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[Qemu-devel] Should memory hotplug work with vhost-user backends?

From: Raphael Norwitz
Subject: [Qemu-devel] Should memory hotplug work with vhost-user backends?
Date: Tue, 2 Jul 2019 22:08:54 +0000

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:
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?

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?


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