Re: [PATCH v2 1/3] hw/net: e1000: Correct the initial value of VET register

[Bin Meng <bmeng.cn@gmail.com>]

On Wed, Jul 14, 2021 at 5:01 PM Peter Maydell <peter.maydell@linaro.org> wrote:
>
> On Wed, 14 Jul 2021 at 04:42, Bin Meng <bmeng.cn@gmail.com> wrote:
> >
> > On Wed, Jul 14, 2021 at 11:10 AM Jason Wang <jasowang@redhat.com> wrote:
> > >
> > >
> > > 在 2021/7/13 下午5:11, Bin Meng 写道:
> > > > Can we get some agreement among maintainers?
> > >
> > >
> > > It's not about the agreement but about to have a stable ABI. I don't
> > > know the case for sd but e1000 is used in various  and we work hard to
> > > unbreak the migration compatibility among downstream versions. Git log
> > > on e1000.c will tell you more.
> >
> > Agreement or stable ABI, whatever we call, but we should be in some consistency.
> >
> > IMHO maintainers should reach an agreement to some extent on how
> > compatibility should be achieved. I just found silly to add a property
> > to fix a real bug in the model, and we preserve the bug all over
> > releases.
> >
> > I can find plenty of examples in the current QEMU tree that were
> > accepted that changed the bugous register behavior, but it was not
> > asked to add new properties to keep the bugos behavior.
> >
> > e.g.: commit ce8e43760e8e ("hw/net: fsl_etsec: Reverse the RCTRL.RSF logic")
>
> There is basically a judgement call going on here about whether the
> device is "significant enough" that it's worth the effort of
> preserving back-compatibility of bugs.
>
> There is at least one clear rule: if the device isn't used by any
> machine with a versioned machine type, then there is no need to
> provide back-compatibility of old buggy behaviour. (There would
> be no way for the user to select the old behaviour by choosing a
> -4.2 machine type.) This is why the fsl_etsec device doesn't need
> to handle that.
>
> For PCI devices it's a bit fuzzier because in theory you can plug
> any PCI card into a versioned x86 PC machine.
>
> We don't want to preserve bug-compatibility for absolutely
> everything, because the codebase would quickly clog up with weird
> behaviour that we never test and which is not of any use to users
> either. So you have to look at:
>  * how easy is providing the back-compat?
>  * how commonly used in production is the device?
>  * how likely is it that guests might care?
>  * would the behaviour change cause odd behaviour across
>    a cross-version migration from the old QEMU?
>
> Migration compat is similar, but not quite the same because the
> compatibility handling tends to be less invasive, so we take the
> "provide compat" choice more often. For non-versioned machine types,
> again, we're OK with breaking back-compat as long as we bump a
> migration version number so the user gets an error message rather
> than weird behaviour.

Thanks Peter. I think the above information can be put in a doc in
docs/devel, and with some real examples to help developers on how to
achieve backward compatibility.

Regarding this series, as I mentioned in an earlier thread, I believe
the possibility of breaking a guest is very low. Adding a back-compat
check is not that hard either. Just not sure which factor weighs more.

Regards,
Bin