Re: [PATCH v1 4/4] virtio-mem: Support "x-ignore-shared" migration

[David Hildenbrand <david@redhat.com>]

On 20.06.23 15:06, Michael S. Tsirkin wrote:
> On Tue, Jun 20, 2023 at 03:03:54PM +0200, David Hildenbrand wrote:
>> To achieve desired "x-ignore-shared" functionality, we should not
>> discard all RAM when realizing the device and not mess with
>> preallocation/postcopy when loading device state. In essence, we should
>> not touch RAM content.
>>
>> As "x-ignore-shared" gets set after realizing the device, we cannot
>> rely on that. Let's simply skip discarding of RAM on incoming migration.
>> Note that virtio_mem_post_load() will call
>> virtio_mem_restore_unplugged() -- unless "x-ignore-shared" is set. So
>> once migration finished we'll have a consistent state.
>>
>> The initial system reset will also not discard any RAM, because
>> virtio_mem_unplug_all() will not call virtio_mem_unplug_all() when no
>> memory is plugged (which is the case before loading the device state).
>>
>> Note that something like VM templating -- see commit b17fbbe55cba
>> ("migration: allow private destination ram with x-ignore-shared") -- is
>> currently incompatible with virtio-mem and ram_block_discard_range() will
>> warn in case a private file mapping is supplied by virtio-mem.
>>
>> For VM templating with virtio-mem, it makes more sense to either
>> (a) Create the template without the virtio-mem device and hotplug a
>>      virtio-mem device to the new VM instances using proper own memory
>>      backend.
>> (b) Use a virtio-mem device that doesn't provide any memory in the
>>      template (requested-size=0) and use private anonymous memory.
>>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>   hw/virtio/virtio-mem.c | 47 ++++++++++++++++++++++++++++++++++--------
>>   1 file changed, 38 insertions(+), 9 deletions(-)
>>
>> diff --git a/hw/virtio/virtio-mem.c b/hw/virtio/virtio-mem.c
>> index 9f6169af32..b013dfbaf0 100644
>> --- a/hw/virtio/virtio-mem.c
>> +++ b/hw/virtio/virtio-mem.c
>> @@ -18,6 +18,7 @@
>>   #include "sysemu/numa.h"
>>   #include "sysemu/sysemu.h"
>>   #include "sysemu/reset.h"
>> +#include "sysemu/runstate.h"
>>   #include "hw/virtio/virtio.h"
>>   #include "hw/virtio/virtio-bus.h"
>>   #include "hw/virtio/virtio-access.h"
>> @@ -886,11 +887,23 @@ static void virtio_mem_device_realize(DeviceState *dev, Error **errp)
>>           return;
>>       }
>>   
>> -    ret = ram_block_discard_range(rb, 0, qemu_ram_get_used_length(rb));
>> -    if (ret) {
>> -        error_setg_errno(errp, -ret, "Unexpected error discarding RAM");
>> -        ram_block_coordinated_discard_require(false);
>> -        return;
>> +    /*
>> +     * We don't know at this point whether shared RAM is migrated using
>> +     * QEMU or migrated using the file content. "x-ignore-shared" will be
>> +     * configurated
> 
> configurated == configured?

Thanks!

-- 
Cheers,

David / dhildenb