Message-ID: <f3837ce1-cea8-043c-a2c6-0ffa48f8f4de@linux.microsoft.com>
Date: Fri, 23 Jan 2026 16:37:34 -0800
From: Mukesh R <mrathor@...ux.microsoft.com>
To: Nuno Das Neves <nunodasneves@...ux.microsoft.com>,
 linux-kernel@...r.kernel.org, linux-hyperv@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
 linux-pci@...r.kernel.org, linux-arch@...r.kernel.org
Cc: kys@...rosoft.com, haiyangz@...rosoft.com, wei.liu@...nel.org,
 decui@...rosoft.com, longli@...rosoft.com, catalin.marinas@....com,
 will@...nel.org, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, hpa@...or.com, joro@...tes.org,
 lpieralisi@...nel.org, kwilczynski@...nel.org, mani@...nel.org,
 robh@...nel.org, bhelgaas@...gle.com, arnd@...db.de, mhklinux@...look.com,
 romank@...ux.microsoft.com
Subject: Re: [PATCH v0 06/15] mshv: Implement mshv bridge device for VFIO

On 1/23/26 10:32, Nuno Das Neves wrote:
> On 1/19/2026 10:42 PM, Mukesh R wrote:
>> From: Mukesh Rathor <mrathor@...ux.microsoft.com>
>>
>> Add a new file to implement 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.
>>
>> Original author: Wei Liu <wei.liu@...nel.org>
>> (Slightly modified from the original version).
>>
>> Signed-off-by: Mukesh Rathor <mrathor@...ux.microsoft.com>
> 
> Since the code is very similar to Wei's original commit, the way I'd
> recommend to do it is:
> 1. Change the commit author to Wei, using git commit --amend --author=
> and
> 2. Put his signed-off line before yours:
> 
> Signed-off-by: Wei Liu <wei.liu@...nel.org>
> Signed-off-by: Mukesh Rathor <mrathor@...ux.microsoft.com>
> 
> This shows he is the author of the commit but you ported it.
> 
> If you feel you changed it enough that it should be considered
> co-authored, you can instead keep your authorship of the commit and
> put:
> 
> Co-developed-by: Wei Liu <wei.liu@...nel.org>
> Signed-off-by: Wei Liu <wei.liu@...nel.org>
> Signed-off-by: Mukesh Rathor <mrathor@...ux.microsoft.com>

Perfect! Thank you, that is exactly the information I was trying to
seek... makes sense.

Thanks,
-Mukesh



>> ---
>>   drivers/hv/Makefile    |   3 +-
>>   drivers/hv/mshv_vfio.c | 210 +++++++++++++++++++++++++++++++++++++++++
>>   2 files changed, 212 insertions(+), 1 deletion(-)
>>   create mode 100644 drivers/hv/mshv_vfio.c
>>
>> diff --git a/drivers/hv/Makefile b/drivers/hv/Makefile
>> index a49f93c2d245..eae003c4cb8f 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_vtl-y := mshv_vtl_main.o
>>   
>>   # Code that must be built-in
>> diff --git a/drivers/hv/mshv_vfio.c b/drivers/hv/mshv_vfio.c
>> new file mode 100644
>> index 000000000000..6ea4d99a3bd2
>> --- /dev/null
>> +++ b/drivers/hv/mshv_vfio.c
>> @@ -0,0 +1,210 @@
>> +// 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 "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);
>> +
>> +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, u32 type)
>> +{
>> +	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(sizeof(*mshv_vfio), GFP_KERNEL_ACCOUNT);
>> +	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);
>> +}
>> +
>> +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,
>> +};
> 

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