Re: [RFC PATCH 1/9] vfio/pci: introduce mediate ops to intercept vfio-pci ops

Yan Zhao
2019-12-06T08:05:29Z

On Fri, Dec 06, 2019 at 07:55:19AM +0800, Alex Williamson wrote:
> On Wed, 4 Dec 2019 22:25:36 -0500
> Yan Zhao <yan.y.zhao@intel.com> wrote:
>
> > when vfio-pci is bound to a physical device, almost all the hardware
> > resources are passthroughed.
> > Sometimes, vendor driver of this physcial device may want to mediate some
> > hardware resource access for a short period of time, e.g. dirty page
> > tracking during live migration.
> >
> > Here we introduce mediate ops in vfio-pci for this purpose.
> >
> > Vendor driver can register a mediate ops to vfio-pci.
> > But rather than directly bind to the passthroughed device, the
> > vendor driver is now either a module that does not bind to any device or
> > a module binds to other device.
> > E.g. when passing through a VF device that is bound to vfio-pci modules,
> > PF driver that binds to PF device can register to vfio-pci to mediate
> > VF's regions, hence supporting VF live migration.
> >
> > The sequence goes like this:
> > 1. Vendor driver register its vfio_pci_mediate_ops to vfio-pci driver
> >
> > 2. vfio-pci maintains a list of those registered vfio_pci_mediate_ops
> >
> > 3. Whenever vfio-pci opens a device, it searches the list and call
> > vfio_pci_mediate_ops->open() to check whether a vendor driver supports
> > mediating this device.
> > Upon a success return value of from vfio_pci_mediate_ops->open(),
> > vfio-pci will stop list searching and store a mediate handle to
> > represent this open into vendor driver.
> > (so if multiple vendor drivers support mediating a device through
> > vfio_pci_mediate_ops, only one will win, depending on their registering
> > sequence)
> >
> > 4. Whenever a VFIO_DEVICE_GET_REGION_INFO ioctl is received in vfio-pci
> > ops, it will chain into vfio_pci_mediate_ops->get_region_info(), so that
> > vendor driver is able to override a region's default flags and caps,
> > e.g. adding a sparse mmap cap to passthrough only sub-regions of a whole
> > region.
> >
> > 5. vfio_pci_rw()/vfio_pci_mmap() first calls into
> > vfio_pci_mediate_ops->rw()/vfio_pci_mediate_ops->mmaps().
> > if pt=true is rteturned, vfio_pci_rw()/vfio_pci_mmap() will further
> > passthrough this read/write/mmap to physical device, otherwise it just
> > returns without touch physical device.
> >
> > 6. When vfio-pci closes a device, vfio_pci_release() chains into
> > vfio_pci_mediate_ops->release() to close the reference in vendor driver.
> >
> > 7. Vendor driver unregister its vfio_pci_mediate_ops when driver exits
> >
> > Cc: Kevin Tian <kevin.tian@intel.com>
> >
> > Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
> > ---
> > drivers/vfio/pci/vfio_pci.c | 146 ++++++++++++++++++++++++++++
> > drivers/vfio/pci/vfio_pci_private.h | 2 +
> > include/linux/vfio.h | 16 +++
> > 3 files changed, 164 insertions(+)
> >
> > diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
> > index 02206162eaa9..55080ff29495 100644
> > --- a/drivers/vfio/pci/vfio_pci.c
> > +++ b/drivers/vfio/pci/vfio_pci.c
> > @@ -54,6 +54,14 @@ module_param(disable_idle_d3, bool, S_IRUGO | S_IWUSR);
> > MODULE_PARM_DESC(disable_idle_d3,
> > "Disable using the PCI D3 low power state for idle, unused devices");
> >
> > +static LIST_HEAD(mediate_ops_list);
> > +static DEFINE_MUTEX(mediate_ops_list_lock);
> > +struct vfio_pci_mediate_ops_list_entry {
> > + struct vfio_pci_mediate_ops *ops;
> > + int refcnt;
> > + struct list_head next;
> > +};
> > +
> > static inline bool vfio_vga_disabled(void)
> > {
> > #ifdef CONFIG_VFIO_PCI_VGA
> > @@ -472,6 +480,10 @@ static void vfio_pci_release(void *device_data)
> > if (!(--vdev->refcnt)) {
> > vfio_spapr_pci_eeh_release(vdev->pdev);
> > vfio_pci_disable(vdev);
> > + if (vdev->mediate_ops && vdev->mediate_ops->release) {
> > + vdev->mediate_ops->release(vdev->mediate_handle);
> > + vdev->mediate_ops = NULL;
> > + }
> > }
> >
> > mutex_unlock(&vdev->reflck->lock);
> > @@ -483,6 +495,7 @@ static int vfio_pci_open(void *device_data)
> > {
> > struct vfio_pci_device *vdev = device_data;
> > int ret = 0;
> > + struct vfio_pci_mediate_ops_list_entry *mentry;
> >
> > if (!try_module_get(THIS_MODULE))
> > return -ENODEV;
> > @@ -495,6 +508,30 @@ static int vfio_pci_open(void *device_data)
> > goto error;
> >
> > vfio_spapr_pci_eeh_open(vdev->pdev);
> > + mutex_lock(&mediate_ops_list_lock);
> > + list_for_each_entry(mentry, &mediate_ops_list, next) {
> > + u64 caps;
> > + u32 handle;
>
> Wouldn't it seem likely that the ops provider might use this handle as
> a pointer, so we'd want it to be an opaque void*?
>
yes, you are right, handle as a pointer is much better. will change it.
Thanks :)

