Re: [Qemu-devel] [PATCH v4 0/7] Fix QEMU crash during memory hotplug with vhost=on

[Igor Mammedov]

On Wed, 15 Jul 2015 19:32:31 +0300
"Michael S. Tsirkin" <address@hidden> wrote:

> On Wed, Jul 15, 2015 at 05:12:01PM +0200, Igor Mammedov wrote:
> > On Thu,  9 Jul 2015 13:47:17 +0200
> > Igor Mammedov <address@hidden> wrote:
> > 
> > there also is yet another issue with vhost-user. It also has
> > very low limit on amount of memory regions (if I recall correctly 8)
> > and it's possible to trigger even without memory hotplug.
> > one just need to start QEMU with a several -numa memdev= options
> > to create a necessary amount of memory regions to trigger it.
> > 
> > lowrisk option to fix it would be increasing limit in vhost-user
> > backend.
> > 
> > another option is disabling vhost and fall-back to virtio,
> > but I don't know much about vhost if it's possible to 
> > to switch it off without loosing packets guest was sending
> > at the moment and if it will work at all with vhost.
> 
> With vhost-user you can't fall back to virtio: it's
> not an accelerator, it's the backend.
> 
> Updating the protocol to support a bigger table
> is possible but old remotes won't be able to support it.
> 
it looks like increasing limit is the only option left.

it's not ideal that old remotes /with hardcoded limit/
might not be able to handle bigger table but at least
new ones and ones that handle VhostUserMsg payload
dynamically would be able to work without crashing.