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Re: [Qemu-devel] [PATCH 37/38] ivshmem: Require master to have ID zero


From: Marc-André Lureau
Subject: Re: [Qemu-devel] [PATCH 37/38] ivshmem: Require master to have ID zero
Date: Thu, 3 Mar 2016 15:11:39 +0100

On Mon, Feb 29, 2016 at 7:40 PM, Markus Armbruster <address@hidden> wrote:
> Migration with ivshmem needs to be carefully orchestrated to work.
> Exactly one peer (the "master") migrates to the destination, all other
> peers need to unplug (and disconnect), migrate, plug back (and
> reconnect).  This is sort of documented in qemu-doc.
>
> If peers connect on the destination before migration completes, the
> shared memory can get messed up.  This isn't documented anywhere.  Fix
> that in qemu-doc.
>
> To avoid messing up register IVPosition on migration, the server must
> assign the same ID on source and destination.  ivshmem-spec.txt leaves
> ID assignment unspecified, however.
>
> Amend ivshmem-spec.txt to require the first client to receive ID zero.
> The example ivshmem-server complies: it always assigns the first
> unused ID.
>
> For a bit of additional safety, enforce ID zero for the master.  This
> does nothing when we're not using a server, because the ID is zero for
> all peers then.
>
> Signed-off-by: Markus Armbruster <address@hidden>
> ---

Reviewed-by: Marc-André Lureau <address@hidden>


>  docs/specs/ivshmem-spec.txt | 2 ++
>  hw/misc/ivshmem.c           | 6 ++++++
>  qemu-doc.texi               | 5 +++++
>  3 files changed, 13 insertions(+)
>
> diff --git a/docs/specs/ivshmem-spec.txt b/docs/specs/ivshmem-spec.txt
> index 047dfc0..b062acd 100644
> --- a/docs/specs/ivshmem-spec.txt
> +++ b/docs/specs/ivshmem-spec.txt
> @@ -164,6 +164,8 @@ For each new client that connects to the server, the 
> server
>  - sends interrupt setup messages to the new client (these contain file
>    descriptors for receiving interrupts).
>
> +The first client to connect to the server receives ID zero.
> +
>  When a client disconnects from the server, the server sends disconnect
>  notifications to the other clients.
>
> diff --git a/hw/misc/ivshmem.c b/hw/misc/ivshmem.c
> index f6fce15..9a292e5 100644
> --- a/hw/misc/ivshmem.c
> +++ b/hw/misc/ivshmem.c
> @@ -869,6 +869,12 @@ static void ivshmem_common_realize(PCIDevice *dev, Error 
> **errp)
>              return;
>          }
>
> +        if (s->master == ON_OFF_AUTO_ON && s->vm_id != 0) {
> +            error_setg(errp,
> +                       "master must connect to the server before any peers");
> +            return;
> +        }
> +
>          qemu_chr_add_handlers(s->server_chr, ivshmem_can_receive,
>                                ivshmem_read, NULL, s);
>
> diff --git a/qemu-doc.texi b/qemu-doc.texi
> index 0dd01c7..79141d3 100644
> --- a/qemu-doc.texi
> +++ b/qemu-doc.texi
> @@ -1295,12 +1295,17 @@ When using the server, the guest will be assigned a 
> VM ID (>=0) that allows gues
>  using the same server to communicate via interrupts.  Guests can read their
>  VM ID from a device register (see ivshmem-spec.txt).
>
> address@hidden Migration with ivshmem
> +
>  With device property @option{master=on}, the guest will copy the shared
>  memory on migration to the destination host.  With @option{master=off},
>  the guest will not be able to migrate with the device attached.  In the
>  latter case, the device should be detached and then reattached after
>  migration using the PCI hotplug support.
>
> +At most one of the devices sharing the same memory can be master.  The
> +master must complete migration before you plug back the other devices.
> +
>  @subsubsection ivshmem and hugepages
>
>  Instead of specifying the <shm size> using POSIX shm, you may specify
> --
> 2.4.3
>
>



-- 
Marc-André Lureau



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