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

[Michael S. Tsirkin]

On Thu, Jul 16, 2015 at 09:26:21AM +0200, Igor Mammedov wrote:
> 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.

I think we need a way for hotplug to fail gracefully.  As long as we
don't implement the hva trick, it's needed for old kernels with vhost in
kernel, too.

-- 
MST