Re: [PATCH v5 3/9] mshv: Introduce basic mshv bridge device for VFIO to build upon

[Jacob Pan <jacob.pan@linux.microsoft.com>]

Hi Mukesh,

On Fri, 31 Jul 2026 15:34:21 -0700
Mukesh R <mrathor@linux.microsoft.com> wrote:

> Add a new file to implement basic VFIO-MSHV bridge pseudo device.
> These functions are called in the VFIO framework, and credits to
> kvm/vfio.c as this file was adapted from it. This is a basic version
> to build upon.
> 
> Co-developed-by: Wei Liu <wei.liu@kernel.org>
> Signed-off-by: Wei Liu <wei.liu@kernel.org>
> Signed-off-by: Mukesh R <mrathor@linux.microsoft.com>
> ---
>  drivers/hv/Makefile       |   3 +-
>  drivers/hv/mshv_vfio.c    | 211
> ++++++++++++++++++++++++++++++++++++++ include/uapi/linux/mshv.h |
> 1 + 3 files changed, 214 insertions(+), 1 deletion(-)
>  create mode 100644 drivers/hv/mshv_vfio.c
> 
> diff --git a/drivers/hv/Makefile b/drivers/hv/Makefile
> index 888a748cc7cb..9ab6fc254c38 100644
> --- a/drivers/hv/Makefile
> +++ b/drivers/hv/Makefile
> @@ -14,7 +14,8 @@ hv_vmbus-y := vmbus_drv.o \
>  hv_vmbus-$(CONFIG_HYPERV_TESTING)	+= hv_debugfs.o
>  hv_utils-y := hv_util.o hv_kvp.o hv_snapshot.o hv_utils_transport.o
>  mshv_root-y := mshv_root_main.o mshv_synic.o mshv_eventfd.o
> mshv_irq.o \
> -	       mshv_root_hv_call.o mshv_portid_table.o mshv_regions.o
> +	       mshv_root_hv_call.o mshv_portid_table.o
> mshv_regions.o  \
> +               mshv_vfio.o
>  mshv_root-$(CONFIG_DEBUG_FS) += mshv_debugfs.o
>  mshv_root-$(CONFIG_TRACEPOINTS) += mshv_trace.o
>  mshv_vtl-y := mshv_vtl_main.o
> diff --git a/drivers/hv/mshv_vfio.c b/drivers/hv/mshv_vfio.c
> new file mode 100644
> index 000000000000..92cfbaef0328
> --- /dev/null
> +++ b/drivers/hv/mshv_vfio.c
> @@ -0,0 +1,211 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * VFIO-MSHV bridge pseudo device
> + *
> + * Heavily inspired by the VFIO-KVM bridge pseudo device.
> + */
> +#include <linux/errno.h>
> +#include <linux/file.h>
> +#include <linux/list.h>
> +#include <linux/module.h>
> +#include <linux/mutex.h>
> +#include <linux/slab.h>
> +#include <linux/vfio.h>
> +#include <asm/mshyperv.h>
> +
> +#include "mshv.h"
> +#include "mshv_root.h"
> +
> +struct mshv_vfio_file {
> +	struct list_head node;
> +	struct file *file;	/* list of struct mshv_vfio_file */
> +};
> +
> +struct mshv_vfio {
> +	struct list_head file_list;
> +	struct mutex lock;
> +};
> +
> +static bool mshv_vfio_file_is_valid(struct file *file)
> +{
> +	bool (*fn)(struct file *file);
> +	bool ret;
> +
> +	fn = symbol_get(vfio_file_is_valid);
> +	if (!fn)
> +		return false;
> +
> +	ret = fn(file);
> +
> +	symbol_put(vfio_file_is_valid);
> +
> +	return ret;
> +}
> +
> +static long mshv_vfio_file_add(struct mshv_device *mshvdev, unsigned
> int fd) +{
> +	struct mshv_vfio *mshv_vfio = mshvdev->device_private;
> +	struct mshv_vfio_file *mvf;
> +	struct file *filp;
> +	long ret = 0;
> +
> +	filp = fget(fd);
> +	if (!filp)
> +		return -EBADF;
> +
> +	/* Ensure the FD is a vfio FD. */
> +	if (!mshv_vfio_file_is_valid(filp)) {
> +		ret = -EINVAL;
> +		goto out_fput;
> +	}
> +
> +	mutex_lock(&mshv_vfio->lock);
> +
> +	list_for_each_entry(mvf, &mshv_vfio->file_list, node) {
> +		if (mvf->file == filp) {
> +			ret = -EEXIST;
> +			goto out_unlock;
> +		}
> +	}
> +
> +	mvf = kzalloc(sizeof(*mvf), GFP_KERNEL_ACCOUNT);
> +	if (!mvf) {
> +		ret = -ENOMEM;
> +		goto out_unlock;
> +	}
> +
> +	mvf->file = get_file(filp);
> +	list_add_tail(&mvf->node, &mshv_vfio->file_list);
> +
Why there is no vfio_device_file_set_kvm equivalent for mshv? since VFIO
device open/bind path will do vfio_device/group_get_kvm_safe() to ensure
lifetime alignment. Otherwise, vfio group/device can outlive mshv
partition.

