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. > > A few questions: > 1. > What exactly is the check `if (vhost_user_one_time_request(msg->hdr.request) > && dev->vq_index != 0)` for? 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? > > 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?
Hi Raphael, David Gilbert and I recently came to the conclusion that memory hotplug is not safe in vhost-user device backends built using libvhost-user. It's likely that memory hotplug hasn't been fully thought through at the protocol specification and QEMU vhost-user master implementation levels either. We didn't investigate deeper for the time being, but I'm not surprised that you've found inconsistencies. The ability to hotplug memory is a valuable feature. It will be necessary to get it working sooner or later. Are you going to work on it? Stefan
signature.asc
Description: PGP signature
