On Mon, Nov 08, 2010 at 10:20:46AM -0700, Alex Williamson wrote: > On Mon, 2010-11-08 at 18:54 +0200, Michael S. Tsirkin wrote: > > On Mon, Nov 08, 2010 at 07:59:57AM -0700, Alex Williamson wrote: > > > On Mon, 2010-11-08 at 13:40 +0200, Michael S. Tsirkin wrote: > > > > On Wed, Oct 06, 2010 at 02:58:57PM -0600, Alex Williamson wrote: > > > > > Our code paths for saving or migrating a VM are full of functions that > > > > > return void, leaving no opportunity for a device to cancel a migration, > > > > > either from error or incompatibility. The ivshmem driver attempted to > > > > > solve this with a no_migrate flag on the save state entry. I think the > > > > > more generic and flexible way to solve this is to allow driver save > > > > > functions to fail. This series implements that and converts ivshmem > > > > > to uses a set_params function to NAK migration much earlier in the > > > > > processes. This touches a lot of files, but bulk of those changes are > > > > > simply s/void/int/ and tacking a "return 0" to the end of functions. > > > > > Thanks, > > > > > > > > > > Alex > > > > > > > > Well error handling is always tricky: it seems easier to > > > > require save handlers to never fail. > > > > > > Sure it's easier, but does that make it robust? > > > > More robust in the face of wwhat kind of failure? > > I really don't understand why we're having a discussion about whether > providing a means to return an error is a good thing or not. These > patches touch a lot of files, but the change is dead simple. I just don't see the motivation. Presumably your patches are there to achieve some kind of goal, right? I am trying to figure out what that goal is. Currently savevm callbacks never fail. So they return void. Why is returing 0 and adding a bunch of code to test the condition that never happens a good idea? It just seems to create more ways for devices to shoot themselves in the foot. > > > > So there's a bunch of code here but what exactly is the benefit? > > > > Since save handlers have no idea what does the remote do, > > > > what is the compatibility you mention? > > > > > > There are two users I currently have in mind. ivshmem currently makes > > > use of the register_device_unmigratable() because it makes use of host > > > specific resources and connections (aiui). This sets the no_migrate > > > flag, which is not dynamic and a bit of a band-aide. > > > The other is > > > device assignment, which needs a way to NAK a migration since physical > > > devices are never migratable. > > > > Well since all these can't be migrated ever, a fixed property actually seems > > a good match. Sure it's not dynamic but all the easier to debug. > > > > > I imagine we could at some point have > > > devices with state tied to other features that can't always be detached > > > from the host, this tries to provide the infrastructure for that to > > > happen. > > > > > > Alex > > > > Let guest control whether you can migrate? > > Sounds like something that is more likely to be abused > > than used constructively. > > s/guest/device/ So you would rather the migration failed on the > incoming side where it may not be detected And incoming migration handlers *must* validate the input, anyway. We should not plaster over this with checks on outgoing side. > or it may be detected too > late to stop the migration? > > Alex So there's a bug and device is in an unexpected state. What can we do? Assert, print an error, notify guest - all these come to mind. But stop migration? Seems arbitrary. -- MST -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
