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Re: [Qemu-block] [Qemu-devel] [PATCH] tcmu: Introduce qemu-tcmu utility

From: Fam Zheng
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH] tcmu: Introduce qemu-tcmu utility
Date: Tue, 12 Mar 2019 10:18:11 +0800

> On Mar 11, 2019, at 19:06, Kevin Wolf <address@hidden> wrote:
> Am 09.03.2019 um 02:46 hat Yaowei Bai geschrieben:
>> Thanks for explaining the background. It comes to my mind that actually we
>> talked about these two cases with Fam a bit long time ago and decided to
>> support both these two cases. The reason why we implement case2 first is
>> currently we care more about exporting new opened images and it's a bit
>> more convenient, exporting from a VM or QMP can be added in the later
>> release. Do you think it's reasonable/acceptable that we support both
>> cases and use case2 for normal new opened images and case1 for the
>> circumstances you mention above?
> I would like to avoid a second code path because it comes with a
> maintenance cost.
> Experience also tells that adding a new way to parse option strings will
> come back at us later because it we must always maintain compatibility
> with yet another format.

If the rule is that cfgstr strictly follows -blockdev syntax and the parsing 
code is mostly shared, it shouldn’t be a problem, right? I suppose this will 
limit the expressiveness of cfgstr but might be a reasonable tradeoff between 
implementation complexity, user friendliness and interface consistency.

Another possibility is to use json: format in cfgstr for anything more complex 
than a single -blockdev.

> So I would prefer not doing this and just passing command line options
> to qemu-tcmu, which can reuse the already existing code paths.

I think the effect of not supporting adding new blockdev from cfgstr is that we 
have to resort to QMP to allow hot-adding targets. It’s not necessarily bad, 
though I’m not sure hwo that aligns with Yaowei’s development plan.


> Kevin

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