Re: [PATCH v3 07/24] vfio/pci: Preserve vfio-pci device files across Live Update

Samiullah Khawaja
2026-03-27T23:40:02Z

On Mon, Mar 23, 2026 at 11:57:59PM +0000, David Matlack wrote:
>From: Vipin Sharma <vipinsh@google.com>
>
>Implement the live update file handler callbacks to preserve a vfio-pci
>device across a Live Update. Subsequent commits will enable userspace to
>then retrieve this file after the Live Update.
>
>Live Update support is scoped only to cdev files (i.e. not
>VFIO_GROUP_GET_DEVICE_FD files).
>
>State about each device is serialized into a new ABI struct
>vfio_pci_core_device_ser. The contents of this struct are preserved
>across the Live Update to the next kernel using a combination of
>Kexec-Handover (KHO) to preserve the page(s) holding the struct and the
>Live Update Orchestrator (LUO) to preserve the physical address of the
>struct.
>
>For now the only contents of struct vfio_pci_core_device_ser the
>device's PCI segment number and BDF, so that the device can be uniquely
>identified after the Live Update.
>
>Require that userspace disables interrupts on the device prior to
>freeze() so that the device does not send any interrupts until new
>interrupt handlers have been set up by the next kernel.
>
>Reset the device and restore its state in the freeze() callback. This
>ensures the device can be received by the next kernel in a consistent
>state. Eventually this will be dropped and the device can be preserved
>across in a running state, but that requires further work in VFIO and
>the core PCI layer.
>
>Note that LUO holds a reference to this file when it is preserved. So
>VFIO is guaranteed that vfio_df_device_last_close() will not be called
>on this device no matter what userspace does.

LUO session holding the reference is also in userspace. Just to be
accurate, maybe say something like that the LUO will hold a reference
until the FD is preserved and vfio_df_device_last_close() will not be
called until file is unpreserved.
>
>Signed-off-by: Vipin Sharma <vipinsh@google.com>
>Co-developed-by: David Matlack <dmatlack@google.com>
>Signed-off-by: David Matlack <dmatlack@google.com>
>---
> drivers/vfio/pci/vfio_pci.c | 2 +-
> drivers/vfio/pci/vfio_pci_core.c | 57 +++++----
> drivers/vfio/pci/vfio_pci_liveupdate.c | 156 ++++++++++++++++++++++++-
> drivers/vfio/pci/vfio_pci_priv.h | 4 +
> drivers/vfio/vfio_main.c | 3 +-
> include/linux/kho/abi/vfio_pci.h | 15 +++
> include/linux/vfio.h | 2 +
> 7 files changed, 213 insertions(+), 26 deletions(-)
>
>diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
>index 41dcbe4ace67..351480d13f6e 100644
>--- a/drivers/vfio/pci/vfio_pci.c
>+++ b/drivers/vfio/pci/vfio_pci.c
>@@ -125,7 +125,7 @@ static int vfio_pci_open_device(struct vfio_device *core_vdev)
> return 0;
> }
>
>-static const struct vfio_device_ops vfio_pci_ops = {
>+const struct vfio_device_ops vfio_pci_ops = {
> .name = "vfio-pci",
> .init = vfio_pci_core_init_dev,
> .release = vfio_pci_core_release_dev,
>diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
>index d43745fe4c84..81f941323641 100644
>--- a/drivers/vfio/pci/vfio_pci_core.c
>+++ b/drivers/vfio/pci/vfio_pci_core.c
>@@ -585,9 +585,42 @@ int vfio_pci_core_enable(struct vfio_pci_core_device *vdev)
> }
> EXPORT_SYMBOL_GPL(vfio_pci_core_enable);
>
>+void vfio_pci_core_try_reset(struct vfio_pci_core_device *vdev)
>+{
>+ struct pci_dev *pdev = vdev->pdev;
>+ struct pci_dev *bridge = pci_upstream_bridge(pdev);
>+
>+ lockdep_assert_held(&vdev->vdev.dev_set->lock);
>+
>+ if (!vdev->reset_works)
>+ return;
>+
>+ /*
>+ * Try to get the locks ourselves to prevent a deadlock. The
>+ * success of this is dependent on being able to lock the device,
>+ * which is not always possible.
>+ *
>+ * We cannot use the "try" reset interface here, since that will
>+ * overwrite the previously restored configuration information.
