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Re: [Qemu-devel] [Questions] about the VHOST_MEMORY_MAX_NREGIONS of vhos

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [Questions] about the VHOST_MEMORY_MAX_NREGIONS of vhost-user backend?
Date: Fri, 24 Nov 2017 23:07:36 +0200

On Fri, Nov 24, 2017 at 10:28:48AM +0000, Gonglei (Arei) wrote:
> 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?

I think we can extend it with a protocol flag.

> static int vhost_user_memslots_limit(struct vhost_dev *dev)
> {
> }
> Which is introduced by 
> commit 5f6f6664bf24dc53f4bf98ba812d55ca93684cd5
> Author: Nikolay Nikolaev <address@hidden>
> 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 <address@hidden>
>     Signed-off-by: Nikolay Nikolaev <address@hidden>
>     Reviewed-by: Michael S. Tsirkin <address@hidden>
>     Signed-off-by: Michael S. Tsirkin <address@hidden>
> Thanks,
> -Gonglei

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