> > +
> > + memset(&caps, 0, sizeof(caps));
>
> @caps has no purpose here, add it if/when we do something with it.
> It's also a standard type, why are we memset'ing it rather than just
> =0??
>
> > + ret = mentry->ops->open(vdev->pdev, &caps, &handle);
> > + if (!ret) {
> > + vdev->mediate_ops = mentry->ops;
> > + vdev->mediate_handle = handle;
> > +
> > + pr_info("vfio pci found mediate_ops %s, caps=%llx, handle=%x for %x:%x\n",
> > + vdev->mediate_ops->name, caps,
> > + handle, vdev->pdev->vendor,
> > + vdev->pdev->device);
>
> Generally not advisable to make user accessible printks.
>
ok.

> > + /*
> > + * only find the first matching mediate_ops,
> > + * and add its refcnt
> > + */
> > + mentry->refcnt++;
> > + break;
> > + }
> > + }
> > + mutex_unlock(&mediate_ops_list_lock);
> > }
> > vdev->refcnt++;
> > error:
> > @@ -736,6 +773,14 @@ static long vfio_pci_ioctl(void *device_data,
> > info.size = pdev->cfg_size;
> > info.flags = VFIO_REGION_INFO_FLAG_READ |
> > VFIO_REGION_INFO_FLAG_WRITE;
> > +
> > + if (vdev->mediate_ops &&
> > + vdev->mediate_ops->get_region_info) {
> > + vdev->mediate_ops->get_region_info(
> > + vdev->mediate_handle,
> > + &info, &caps, NULL);
> > + }
>
> These would be a lot cleaner if we could just call a helper function:
>
> void vfio_pci_region_info_mediation_hook(vdev, info, caps, etc...)
> {
> if (vdev->mediate_ops
> vdev->mediate_ops->get_region_info)
> vdev->mediate_ops->get_region_info(vdev->mediate_handle,
> &info, &caps, NULL);
> }
>
> I'm not thrilled with all these hooks, but not open coding every one of
> them might help.

