[PATCH v3 1/2] qemu: add support for shmem-{plain, doorbell} role

[Martin Kletzander]

On Mon, Aug 03, 2020 at 09:26:19AM +0000, Wangxin (Alexander) wrote:
>> On Fri, Jul 24, 2020 at 11:34:11AM +0800, Wang Xin wrote:
>> >Role(master or peer) controls how the domain behaves on migration.
>> >For more details about migration with ivshmem, see
>> >https://git.qemu.org/?p=qemu.git;a=blob_plain;f=docs/system/ivshmem.rst;hb=HEAD
>> >
>> >It's a optional attribute in libvirt, and qemu will choose default
>> >role for ivshmem device if the user is not specified.
>> >
>> >With device property 'role', the value can be 'master' or 'peer'.
>> > - 'master' (means 'master=on' in qemu), the guest will copy
>> >   the shared memory on migration to the destination host.
>> > - 'peer' (means 'master=off' in qemu), the migration is disabled.
>> >
>> >Signed-off-by: Martin Kletzander <mkletzan at redhat.com>
>> >Signed-off-by: Yang Hang <yanghang44 at huawei.com>
>> >Signed-off-by: Wang Xin <wangxinxin.wang at huawei.com>
>> >
>> >diff --git a/src/qemu/qemu_migration.c b/src/qemu/qemu_migration.c
>> >index 2c7bf349c3..e588b74c2f 100644
>> >--- a/src/qemu/qemu_migration.c
>> >+++ b/src/qemu/qemu_migration.c
>> >@@ -1261,10 +1261,22 @@ qemuMigrationSrcIsAllowed(virQEMUDriverPtr driver,
>> >             }
>> >         }
>> >
>> >-        if (vm->def->nshmems) {
>> >-            virReportError(VIR_ERR_OPERATION_INVALID, "%s",
>> >-                           _("migration with shmem device is not supported"));
>> >-            return false;
>> >+        for (i = 0; i < vm->def->nshmems; i++) {
>> >+            virDomainShmemDefPtr shmem = vm->def->shmems[i];
>> >+
>> >+            if (shmem->model == VIR_DOMAIN_SHMEM_MODEL_IVSHMEM) {
>> >+                virReportError(VIR_ERR_OPERATION_INVALID, "%s",
>> >+                               _("migration with legacy shmem device is not supported"));
>> >+                return false;
>> >+            }
>> >+            if (shmem->role == VIR_DOMAIN_SHMEM_ROLE_PEER) {
>>
>> This allows migration with the default role which you leave to be chosen by the
>> hypervisor.  In that case we need to change this to:
>>
>>    if (shmem->role != VIR_DOMAIN_SHMEM_ROLE_MASTER) {
>
>In fact, the hypervisor will also check weather the ivshmem device is migratable(by
>using migration blocker) or not.
>If 'role' is not specified in libvirt, the default role may 'master' or 'peer', we don't kown it,
>so, I think it's better to check it in hypervisor.
>

We strive to prevent any errors in QEMU "just in case".  Users could start
doubting our backwards compatibility if their `default` role shmem stopped
migrating in the future, for example.  And by being strict we avoid any possible
future issues like this.

>>
>> >+                virReportError(VIR_ERR_OPERATION_INVALID,
>> >+                               _("shmem device '%s' with role='%s', "
>> >+                                 "cannot be migrated"),
>>
>> and this accordingly.
>>
>> But that's fine, I can change that before pushing.  With that change:
>>
>> Reviewed-by: Martin Kletzander <mkletzan at redhat.com>
>>
>> but I'll wait after the release since this is a feature.
>
>
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