If we do want to support this kvm-vfio bridge semantics beyond kvm,
maybe this should be abstracted as a generic VFIO "hypervisor
partition" association, with hypervisor-specific get/put callbacks,
rather than adding an MSHV-only copy of the KVM hook.
+Alex

> +out_unlock:
> +	mutex_unlock(&mshv_vfio->lock);
> +out_fput:
> +	fput(filp);
> +	return ret;
> +}
> +
> +static long mshv_vfio_file_del(struct mshv_device *mshvdev, unsigned
> int fd) +{
> +	struct mshv_vfio *mshv_vfio = mshvdev->device_private;
> +	struct mshv_vfio_file *mvf;
> +	long ret;
> +
> +	CLASS(fd, f)(fd);
> +
> +	if (fd_empty(f))
> +		return -EBADF;
> +
> +	ret = -ENOENT;
> +	mutex_lock(&mshv_vfio->lock);
> +
> +	list_for_each_entry(mvf, &mshv_vfio->file_list, node) {
> +		if (mvf->file != fd_file(f))
> +			continue;
> +
> +		list_del(&mvf->node);
> +		fput(mvf->file);
> +		kfree(mvf);
> +		ret = 0;
> +		break;
> +	}
> +
> +	mutex_unlock(&mshv_vfio->lock);
> +	return ret;
> +}
> +
> +static long mshv_vfio_set_file(struct mshv_device *mshvdev, long
> attr,
> +			      void __user *arg)
> +{
> +	int32_t __user *argp = arg;
> +	int32_t fd;
> +
> +	switch (attr) {
> +	case MSHV_DEV_VFIO_FILE_ADD:
> +		if (get_user(fd, argp))
> +			return -EFAULT;
> +		return mshv_vfio_file_add(mshvdev, fd);
> +
> +	case MSHV_DEV_VFIO_FILE_DEL:
> +		if (get_user(fd, argp))
> +			return -EFAULT;
> +		return mshv_vfio_file_del(mshvdev, fd);
> +	}
> +
> +	return -ENXIO;
> +}
> +
> +static long mshv_vfio_set_attr(struct mshv_device *mshvdev,
> +			      struct mshv_device_attr *attr)
> +{
> +	switch (attr->group) {
> +	case MSHV_DEV_VFIO_FILE:
> +		return mshv_vfio_set_file(mshvdev, attr->attr,
> +
> u64_to_user_ptr(attr->addr));
> +	}
> +
> +	return -ENXIO;
> +}
> +
> +static long mshv_vfio_has_attr(struct mshv_device *mshvdev,
> +			      struct mshv_device_attr *attr)
> +{
> +	switch (attr->group) {
> +	case MSHV_DEV_VFIO_FILE:
> +		switch (attr->attr) {
> +		case MSHV_DEV_VFIO_FILE_ADD:
> +		case MSHV_DEV_VFIO_FILE_DEL:
> +			return 0;
> +		}
> +
> +		break;
> +	}
> +
> +	return -ENXIO;
> +}
> +
> +static long mshv_vfio_create_device(struct mshv_device *mshvdev)
> +{
> +	struct mshv_device *tmp;
> +	struct mshv_vfio *mshv_vfio;
> +
> +	/* Only one VFIO "device" per VM */
> +	hlist_for_each_entry(tmp, &mshvdev->device_pt->pt_devices,
> +			     device_ptnode)
> +		if (tmp->device_ops == &mshv_vfio_device_ops)
> +			return -EBUSY;
> +
> +	mshv_vfio = kzalloc_obj(*mshv_vfio);
> +	if (mshv_vfio == NULL)
> +		return -ENOMEM;
> +
> +	INIT_LIST_HEAD(&mshv_vfio->file_list);
> +	mutex_init(&mshv_vfio->lock);
> +
> +	mshvdev->device_private = mshv_vfio;
> +
> +	return 0;
> +}
> +
> +/* This is called from mshv_device_fop_release() */
> +static void mshv_vfio_release_device(struct mshv_device *mshvdev)
> +{
> +	struct mshv_vfio *mv = mshvdev->device_private;
> +	struct mshv_vfio_file *mvf, *tmp;
> +
> +	list_for_each_entry_safe(mvf, tmp, &mv->file_list, node) {
> +		fput(mvf->file);
> +		list_del(&mvf->node);
> +		kfree(mvf);
> +	}
> +
> +	kfree(mv);
> +	kfree(mshvdev);
> +}
> +
> +const struct mshv_device_ops mshv_vfio_device_ops = {
> +	.device_name = "mshv-vfio",
> +	.device_create = mshv_vfio_create_device,
> +	.device_release = mshv_vfio_release_device,
> +	.device_set_attr = mshv_vfio_set_attr,
> +	.device_has_attr = mshv_vfio_has_attr,
> +};
> diff --git a/include/uapi/linux/mshv.h b/include/uapi/linux/mshv.h
> index be6fe3ee8707..b038a79786d2 100644
> --- a/include/uapi/linux/mshv.h
> +++ b/include/uapi/linux/mshv.h
> @@ -254,6 +254,7 @@ struct mshv_root_hvcall {
>  #define MSHV_GET_GPAP_ACCESS_BITMAP	_IOWR(MSHV_IOCTL, 0x06,
> struct mshv_gpap_access_bitmap) /* Generic hypercall */
>  #define MSHV_ROOT_HVCALL		_IOWR(MSHV_IOCTL, 0x07,
> struct mshv_root_hvcall) +#define MSHV_CREATE_DEVICE
> _IOWR(MSHV_IOCTL, 0x08, struct mshv_create_device) 
>  /*
>   ********************************