Re: [Qemu-devel] [PATCH v2 0/2] e1000e: Reimplement e1000 as a variant of e1000e

[Ed Swierk]

On Thu, Nov 9, 2017 at 5:53 AM, Daniel P. Berrange <address@hidden> wrote:
> My fear is that this approach of building a new e1000-ng device in
> parallel with having the existing e1000 device is going to cause
> long term pain, possibly never getting to a state where the e1000-ng
> device can replace the e1000 device. Any time there needs to be a
> "big bang" to switch from one impl to another completely different
> impl always causes trouble IME. With need for migration wire format
> & state compatibility, this is even more difficult. From a code review
> POV it will be essentially impossible to have confidence that the new
> impl can be a viable drop-in replacement for the old impl.
>
> Is there really no way that you can change the approach to do a more
> incremental conversion of the existing code base, but still end up in
> the same place at the very end ?
>
> eg just copy all the e1000.c code into the e1000e.c file to start with.
> Then gradually merge functional areas over a longish series of patches
> to eliminate the duplication. This would make it far more practical to
> identify where any regressions come from, and will give reviewers more
> confidence that we're not breaking migration compat.

I agree an incremental conversion is the only realistic way to unearth
potential regressions; testing won't be enough. In the past couple of
days I've run into several cases where e1000 works but e1000-ng
doesn't. Apparently e1000e guest drivers just happen not to tickle
these bugs. So e1000-ng isn't ready for prime time anyway. I'll shelve
this patch series for the moment.

Meanwhile I'll post separate patches for the bugs I've encountered
with e1000 UDP checksum offload.

--Ed