>+ */
>+ if (bridge && !pci_dev_trylock(bridge))
>+ return;
>+
>+ if (!pci_dev_trylock(pdev))
>+ goto out;
>+
>+ if (!__pci_reset_function_locked(pdev))
>+ vdev->needs_reset = false;
>+
>+ pci_dev_unlock(pdev);
>+out:
>+ if (bridge)
>+ pci_dev_unlock(bridge);
>+}
>+EXPORT_SYMBOL_GPL(vfio_pci_core_try_reset);
>+
> void vfio_pci_core_disable(struct vfio_pci_core_device *vdev)
> {
>- struct pci_dev *bridge;
> struct pci_dev *pdev = vdev->pdev;
> struct vfio_pci_dummy_resource *dummy_res, *tmp;
> struct vfio_pci_ioeventfd *ioeventfd, *ioeventfd_tmp;
>@@ -687,27 +720,7 @@ void vfio_pci_core_disable(struct vfio_pci_core_device *vdev)
> */
> pci_write_config_word(pdev, PCI_COMMAND, PCI_COMMAND_INTX_DISABLE);
>
>- /*
>- * Try to get the locks ourselves to prevent a deadlock. The
>- * success of this is dependent on being able to lock the device,
>- * which is not always possible.
>- * We can not use the "try" reset interface here, which will
>- * overwrite the previously restored configuration information.
>- */
>- if (vdev->reset_works) {
>- bridge = pci_upstream_bridge(pdev);
>- if (bridge && !pci_dev_trylock(bridge))
>- goto out_restore_state;
>- if (pci_dev_trylock(pdev)) {
>- if (!__pci_reset_function_locked(pdev))
>- vdev->needs_reset = false;
>- pci_dev_unlock(pdev);
>- }
>- if (bridge)
>- pci_dev_unlock(bridge);
>- }
>-
>-out_restore_state:
>+ vfio_pci_core_try_reset(vdev);
> pci_restore_state(pdev);
> out:
> pci_disable_device(pdev);
>diff --git a/drivers/vfio/pci/vfio_pci_liveupdate.c b/drivers/vfio/pci/vfio_pci_liveupdate.c
>index 5ea5af46b159..c4ebc7c486e5 100644
>--- a/drivers/vfio/pci/vfio_pci_liveupdate.c
>+++ b/drivers/vfio/pci/vfio_pci_liveupdate.c
>@@ -6,27 +6,178 @@
> * David Matlack <dmatlack@google.com>
> */
>
>+/**
>+ * DOC: VFIO PCI Preservation via LUO
>+ *
>+ * VFIO PCI devices can be preserved over a kexec using the Live Update
>+ * Orchestrator (LUO) file preservation. This allows userspace (such as a VMM)
>+ * to transfer an in-use device to the next kernel.
>+ *
>+ * .. note::
>+ * The support for preserving VFIO PCI devices is currently *partial* and
>+ * should be considered *experimental*. It should only be used by developers
>+ * working on expanding the support for the time being.
>+ *
>+ * To avoid accidental usage while the support is still experimental, this
>+ * support is hidden behind a default-disable config option
>+ * ``CONFIG_VFIO_PCI_LIVEUPDATE``. Once the kernel support has stabilized and
>+ * become complete, this option will be enabled by default when
>+ * ``CONFIG_VFIO_PCI`` and ``CONFIG_LIVEUPDATE`` are enabled.
>+ *
>+ * Usage Example
>+ * =============
>+ *
>+ * VFIO PCI devices can be preserved across a kexec by preserving the file
>+ * associated with the device in a LUO session::
>+ *
>+ * device_fd = open("/dev/vfio/devices/X");
>+ * ...
>+ * ioctl(session_fd, LIVEUPDATE_SESSION_PRESERVE_FD, { ..., device_fd, ...});
>+ *
>+ * .. note::
>+ * LUO will hold an extra reference to the device file for as long as it is
>+ * preserved, so there is no way for the file to be destroyed or the device
>+ * to be unbound from the vfio-pci driver while it is preserved.
>+ *
>+ * Retrieving the file after kexec is not yet supported.
>+ *
>+ * Restrictions
>+ * ============
>+ *
>+ * The kernel imposes the following restrictions when preserving VFIO devices:
>+ *
>+ * * The device must be bound to the ``vfio-pci`` driver.
>+ *
>+ * * ``CONFIG_VFIO_PCI_ZDEV_KVM`` must not be enabled. This may be relaxed in
>+ * the future.
>+ *
>+ * * The device not be an Intel display device. This may be relaxed in the
>+ * future.

Seems there is a typo. The device "may"/"must" not be?
>+ *
>+ * * The device file must have been acquired from the VFIO character device,
>+ * not ``VFIO_GROUP_GET_DEVICE_FD``.