ok. got it.
>
> > +
> > break;
> > case VFIO_PCI_BAR0_REGION_INDEX ... VFIO_PCI_BAR5_REGION_INDEX:
> > info.offset = VFIO_PCI_INDEX_TO_OFFSET(info.index);
> > @@ -756,6 +801,13 @@ static long vfio_pci_ioctl(void *device_data,
> > }
> > }
> >
> > + if (vdev->mediate_ops &&
> > + vdev->mediate_ops->get_region_info) {
> > + vdev->mediate_ops->get_region_info(
> > + vdev->mediate_handle,
> > + &info, &caps, NULL);
> > + }
> > +
> > break;
> > case VFIO_PCI_ROM_REGION_INDEX:
> > {
> > @@ -794,6 +846,14 @@ static long vfio_pci_ioctl(void *device_data,
> > }
> >
> > pci_write_config_word(pdev, PCI_COMMAND, orig_cmd);
> > +
> > + if (vdev->mediate_ops &&
> > + vdev->mediate_ops->get_region_info) {
> > + vdev->mediate_ops->get_region_info(
> > + vdev->mediate_handle,
> > + &info, &caps, NULL);
> > + }
> > +
> > break;
> > }
> > case VFIO_PCI_VGA_REGION_INDEX:
> > @@ -805,6 +865,13 @@ static long vfio_pci_ioctl(void *device_data,
> > info.flags = VFIO_REGION_INFO_FLAG_READ |
> > VFIO_REGION_INFO_FLAG_WRITE;
> >
> > + if (vdev->mediate_ops &&
> > + vdev->mediate_ops->get_region_info) {
> > + vdev->mediate_ops->get_region_info(
> > + vdev->mediate_handle,
> > + &info, &caps, NULL);
> > + }
> > +
> > break;
> > default:
> > {
> > @@ -839,6 +906,13 @@ static long vfio_pci_ioctl(void *device_data,
> > if (ret)
> > return ret;
> > }
> > +
> > + if (vdev->mediate_ops &&
> > + vdev->mediate_ops->get_region_info) {
> > + vdev->mediate_ops->get_region_info(
> > + vdev->mediate_handle,
> > + &info, &caps, &cap_type);
> > + }
> > }
> > }
> >
> > @@ -1151,6 +1225,16 @@ static ssize_t vfio_pci_rw(void *device_data, char __user *buf,
> > if (index >= VFIO_PCI_NUM_REGIONS + vdev->num_regions)
> > return -EINVAL;
> >
> > + if (vdev->mediate_ops && vdev->mediate_ops->rw) {
> > + int ret;
> > + bool pt = true;
> > +
> > + ret = vdev->mediate_ops->rw(vdev->mediate_handle,
> > + buf, count, ppos, iswrite, &pt);
> > + if (!pt)
> > + return ret;
> > + }
> > +
> > switch (index) {
> > case VFIO_PCI_CONFIG_REGION_INDEX:
> > return vfio_pci_config_rw(vdev, buf, count, ppos, iswrite);
> > @@ -1200,6 +1284,15 @@ static int vfio_pci_mmap(void *device_data, struct vm_area_struct *vma)
> > u64 phys_len, req_len, pgoff, req_start;
> > int ret;
> >
> > + if (vdev->mediate_ops && vdev->mediate_ops->mmap) {
> > + int ret;
> > + bool pt = true;
> > +
> > + ret = vdev->mediate_ops->mmap(vdev->mediate_handle, vma, &pt);
> > + if (!pt)
> > + return ret;
> > + }
>
> There must be a better way to do all these. Do we really want to call
> into ops for every rw or mmap, have the vendor code decode a region,
> and maybe or maybe not have it handle it? It's pretty ugly. Do we

do you think below flow is good ?
1. in mediate_ops->open(), return
(1) region[] indexed by region index, if a mediate driver supports mediating
region[i], region[i].ops->get_region_info, regions[i].ops->rw, or
regions[i].ops->mmap is not null.
(2) irq_info[] indexed by irq index, if a mediate driver supports mediating
irq_info[i], irq_info[i].ops->get_irq_info or irq_info[i].ops->set_irq_info
is not null.

Then, vfio_pci_rw/vfio_pci_mmap/vfio_pci_ioctl only call into those
non-null hooks.

> need the mediation provider to be able to dynamically setup the ops per
May I confirm that you are not saying dynamic registering mediate ops
after vfio-pci already opened a device, right?

> region and export the default handlers out for them to call?
>
could we still keep checking return value of the hooks rather than
export default handlers? Otherwise at least vfio_pci_default_ioctl(),
vfio_pci_default_rw(), and vfio_pci_default_mmap() need to be exported.

> > +
> > index = vma->vm_pgoff >> (VFIO_PCI_OFFSET_SHIFT - PAGE_SHIFT);
> >
> > if (vma->vm_end < vma->vm_start)
> > @@ -1629,8 +1722,17 @@ static void vfio_pci_try_bus_reset(struct vfio_pci_device *vdev)
> >
> > static void __exit vfio_pci_cleanup(void)
> > {
> > + struct vfio_pci_mediate_ops_list_entry *mentry, *n;
> > +
> > pci_unregister_driver(&vfio_pci_driver);
> > vfio_pci_uninit_perm_bits();
> > +
> > + mutex_lock(&mediate_ops_list_lock);
> > + list_for_each_entry_safe(mentry, n, &mediate_ops_list, next) {
> > + list_del(&mentry->next);
> > + kfree(mentry);
> > + }
> > + mutex_unlock(&mediate_ops_list_lock);
>
> Is it even possible to unload vfio-pci while there are mediation
> drivers registered? I don't think the module interactions are well
> thought out here, ex. do you really want i40e to have build and runtime
> dependencies on vfio-pci? I don't think so.
>
Currently, yes, i40e has build dependency on vfio-pci.
It's like this, if i40e decides to support SRIOV and compiles in vf
related code who depends on vfio-pci, it will also have build dependency
on vfio-pci. isn't it natural?

