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Re: [Qemu-devel] Why I advise against using ivshmem


From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] Why I advise against using ivshmem
Date: Tue, 17 Jun 2014 10:54:04 +0800

On Fri, Jun 13, 2014 at 10:10 PM, Paolo Bonzini <address@hidden> wrote:
> Il 13/06/2014 15:41, Vincent JARDIN ha scritto:
>> I do repeat this use case that you had removed because vhost-user does
>> not solve it yet:
>>
>>>>  - ivshmem -> framework to be generic to have shared memory for many
>>>> use cases (HPC, in-memory-database, a network too like memnic).
>
>
> Right, ivshmem is better for guest-to-guest.  vhost-user is not restricted
> to networking, but it is indeed more focused on guest-to-host.  ivshmem is
> usable for guest-to-host, but I would prefer still some "hybrid" that uses
> vhost-like messages to pass the shared memory fds to the external program.

ivshmem has a performance disadvantage for guest-to-host
communication.  Since the shared memory is exposed as PCI BARs, the
guest has to memcpy into the shared memory.

vhost-user can access guest memory directly and avoid the copy inside the guest.

Unless someone steps up and maintains ivshmem, I think it should be
deprecated and dropped from QEMU.

Stefan



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