Re: [Qemu-devel] [PATCH] ivshmem: Store file descriptor for vhost-user negotiation

[Tetsuya Mukawa <mukawa@igel.co.jp>]

On 2016/01/12 4:20, Markus Armbruster wrote:
> Marc-André Lureau <marcandre.lureau@gmail.com> writes:
>
>> Hi
>>
>> On Fri, Dec 4, 2015 at 3:55 PM, Tetsuya Mukawa <mukawa@igel.co.jp> wrote:
>>>> What's the use case or test case?
>>> One of example is userspace device driver like DPDK PMD.
>>> Actually, I've found this fd related behavior using DPDK virtio-net PMD.
>>> Could you please check below to know my use case more?
>>> http://thread.gmane.org/gmane.comp.networking.dpdk.devel/28467/focus=28493
>> Thanks. Why not use memory-backend-file with share=on ? I think I
>> understand you want to use the same BAR address as the host shared
>> memory location to avoid extra offset computation. I don't know about
>> the limitations here. Is this extra offset really worth saving?
>> Finally, I wonder what qemu is really doing in the end ;)
> Tetsuya, I still don't understand what you're trying to accomplish.

Hi Markus,

Please let me describe it more.

We have below test program for vhost-user.
 - ./tests/vhost-user-test.c

This program doesn't share memory between test process and QEMU process.
So all memory accesses are sent as qtest command.
If both process has a shared memory that can be accessed using same
address by both processes, we can reduce above memory accessing overhead.
This is what I want to do.

So far, when vhost-user is used, the memory will not be shared because
fd isn't kept.
This is what I want to fix.

> I concur with Marc-André that -object memory-backend-FOO is how we
> allocate a memory object for something else to use.

I am not so familiar with QEMU memory architecture. So please let me
make sure.
Does vhost-user only works with memory backend objects?

If it's correct, in my case, I want to use an unix domain socket to
transfer fd of the shared memory.
To do that, do you suggest me to add 'chardev' parameter to
./backend/hostmem-file.c?

Tetsuya

>
>> Michael, could you comment about this vhost-user use case? (see above
>> linked thread)