On Thu, 24 Nov 2022 19:39:20 +0200
Yishai Hadas <yishaih@xxxxxxxxxx> wrote:
> +/**
> + * VFIO_MIG_GET_PRECOPY_INFO - _IO(VFIO_TYPE, VFIO_BASE + 21)
> + *
> + * This ioctl is used on the migration data FD in the precopy phase of the
> + * migration data transfer. It returns an estimate of the current data sizes
> + * remaining to be transferred. It allows the user to judge when it is
> + * appropriate to leave PRE_COPY for STOP_COPY.
> + *
> + * This ioctl is valid only in PRE_COPY states and kernel driver should
> + * return -EINVAL from any other migration state.
> + *
> + * The vfio_precopy_info data structure returned by this ioctl provides
> + * estimates of data available from the device during the PRE_COPY states.
> + * This estimate is split into two categories, initial_bytes and
> + * dirty_bytes.
> + *
> + * The initial_bytes field indicates the amount of initial mandatory precopy
> + * data available from the device. This field should have a non-zero initial
> + * value and decrease as migration data is read from the device.
> + * It is a must to leave PRE_COPY for STOP_COPY only after this field reach
> + * zero.
Is this actually a requirement that's compatible with current QEMU
behavior? It's my impression that a user can force the migration to
move to STOP_COPY at any point in time. Thanks,
Alex
> + *
> + * The dirty_bytes field tracks device state changes relative to data
> + * previously retrieved. This field starts at zero and may increase as
> + * the internal device state is modified or decrease as that modified
> + * state is read from the device.
> + *
> + * Userspace may use the combination of these fields to estimate the
> + * potential data size available during the PRE_COPY phases, as well as
> + * trends relative to the rate the device is dirtying its internal
> + * state, but these fields are not required to have any bearing relative
> + * to the data size available during the STOP_COPY phase.
> + *
> + * Drivers have a lot of flexibility in when and what they transfer during the
> + * PRE_COPY phase, and how they report this from VFIO_MIG_GET_PRECOPY_INFO.
> + *
> + * During pre-copy the migration data FD has a temporary "end of stream" that is
> + * reached when both initial_bytes and dirty_byte are zero. For instance, this
> + * may indicate that the device is idle and not currently dirtying any internal
> + * state. When read() is done on this temporary end of stream the kernel driver
> + * should return ENOMSG from read(). Userspace can wait for more data (which may
> + * never come) by using poll.
> + *
> + * Once in STOP_COPY the migration data FD has a permanent end of stream
> + * signaled in the usual way by read() always returning 0 and poll always
> + * returning readable. ENOMSG may not be returned in STOP_COPY. Support
> + * for this ioctl is optional.
> + *
> + * Return: 0 on success, -1 and errno set on failure.
> + */
> +struct vfio_precopy_info {
> + __u32 argsz;
> + __u32 flags;
> + __aligned_u64 initial_bytes;
> + __aligned_u64 dirty_bytes;
> };
>
> +#define VFIO_MIG_GET_PRECOPY_INFO _IO(VFIO_TYPE, VFIO_BASE + 21)
> +
> /*
> * Upon VFIO_DEVICE_FEATURE_SET, allow the device to be moved into a low power
> * state with the platform-based power management. Device use of lower power
