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Wed, 14 Jul 2021 02:24:54 -0700 (PDT) X-Google-Smtp-Source: ABdhPJxGHDjt6e/+MSV2P4ZJ8V/LmSxTfmQN3xfdoAlep5MJiCrekEo0y6Rsn5pE2SMTGyUu1JWqsw== X-Received: by 2002:a62:b414:0:b029:32e:3ef0:7735 with SMTP id h20-20020a62b4140000b029032e3ef07735mr6579861pfn.61.1626254694278; Wed, 14 Jul 2021 02:24:54 -0700 (PDT) Received: from wangxiaodeMacBook-Air.local ([209.132.188.80]) by smtp.gmail.com with ESMTPSA id t10sm2109528pgv.52.2021.07.14.02.24.51 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 14 Jul 2021 02:24:53 -0700 (PDT) Subject: Re: [PATCH v2 1/3] hw/net: e1000: Correct the initial value of VET register To: Bin Meng , Peter Maydell References: <20210702092427.1323667-1-bmeng.cn@gmail.com> <434daef6-4afb-c796-9b63-f72cca403314@redhat.com> <63ff5849-d830-87cc-486c-7fc292220424@redhat.com> <891042a0-10d5-c1df-ae85-7950ae387a41@redhat.com> <62d56b3b-ef11-2384-d3cd-0d34046400ee@redhat.com> From: Jason Wang Message-ID: 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 MIME-Version: 1.0 In-Reply-To: Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=jasowang@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Received-SPF: pass client-ip=216.205.24.124; envelope-from=jasowang@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -38 X-Spam_score: -3.9 X-Spam_bar: --- X-Spam_report: (-3.9 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.7, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.368, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Bin Meng , Christina Wang , =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= , Markus Carlstedt , "qemu-devel@nongnu.org Developers" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" 在 2021/7/14 下午5:14, Bin Meng 写道: > On Wed, Jul 14, 2021 at 5:01 PM Peter Maydell wrote: >> On Wed, 14 Jul 2021 at 04:42, Bin Meng wrote: >>> On Wed, Jul 14, 2021 at 11:10 AM Jason Wang 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. Just few lines of code for the compatibility. Let's do that. Thanks > > Regards, > Bin >