[dpdk-dev] [Questions] about the VHOST_MEMORY_MAX_NREGIONS of vhost-user backend?

["Gonglei (Arei)" <arei.gonglei@huawei.com>]

Hi,

Currently, the maximum number of supported memory regions for vhost-user backends is 8, 
and the maximum supported memory regions for vhost-net backends is determined by 
" /sys/module/vhost/parameters/max_mem_regions". 

In many scenarios, the vhost-user NIC will cause the memory region to become a bottleneck, reports
 "a used vhost backend has no free memory slots left". 

Such as memory hotplug (need to support multiple memory slots), 
and GPU pass-through (need to register multiple bar regions) and so on.

So, my questions are: Why definition vhost-user memory card memory region up to 8?
Does it have any side effects if we increase the VHOST_MEMORY_MAX_NREGIONS? 
What about cross-version migration?

#define VHOST_MEMORY_MAX_NREGIONS    8

static int vhost_user_memslots_limit(struct vhost_dev *dev)
{
    return VHOST_MEMORY_MAX_NREGIONS;
}

Which is introduced by 
commit 5f6f6664bf24dc53f4bf98ba812d55ca93684cd5
Author: Nikolay Nikolaev <n.nikolaev@virtualopensystems.com>
Date:   Tue May 27 15:06:02 2014 +0300

    Add vhost-user as a vhost backend.
    
    The initialization takes a chardev backed by a unix domain socket.
    It should implement qemu_fe_set_msgfds in order to be able to pass
    file descriptors to the remote process.
    
    Each ioctl request of vhost-kernel has a vhost-user message equivalent,
    which is sent over the control socket.
    
    The general approach is to copy the data from the supplied argument
    pointer to a designated field in the message. If a file descriptor is
    to be passed it will be placed in the fds array for inclusion in
    the sendmsg control header.
    
    VHOST_SET_MEM_TABLE ignores the supplied vhost_memory structure and scans
    the global ram_list for ram blocks with a valid fd field set. This would
    be set when the '-object memory-file' option with share=on property is used.
    
    Signed-off-by: Antonios Motakis <a.motakis@virtualopensystems.com>
    Signed-off-by: Nikolay Nikolaev <n.nikolaev@virtualopensystems.com>
    Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
    Signed-off-by: Michael S. Tsirkin <mst@redhat.com>


Thanks,
-Gonglei