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

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] Should memory hotplug work with vhost-user backends?
Date: Wed, 3 Jul 2019 14:57:53 -0400

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

> 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.


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