On Tue, Jul 02, 2019 at 10:08:54PM +0000, Raphael Norwitz wrote: > For background I am trying to work around a ram slot limit imposed by the > vhost-user protocol. We are having trouble reconciling the comment here: > https: > //github.com/qemu/qemu/blob/master/hw/virtio/vhost-user.c#L333 that “For > non-vring specific requests, like VHOST_USER_SET_MEM_TABLE., we just need to > send it once the first time” and the high level implementation of memory > hot-add, which calls set_mem_table every time a VM hot adds memory.
IIUC the comment refers to multiple virtqueue. It is trying to say that we do not need to send VHOST_USER_SET_MEM_TABLE for each virtqueue. > > > A few questions: > > 1. > > What exactly is the check `if (vhost_user_one_time_request(msg->hdr.request) > && > dev->vq_index != 0)` for? Some backends register multiple dev instances per backend: one for each virtqueue. This check avoids sending VHOST_USER_SET_MEM_TABLE more than once in this case. > In the message for commit > b931bfbf042983f311b3b09894d8030b2755a638, which introduced the check, I see it > says “non-vring specific messages[, which should] be sent only once” and gives > VHOST_USER_SET_MEM_TABLE as an example one such message. The > `vhost_user_one_time_request()` call clearly checks whether this type of > message is the kind of message is supposed to be sent once of which > VHOST_USER_SET_MEM_TABLE is one. Why, then, does this commit add the check if > `dev->vq_index != 0`? It seems like there is a latent assumption that after > the > first call dev->vq_index should be set to some value greater than one, however > for many cases such as vhost-user-scsi devices we can see this is clearly not > the case https://github.com/qemu/qemu/blob/master/hw/scsi/vhost-user-scsi.c# > L95. Is this check then ‘broken’ for such devices? I think vhost-scsi has a single instance per backend, that is why vq_index is 0. > > > 2. > > If this check is indeed broken for such devices, and set_mem_table call is > only > supposed to be run once for such devices, is the ability to call it multiple > times technically a bug for devices such as vhost-user-scsci devices? If so, > this would imply that the existing ability to hot add memory to > vhost-user-scsi > devices is by extension technically a bug/unintended behavior. Is this the > case? > > > > Thanks, > > Raphael I don't think that is the case. It's possible that memory hotplug has bugs with vhost-user, but I don't think it's anything fundamental. -- MST
