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

[Stefan Hajnoczi]

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

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