> > }
> >
> > static void __init vfio_pci_fill_ids(void)
> > @@ -1697,6 +1799,50 @@ static int __init vfio_pci_init(void)
> > return ret;
> > }
> >
> > +int vfio_pci_register_mediate_ops(struct vfio_pci_mediate_ops *ops)
> > +{
> > + struct vfio_pci_mediate_ops_list_entry *mentry;
> > +
> > + mutex_lock(&mediate_ops_list_lock);
> > + mentry = kzalloc(sizeof(*mentry), GFP_KERNEL);
> > + if (!mentry) {
> > + mutex_unlock(&mediate_ops_list_lock);
> > + return -ENOMEM;
> > + }
> > +
> > + mentry->ops = ops;
> > + mentry->refcnt = 0;
>
> It's kZalloc'd, this is unnecessary.
>
right :)
> > + list_add(&mentry->next, &mediate_ops_list);
>
> Check for duplicates?
>
ok. will do it.
> > +
> > + pr_info("registered dm ops %s\n", ops->name);
> > + mutex_unlock(&mediate_ops_list_lock);
> > +
> > + return 0;
> > +}
> > +EXPORT_SYMBOL(vfio_pci_register_mediate_ops);
> > +
> > +void vfio_pci_unregister_mediate_ops(struct vfio_pci_mediate_ops *ops)
> > +{
> > + struct vfio_pci_mediate_ops_list_entry *mentry, *n;
> > +
> > + mutex_lock(&mediate_ops_list_lock);
> > + list_for_each_entry_safe(mentry, n, &mediate_ops_list, next) {
> > + if (mentry->ops != ops)
> > + continue;
> > +
> > + mentry->refcnt--;
>
> Whose reference is this removing?
>
I intended to prevent mediate driver from calling unregister mediate ops
while there're still opened devices in it.
after a successful mediate_ops->open(), mentry->refcnt++.
after calling mediate_ops->release(). mentry->refcnt--.

(seems in this RFC, I missed a mentry->refcnt-- after calling
mediate_ops->release())

> > + if (!mentry->refcnt) {
> > + list_del(&mentry->next);
> > + kfree(mentry);
> > + } else
> > + pr_err("vfio_pci unregister mediate ops %s error\n",
> > + mentry->ops->name);
>
> This is bad, we should hold a reference to the module providing these
> ops for each use of it such that the module cannot be removed while
> it's in use. Otherwise we enter a very bad state here and it's
> trivially accessible by an admin remove the module while in use.
mediate driver is supposed to ref its own module on a success
mediate_ops->open(), and deref its own module on mediate_ops->release().
so, it can't be accidentally removed.

Thanks

Yan
> Thanks,
>
> Alex
>
> > + }
> > + mutex_unlock(&mediate_ops_list_lock);
> > +
> > +}
> > +EXPORT_SYMBOL(vfio_pci_unregister_mediate_ops);
> > +
> > module_init(vfio_pci_init);
> > module_exit(vfio_pci_cleanup);
> >
> > diff --git a/drivers/vfio/pci/vfio_pci_private.h b/drivers/vfio/pci/vfio_pci_private.h
> > index ee6ee91718a4..bad4a254360e 100644
> > --- a/drivers/vfio/pci/vfio_pci_private.h
> > +++ b/drivers/vfio/pci/vfio_pci_private.h
> > @@ -122,6 +122,8 @@ struct vfio_pci_device {
> > struct list_head dummy_resources_list;
> > struct mutex ioeventfds_lock;
> > struct list_head ioeventfds_list;
> > + struct vfio_pci_mediate_ops *mediate_ops;
> > + u32 mediate_handle;
> > };
> >
> > #define is_intx(vdev) (vdev->irq_type == VFIO_PCI_INTX_IRQ_INDEX)
> > diff --git a/include/linux/vfio.h b/include/linux/vfio.h
> > index e42a711a2800..0265e779acd1 100644
> > --- a/include/linux/vfio.h
> > +++ b/include/linux/vfio.h
> > @@ -195,4 +195,20 @@ extern int vfio_virqfd_enable(void *opaque,
> > void *data, struct virqfd **pvirqfd, int fd);
> > extern void vfio_virqfd_disable(struct virqfd **pvirqfd);
> >
> > +struct vfio_pci_mediate_ops {
> > + char *name;
> > + int (*open)(struct pci_dev *pdev, u64 *caps, u32 *handle);
> > + void (*release)(int handle);
> > + void (*get_region_info)(int handle,
> > + struct vfio_region_info *info,
> > + struct vfio_info_cap *caps,
> > + struct vfio_region_info_cap_type *cap_type);
> > + ssize_t (*rw)(int handle, char __user *buf,
> > + size_t count, loff_t *ppos, bool iswrite, bool *pt);
> > + int (*mmap)(int handle, struct vm_area_struct *vma, bool *pt);
> > +
> > +};
> > +extern int vfio_pci_register_mediate_ops(struct vfio_pci_mediate_ops *ops);
> > +extern void vfio_pci_unregister_mediate_ops(struct vfio_pci_mediate_ops *ops);
> > +
> > #endif /* VFIO_H */
>