>+ *
>+ * * The device must have interrupt disable prior to kexec. Failure to disable
>+ * interrupts on the device will cause the ``reboot(LINUX_REBOOT_CMD_KEXEC)``
>+ * syscall (to initiate the kexec) to fail.
>+ *
>+ * Preservation Behavior
>+ * =====================
>+ *
>+ * The eventual goal of this support is to avoid disrupting the workload, state,
>+ * or configuration of each preserved device during a Live Update. This would
>+ * include allowing the device to perform DMA to preserved memory buffers and
>+ * perform P2P DMA to other preserved devices. However, there are many pieces
>+ * that still need to land in the kernel.
>+ *
>+ * For now, VFIO only preserves the following state for for devices:
>+ *
>+ * * The PCI Segment, Bus, Device, and Function numbers of the device. The
>+ * kernel guarantees the these will not change across a kexec when a device
>+ * is preserved.
>+ *
>+ * Since the kernel is not yet prepared to preserve all parts of the device and
>+ * its dependencies (such as DMA mappings), VFIO currently resets and restores
>+ * preserved devices back into an idle state during kexec, before handing off
>+ * control to the next kernel. This will be relaxed in future versions of the
>+ * kernel once it is safe to allow the device to keep running across kexec.
>+ */
>+
> #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
>
>+#include <linux/kexec_handover.h>
> #include <linux/kho/abi/vfio_pci.h>
> #include <linux/liveupdate.h>
> #include <linux/errno.h>
>+#include <linux/vfio.h>
>
> #include "vfio_pci_priv.h"
>
> static bool vfio_pci_liveupdate_can_preserve(struct liveupdate_file_handler *handler,
> struct file *file)
> {
>- return false;
>+ struct vfio_device *device = vfio_device_from_file(file);
>+ struct vfio_pci_core_device *vdev;
>+ struct pci_dev *pdev;
>+
>+ if (!device)
>+ return false;
>+
>+ /* Live Update support is limited to cdev files. */
>+ if (!vfio_device_cdev_opened(device))
>+ return false;
>+
>+ if (device->ops != &vfio_pci_ops)
>+ return false;
>+
>+ vdev = container_of(device, struct vfio_pci_core_device, vdev);
>+ pdev = vdev->pdev;
>+
>+ /*
>+ * Don't support specialized vfio-pci devices for now since they haven't
>+ * been tested.
>+ */
>+ if (IS_ENABLED(CONFIG_VFIO_PCI_ZDEV_KVM) || vfio_pci_is_intel_display(pdev))
>+ return false;
>+
>+ return true;
> }
>
> static int vfio_pci_liveupdate_preserve(struct liveupdate_file_op_args *args)
> {
>- return -EOPNOTSUPP;
>+ struct vfio_device *device = vfio_device_from_file(args->file);
>+ struct vfio_pci_core_device_ser *ser;
>+ struct vfio_pci_core_device *vdev;
>+ struct pci_dev *pdev;
>+
>+ vdev = container_of(device, struct vfio_pci_core_device, vdev);
>+ pdev = vdev->pdev;
>+
>+ ser = kho_alloc_preserve(sizeof(*ser));
>+ if (IS_ERR(ser))
>+ return PTR_ERR(ser);
>+
>+ ser->bdf = pci_dev_id(pdev);
>+ ser->domain = pci_domain_nr(pdev->bus);
>+
>+ args->serialized_data = virt_to_phys(ser);
>+ return 0;
> }
>
> static void vfio_pci_liveupdate_unpreserve(struct liveupdate_file_op_args *args)
> {
>+ kho_unpreserve_free(phys_to_virt(args->serialized_data));
>+}
>+
>+static int vfio_pci_liveupdate_freeze(struct liveupdate_file_op_args *args)
>+{
>+ struct vfio_device *device = vfio_device_from_file(args->file);
>+ struct vfio_pci_core_device *vdev;
>+ struct pci_dev *pdev;
>+ int ret;
>+
>+ vdev = container_of(device, struct vfio_pci_core_device, vdev);
>+ pdev = vdev->pdev;
>+
>+ guard(mutex)(&device->dev_set->lock);
>+
>+ /*
>+ * Userspace must disable interrupts on the device prior to freeze so
>+ * that the device does not send any interrupts until new interrupt
>+ * handlers have been established by the next kernel.
