))
+ return POLLHUP;
+
+ poll_wait(fp, &st->disrupt_wait, wait);
+
+ seq = le32_to_cpu(st->clk->seq_count);
+ if (atomic_read(&fst->seq) != seq)
+ return POLLIN | POLLRDNORM;
+
+ return 0;
+}
+
+static int vmclock_miscdev_open(struct inode *inode, struct file *fp)
+{
+ struct vmclock_state *st = container_of(fp->private_data,
+ struct vmclock_state, miscdev);
+ struct vmclock_file_state *fst = kzalloc(sizeof(*fst), GFP_KERNEL);
+
+ if (!fst)
+ return -ENOMEM;
+
+ fst->st = st;
+ atomic_set(&fst->seq, 0);
+
+ fp->private_data = fst;
+
+ return 0;
+}
+
+static int vmclock_miscdev_release(struct inode *inode, struct file *fp)
+{
+ kfree(fp->private_data);
+ return 0;
+}
+
static const struct file_operations vmclock_miscdev_fops = {
.owner = THIS_MODULE,
+ .open = vmclock_miscdev_open,
+ .release = vmclock_miscdev_release,
.mmap = vmclock_miscdev_mmap,
.read = vmclock_miscdev_read,
+ .poll = vmclock_miscdev_poll,
};

/* module operations */
@@ -459,6 +525,44 @@ static acpi_status vmclock_acpi_resources(struct acpi_resource *ares, void *data
return AE_ERROR;
}

+static void
+vmclock_acpi_notification_handler(acpi_handle __always_unused handle,
+ u32 __always_unused event, void *dev)
+{
+ struct device *device = dev;
+ struct vmclock_state *st = device->driver_data;
+
+ wake_up_interruptible(&st->disrupt_wait);
+}
+
+static int vmclock_setup_notification(struct device *dev, struct vmclock_state *st)
+{
+ struct acpi_device *adev = ACPI_COMPANION(dev);
+ acpi_status status;
+
+ /*
+ * This should never happen as this function is only called when
+ * has_acpi_companion(dev) is true, but the logic is sufficiently
+ * complex that Coverity can't see the tautology.
+ */
+ if (!adev)
+ return -ENODEV;
+
+ /* The device does not support notifications. Nothing else to do */
+ if (!(le64_to_cpu(st->clk->flags) & VMCLOCK_FLAG_NOTIFICATION_PRESENT))
+ return 0;
+
+ status = acpi_install_notify_handler(adev->handle, ACPI_DEVICE_NOTIFY,
+ vmclock_acpi_notification_handler,
+ dev);
+ if (ACPI_FAILURE(status)) {
+ dev_err(dev, "failed to install notification handler");
+ return -ENODEV;
+ }
+
+ return 0;
+}
+
static int vmclock_probe_acpi(struct device *dev, struct vmclock_state *st)
{
struct acpi_device *adev = ACPI_COMPANION(dev);
@@ -549,6 +653,11 @@ static int vmclock_probe(struct platform_device *pdev)
if (ret)
return ret;

+ init_waitqueue_head(&st->disrupt_wait);
+ ret = vmclock_setup_notification(dev, st);
+ if (ret)
+ return ret;
+
/*
* If the structure is big enough, it can be mapped to userspace.
* Theoretically a guest OS even using larger pages could still
@@ -581,6 +690,8 @@ static int vmclock_probe(struct platform_device *pdev)
return -ENODEV;
}

+ dev->driver_data = st;
+
dev_info(dev, "%s: registered %s%s%s\n", st->name,
st->miscdev.minor ? "miscdev" : "",
(st->miscdev.minor && st->ptp_clock) ? ", " : "",
diff --git a/include/uapi/linux/vmclock-abi.h b/include/uapi/linux/vmclock-abi.h
index 937fe00e4f33..d320623b0118 100644
--- a/include/uapi/linux/vmclock-abi.h
+++ b/include/uapi/linux/vmclock-abi.h
@@ -121,6 +121,11 @@ struct vmclock_abi {
* loaded from some save state (restored from a snapshot).
*/
#define VMCLOCK_FLAG_VM_GEN_COUNTER_PRESENT (1 << 8)
+ /*
+ * If the NOTIFICATION_PRESENT flag is set, the hypervisor will send
+ * a notification every time it updates seq_count to a new even number.
+ */
+#define VMCLOCK_FLAG_NOTIFICATION_PRESENT (1 << 9)

__u8 pad[2];
__u8 clock_status;
--
2.34.1