Re: [Qemu-devel] [PATCH 2/4] e1000e: No need to validate configuration on migration

[Eduardo Habkost]

On Thu, Oct 27, 2016 at 01:22:29PM +0100, Dr. David Alan Gilbert wrote:
> * Eduardo Habkost (address@hidden) wrote:
> > On Thu, Oct 27, 2016 at 09:42:53AM +0300, Dmitry Fleytman wrote:
> > > 
> > > > On 26 Oct 2016, at 22:21 PM, Eduardo Habkost <address@hidden> wrote:
> > > > 
> > > > The user (or management software) is responsible for keeping the
> > > > same configuration on both sides while migrating. Remove the
> > > > configuration validation code at e1000e_post_load, and the
> > > > unnecessary subsys_used/subsys_ven_used fields.
> > > 
> > > Migration with different subsys/subsys_ven may have a
> > > really cumbersome consequences that are very hard to debug,
> > > so I believe such a verification may save a lot of time,
> > > however it’s a matter of convention.
> > 
> > The same issues apply to almost every other property on every
> > other device. But the convention is to not expect devices to
> > validate configuration changes on migration.
> > 
> > (It would be nice to have a more generic mechanism that detects
> > configuration mismatch on migration, though. Having each device
> > manually checking each of its properties wouldn't work).
> 
> Without knowing the e1000e code, isn't the easy way to do this
> to change the:
> 
>         VMSTATE_UINT16(subsys, E1000EState),
>         VMSTATE_UINT16(subsys_ven, E1000EState),
> to
> 
>         VMSTATE_UINT16_EQUAL(subsys, E1000EState),
>         VMSTATE_UINT16_EQUAL(subsys_ven, E1000EState),
> 
> and the migration code will flag if they don't match for you.

That's very useful. I didn't know about it. I will submit a new
version of the series that uses VMSTATE_UINT16_EQUAL.

-- 
Eduardo