[libvirt] [RFC] require for suggestions on support for ivshmem device

[Martin Kletzander]

On Wed, May 14, 2014 at 08:23:21AM +0000, Wangrui (K) wrote:
>Hi,
>
>Libvirt does not support ivshmem(Inter-VM Shared Memory) device recently,
>thus, I would like to know if there's any plan to support it in the future?
>If not, I would like to contribute a serial of patches to do so.
>
>On Jan 28, Wangyufei (James) asked about this question, and Daniel replied with 2 suggestions:
>1. Libvirt should be capable of configuring the guest's xml on ivshmem.
>2.An ivshmem daemon is needed to run on the host to support it, libvirt is recommended to provide such a daemon.
>Please refer to  https://www.redhat.com/archives/libvir-list/2014-January/msg01335.html for details.
>
>What I'll do later is the 1st suggestion, the 2nd one is left to be accomplished by someone else.
>
>Here is the detailed work I'll do to support configuration of the guest in libvirt:
>virDomainDefParseXML:                 parse ivshmem device xml when defining dom.xml
>virDomainDeviceInfoIterateInternal:       iterate ivshmem devices
>qemuAssignDevicePCISlots:              assign ivshmem device pci slots
>virDomainDefFormatInternal:             format ivshmem device xml (Eg. virsh edit dom)
>virDomainDefFree:                     free ivshmem device def
>
>qemuBuildCommandLine:               build ivshmem device command line when vm starts
>qemuAssignDeviceAliases:              assign ivshmem device aliases when vm starts
>
>virDomainDeviceDefParse:               attach and parse ivshmem device xml
>qemuDomainAttachDeviceConfig:         attach ivshmem device xml persistently
>qemuDomainAttachDeviceLive:           attach ivshmem device online
>
>qemuDomainDetachDeviceConfig:         detach ivshmem device xml persistently
>qemuDomainDetachDeviceLive:           detach ivshmem device online
>

Don't forget about checking for the qemu capability and error-ing out
properly in case it's not supported, you probably know you can use
qemuBuildChrChardevStr() for the '-chardev' part of the commandline,
various backends are supported and the code is in already.

The idea looks good, it would be nice improvement to have.  About the
daemon, you mean it would be another daemon we have in the repo like
virtlockd, I suppose.

>
>There are two ways to use ivshmem with qemu
>(please refer to http://qemu.weilnetz.de/qemu-doc.html#pcsys_005fother_005fdevs ):
>1.Guests map a POSIX shared memory region into the guest as a PCI device
>that enables zero-copy communication to the application level of the guests, The basic syntax is:
>
>  qemu-system-i386-device ivshmem, size = <size in format accepted by -m> [, shm = <shm name>]
>
>2.If desired, interrupts can be sent between guest VMs accessing the same shared memory region.
>Interrupt support requires using a shared memory server and using a chardev socket to connect to it.
>An example syntax when using the shared memory server is:
>
>  qemu-system-i386-device ivshmem, size = <size in format accepted by -m> [, chardev = <id>] [, msi = on]
>                               [, ioeventfd = on] [, vectors = n] [, role = peer | master]
>  qemu-system-i386-chardev socket, path = <path>, id = <id>
>
>The respective xml configuration for the above 2 qemu command lines are shown as below:
>
>Example1: automatically attach device with KVM
>
>  <devices>
>    <ivshmem role='master'>
>      <memory name='dom-ivshmem' size='2'/>
>    </ivshmem>
>  </devices>
>
>NOTE: "size" means ivshmem size in unit MB, "name" means shm name
>      "role" is optional, it may be set to "master" or "peer", the default one is "master"
>

What do these roles mean, I mean what's the difference between master
and peer and why is it only used with the chardev?  Does it mean
master can only send interrupts or...?  Just curious.

>Example2: manually attach device with static PCI slot 4 requested
>
>  <devices>
>    <ivshmem role='master'>
>      <memory name='dom-ivshmem' size='2'/>
>      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
>    </ivshmem>
>  </devices>
>
>Example3: automatically attach device with KVM
>
>  <devices>
>    <ivshmem role='master' type='unix'>
>      <source mode='connect' path='/tmp/ivshmem'/>
>      <memory name='dom-ivshmem' size='2'/>
>    </ivshmem>
>  </devices>
>
>NOTE: "path" means shared memory socket path which is set by the daemon.
>      " source mode " and "type" is similar with vmchannel.
>
>
>I'm looking forward to your suggestions, thank you very much.
>
>--
>libvir-list mailing list
>libvir-list at redhat.com
>https://www.redhat.com/mailman/listinfo/libvir-list
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