Re: [RFC PATCH 4/9] vfio-pci: register default dynamic-trap-bar-info region

[Alex Williamson <alex.williamson@redhat.com>]

On Wed,  4 Dec 2019 22:26:50 -0500
Yan Zhao <yan.y.zhao@intel.com> wrote:

> Dynamic trap bar info region is a channel for QEMU and vendor driver to
> communicate dynamic trap info. It is of type
> VFIO_REGION_TYPE_DYNAMIC_TRAP_BAR_INFO and subtype
> VFIO_REGION_SUBTYPE_DYNAMIC_TRAP_BAR_INFO.
> 
> This region has two fields: dt_fd and trap.
> When QEMU detects a device regions of this type, it will create an
> eventfd and write its eventfd id to dt_fd field.
> When vendor drivre signals this eventfd, QEMU reads trap field of this
> info region.
> - If trap is true, QEMU would search the device's PCI BAR
> regions and disable all the sparse mmaped subregions (if the sparse
> mmaped subregion is disablable).
> - If trap is false, QEMU would re-enable those subregions.
> 
> A typical usage is
> 1. vendor driver first cuts its bar 0 into several sections, all in a
> sparse mmap array. So initally, all its bar 0 are passthroughed.
> 2. vendor driver specifys part of bar 0 sections to be disablable.
> 3. on migration starts, vendor driver signals dt_fd and set trap to true
> to notify QEMU disabling the bar 0 sections of disablable flags on.
> 4. QEMU disables those bar 0 section and hence let vendor driver be able
> to trap access of bar 0 registers and make dirty page tracking possible.
> 5. on migration failure, vendor driver signals dt_fd to QEMU again.
> QEMU reads trap field of this info region which is false and QEMU
> re-passthrough the whole bar 0 region.
> 
> Vendor driver specifies whether it supports dynamic-trap-bar-info region
> through cap VFIO_PCI_DEVICE_CAP_DYNAMIC_TRAP_BAR in
> vfio_pci_mediate_ops->open().
> 
> If vfio-pci detects this cap, it will create a default
> dynamic_trap_bar_info region on behalf of vendor driver with region len=0
> and region->ops=null.
> Vvendor driver should override this region's len, flags, rw, mmap in its
> vfio_pci_mediate_ops.

TBH, I don't like this interface at all.  Userspace doesn't pass data
to the kernel via INFO ioctls.  We have a SET_IRQS ioctl for
configuring user signaling with eventfds.  I think we only need to
define an IRQ type that tells the user to re-evaluate the sparse mmap
information for a region.  The user would enumerate the device IRQs via
GET_IRQ_INFO, find one of this type where the IRQ info would also
indicate which region(s) should be re-evaluated on signaling.  The user
would enable that signaling via SET_IRQS and simply re-evaluate the
sparse mmap capability for the associated regions when signaled.
Thanks,

Alex

> 
> Cc: Kevin Tian <kevin.tian@intel.com>
> 
> Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
> ---
>  drivers/vfio/pci/vfio_pci.c | 16 ++++++++++++++++
>  include/linux/vfio.h        |  3 ++-
>  include/uapi/linux/vfio.h   | 11 +++++++++++
>  3 files changed, 29 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
> index 059660328be2..62b811ca43e4 100644
> --- a/drivers/vfio/pci/vfio_pci.c
> +++ b/drivers/vfio/pci/vfio_pci.c
> @@ -127,6 +127,19 @@ void init_migration_region(struct vfio_pci_device *vdev)
>  		NULL);
>  }
>  
> +/**
> + * register a region to hold info for dynamically trap bar regions
> + */
> +void init_dynamic_trap_bar_info_region(struct vfio_pci_device *vdev)
> +{
> +	vfio_pci_register_dev_region(vdev,
> +		VFIO_REGION_TYPE_DYNAMIC_TRAP_BAR_INFO,
> +		VFIO_REGION_SUBTYPE_DYNAMIC_TRAP_BAR_INFO,
> +		NULL, 0,
> +		VFIO_REGION_INFO_FLAG_READ | VFIO_REGION_INFO_FLAG_WRITE,
> +		NULL);
> +}
> +
>  static void vfio_pci_probe_mmaps(struct vfio_pci_device *vdev)
>  {
>  	struct resource *res;
> @@ -538,6 +551,9 @@ static int vfio_pci_open(void *device_data)
>  				if (caps & VFIO_PCI_DEVICE_CAP_MIGRATION)
>  					init_migration_region(vdev);
>  
> +				if (caps & VFIO_PCI_DEVICE_CAP_DYNAMIC_TRAP_BAR)
> +					init_dynamic_trap_bar_info_region(vdev);
> +
>  				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,
> diff --git a/include/linux/vfio.h b/include/linux/vfio.h
> index cddea8e9dcb2..cf8ecf687bee 100644
> --- a/include/linux/vfio.h
> +++ b/include/linux/vfio.h
> @@ -197,7 +197,8 @@ extern void vfio_virqfd_disable(struct virqfd **pvirqfd);
>  
>  struct vfio_pci_mediate_ops {
>  	char	*name;
> -#define VFIO_PCI_DEVICE_CAP_MIGRATION (0x01)
> +#define VFIO_PCI_DEVICE_CAP_MIGRATION		(0x01)
> +#define VFIO_PCI_DEVICE_CAP_DYNAMIC_TRAP_BAR	(0x02)
>  	int	(*open)(struct pci_dev *pdev, u64 *caps, u32 *handle);
>  	void	(*release)(int handle);
>  	void	(*get_region_info)(int handle,
> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> index caf8845a67a6..74a2d0b57741 100644
> --- a/include/uapi/linux/vfio.h
> +++ b/include/uapi/linux/vfio.h
> @@ -258,6 +258,9 @@ struct vfio_region_info {
>  struct vfio_region_sparse_mmap_area {
>  	__u64	offset;	/* Offset of mmap'able area within region */
>  	__u64	size;	/* Size of mmap'able area */
> +	__u32	disablable;	/* whether this mmap'able are able to
> +				 *  be dynamically disabled
> +				 */
>  };
>  
>  struct vfio_region_info_cap_sparse_mmap {
> @@ -454,6 +457,14 @@ struct vfio_device_migration_info {
>  #define VFIO_DEVICE_DIRTY_PFNS_ALL	(~0ULL)
>  } __attribute__((packed));
>  
> +/* Region type and sub-type to hold info to dynamically trap bars */
> +#define VFIO_REGION_TYPE_DYNAMIC_TRAP_BAR_INFO		(4)
> +#define VFIO_REGION_SUBTYPE_DYNAMIC_TRAP_BAR_INFO	(1)
> +
> +struct vfio_device_dt_bar_info_region {
> +	__u32 dt_fd; /* fd of eventfd to notify qemu trap/untrap bars*/
> +	__u32 trap;   /* trap/untrap bar regions */
> +};
>  
>  /* sub-types for VFIO_REGION_TYPE_PCI_* */
>