Re: [Qemu-devel] [PATCH 1/5] VFIO KABI for migration interface

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Kirti Wankhede (kwankhede@nvidia.com) wrote:
> - Defined MIGRATION region type and sub-type.
> - Defined VFIO device states during migration process.
> - Defined vfio_device_migration_info structure which will be placed at 0th
>   offset of migration region to get/set VFIO device related information.
>   Defined actions and members of structure usage for each action:
>     * To convey VFIO device state to be transitioned to.
>     * To get pending bytes yet to be migrated for VFIO device
>     * To ask driver to write data to migration region and return number of bytes
>       written in the region
>     * In migration resume path, user space app writes to migration region and
>       communicates it to vendor driver.
>     * Get bitmap of dirty pages from vendor driver from given start address
> 
> Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
> Reviewed-by: Neo Jia <cjia@nvidia.com>

<snip>

> + * Action Get buffer:
> + *      On this action, vendor driver should write data to migration region and
> + *      return number of bytes written in the region.
> + *      data.offset [output] : offset in the region from where data is written.
> + *      data.size [output] : number of bytes written in migration buffer by
> + *          vendor driver.

<snip>

> + */
> +
> +struct vfio_device_migration_info {
> +        __u32 device_state;         /* VFIO device state */
> +        struct {
> +            __u64 precopy_only;
> +            __u64 compatible;
> +            __u64 postcopy_only;
> +            __u64 threshold_size;
> +        } pending;
> +        struct {
> +            __u64 offset;           /* offset */
> +            __u64 size;             /* size */
> +        } data;

I'm curious how the offsets/size work; how does the 
kernel driver know the maximum size of state it's allowed to write?
Why would it pick a none-0 offset into the output region?

Without having dug further these feel like i/o rather than just output;
i.e. the calling process says 'put it at that offset and you've got size
bytes' and the kernel replies with 'I did put it at offset and I wrote
only this size bytes'

Dave

> +        struct {
> +            __u64 start_addr;
> +            __u64 total;
> +            __u64 copied;
> +        } dirty_pfns;
> +} __attribute__((packed));
> +
>  /* -------- API for Type1 VFIO IOMMU -------- */
>  
>  /**
> -- 
> 2.7.0
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK