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

From: Jason Wang
Subject: Re: [PATCH v2 1/3] hw/net: e1000: Correct the initial value of VET register
Date: Wed, 14 Jul 2021 17:24:49 +0800
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.11.0

在 2021/7/14 下午5:14, Bin Meng 写道:
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

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.

Just few lines of code for the compatibility. Let's do that.



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