>+ */
>+ if (vdev->irq_type != VFIO_PCI_NUM_IRQS) {
>+ pci_err(pdev, "Freeze failed! Interrupts are still enabled.\n");
>+ return -EINVAL;
>+ }
>+
>+ ret = pci_load_saved_state(pdev, vdev->pci_saved_state);
>+ if (ret)
>+ return ret;
>+
>+ vfio_pci_core_try_reset(vdev);

nit: I see you have already added a comment earlier saying that device will
be reset to a clean state as all the of the dependencies to preserve the
state for continous DMA are not there. Maybe we can add a comment here
to specify that this will go away once we have full preservation
support.
>+ pci_restore_state(pdev);
>+ return 0;
> }
>
> static int vfio_pci_liveupdate_retrieve(struct liveupdate_file_op_args *args)
>@@ -42,6 +193,7 @@ static const struct liveupdate_file_ops vfio_pci_liveupdate_file_ops = {
> .can_preserve = vfio_pci_liveupdate_can_preserve,
> .preserve = vfio_pci_liveupdate_preserve,
> .unpreserve = vfio_pci_liveupdate_unpreserve,
>+ .freeze = vfio_pci_liveupdate_freeze,
> .retrieve = vfio_pci_liveupdate_retrieve,
> .finish = vfio_pci_liveupdate_finish,
> .owner = THIS_MODULE,
>diff --git a/drivers/vfio/pci/vfio_pci_priv.h b/drivers/vfio/pci/vfio_pci_priv.h
>index cbf46e09da30..fa5c7f544f8a 100644
>--- a/drivers/vfio/pci/vfio_pci_priv.h
>+++ b/drivers/vfio/pci/vfio_pci_priv.h
>@@ -11,6 +11,10 @@
> /* Cap maximum number of ioeventfds per device (arbitrary) */
> #define VFIO_PCI_IOEVENTFD_MAX 1000
>
>+extern const struct vfio_device_ops vfio_pci_ops;
>+
>+void vfio_pci_core_try_reset(struct vfio_pci_core_device *vdev);
>+
> struct vfio_pci_ioeventfd {
> struct list_head next;
> struct vfio_pci_core_device *vdev;
>diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
>index 742477546b15..8b222f71bbab 100644
>--- a/drivers/vfio/vfio_main.c
>+++ b/drivers/vfio/vfio_main.c
>@@ -1436,7 +1436,7 @@ const struct file_operations vfio_device_fops = {
> #endif
> };
>
>-static struct vfio_device *vfio_device_from_file(struct file *file)
>+struct vfio_device *vfio_device_from_file(struct file *file)
> {
> struct vfio_device_file *df = file->private_data;
>
>@@ -1444,6 +1444,7 @@ static struct vfio_device *vfio_device_from_file(struct file *file)
> return NULL;
> return df->device;
> }
>+EXPORT_SYMBOL_GPL(vfio_device_from_file);
>
> /**
> * vfio_file_is_valid - True if the file is valid vfio file
>diff --git a/include/linux/kho/abi/vfio_pci.h b/include/linux/kho/abi/vfio_pci.h
>index e2412b455e61..876aaf81dd92 100644
>--- a/include/linux/kho/abi/vfio_pci.h
>+++ b/include/linux/kho/abi/vfio_pci.h
>@@ -9,6 +9,9 @@
> #ifndef _LINUX_LIVEUPDATE_ABI_VFIO_PCI_H
> #define _LINUX_LIVEUPDATE_ABI_VFIO_PCI_H
>
>+#include <linux/compiler.h>
>+#include <linux/types.h>
>+
> /**
> * DOC: VFIO PCI Live Update ABI
> *
>@@ -25,4 +28,16 @@
>
> #define VFIO_PCI_LUO_FH_COMPATIBLE "vfio-pci-v1"
>
>+/**
>+ * struct vfio_pci_core_device_ser - Serialized state of a single VFIO PCI
>+ * device.
>+ *
>+ * @domain: The device's PCI domain number (segment).
>+ * @bdf: The device's PCI bus, device, and function number.
>+ */
>+struct vfio_pci_core_device_ser {
>+ u32 domain;
>+ u16 bdf;
>+} __packed;
>+
> #endif /* _LINUX_LIVEUPDATE_ABI_VFIO_PCI_H */
>diff --git a/include/linux/vfio.h b/include/linux/vfio.h
>index e90859956514..e9d3ddb715c5 100644
>--- a/include/linux/vfio.h
>+++ b/include/linux/vfio.h
>@@ -81,6 +81,8 @@ struct vfio_device {
> #endif
> };
>
>+struct vfio_device *vfio_device_from_file(struct file *file);
>+
> /**
> * struct vfio_device_ops - VFIO bus driver device callbacks
> *
>--
>2.53.0.983.g0bb29b3bc5-goog
>