Subject: Re: [PATCH v19 02/13] x86/setup: Use parse_crashkernel_high_low() to
 simplify code
To:     Borislav Petkov <bp@alien8.de>
CC:     Dave Young <dyoung@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Ingo Molnar" <mingo@redhat.com>, <x86@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>, <linux-kernel@vger.kernel.org>,
        Baoquan He <bhe@redhat.com>, Vivek Goyal <vgoyal@redhat.com>,
        Eric Biederman <ebiederm@xmission.com>,
        <kexec@lists.infradead.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        "Will Deacon" <will@kernel.org>,
        <linux-arm-kernel@lists.infradead.org>,
        Rob Herring <robh+dt@kernel.org>,
        Frank Rowand <frowand.list@gmail.com>,
        <devicetree@vger.kernel.org>, Jonathan Corbet <corbet@lwn.net>,
        <linux-doc@vger.kernel.org>, Randy Dunlap <rdunlap@infradead.org>,
        Feng Zhou <zhoufeng.zf@bytedance.com>,
        Kefeng Wang <wangkefeng.wang@huawei.com>,
        "Chen Zhou" <dingguo.cz@antgroup.com>,
        John Donnelly <John.p.donnelly@oracle.com>
References: <20211228132612.1860-1-thunder.leizhen@huawei.com>
 <20211228132612.1860-3-thunder.leizhen@huawei.com> <Ycs3kpZD/vpoo1AX@zn.tnic>
 <b017a8ea-989b-c251-f5c8-a8a7940877cf@huawei.com>
 <YcwN9Mfwsh/lPbbd@dhcp-128-65.nay.redhat.com> <YcwyZRDJUMniSaY9@zn.tnic>
 <Ycw8n2BvJzH9wJKG@dhcp-128-65.nay.redhat.com>
 <21736ba2-883d-1037-dbe8-299e40f7ad13@huawei.com> <YcySEdyhXysDSKn/@zn.tnic>
From:   "Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>
Message-ID: <933554c7-1fc6-8e7a-9569-9f8441e50ddf@huawei.com>
Date:   Thu, 30 Dec 2021 10:39:12 +0800
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101
 Thunderbird/60.7.0
MIME-Version: 1.0
In-Reply-To: <YcySEdyhXysDSKn/@zn.tnic>
Content-Type: text/plain; charset="utf-8"
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Precedence: bulk


On 2021/12/30 0:51, Borislav Petkov wrote:
> On Wed, Dec 29, 2021 at 11:04:21PM +0800, Leizhen (ThunderTown) wrote:
>> Chen Zhou and I tried to share the code because of a suggestion. After so many
>> attempts, it doesn't seem to fit to make generic. Or maybe I haven't figured
>> out a good solution yet.
> 
> Well, you learned a very important lesson and the many attempts are not
> in vain: code sharing does not make sense in every case.
> 
>> I will put the patches that make arm64 support crashkernel...high,low to
>> the front, then the parse_crashkernel() unification patches. Even if the
>> second half of the patches is not ready for v5.18, the first half of the
>> patches is ready.
> 
> I think you should concentrate on the arm64 side which is, AFAICT, what
> you're trying to achieve.

Right, a patchset should focus on just one thing.

> 
> The "parse_crashkernel() unification" needs more thought because, as I
> said already, that doesn't make a whole lot of sense to me.

Yes, because it's not a functional improvement, it's not a performance optimization,
it's also not a fix for a known bug, it's just a programmer's artistic pursuit.

> 
> If you want to enforce the fact that "low" makes sense only when "high"
> is supplied, parse_crashkernel_high_low() is not the right thing to do.
> You need to have a *single* function which does all the parsing where
> you can decide what to do: "if high, parse low", "if no high supplied,
> ignore low" and so on.

I understand your proposal, but parse_crashkernel_high_low() is a cost-effective
and profitable change, it makes the current code a little clearer, and avoid passing
unnecessary parameters "system_ram" and "crash_base" when other architectures use
parse_crashkernel_{high|low}().

I actually followed your advice in the beginning to do "parse_crashkernel() and
parse_crashkernel_{high|low}() unification". But I found it's difficult and the
end result may not be as good as expected. So I introduced parse_crashkernel_high_low().

The parameter "system_ram" and "crash_base" of parse_crashkernel() is not need by
"crashkernel=X,[high,low]". And parameter "low_size" of parse_crashkernel_high_low()
is not need by "crashkernel=X[@offset]". The "parse_crashkernel() unification"
complicates things. For example, the parameter "crash_size" means "low or high" memory
size for "crashkernel=X[@offset]", but only means "high" memory size for "crashkernel=X,high".
So we'd better give it two names with union.

> 
> And if those are supported on certain architectures only, you can do
> ifdeffery...

I don't think so. These __init functions are small and architecture-independent, and do not
affect compilation of other architectures. There may be other architectures that use
it in the future, such as the current arm64.

> 
> But I think I already stated that I don't like such unifications which
> introduce unnecessary dependencies between architectures. Therefore, I
> won't accept them into x86 unless there's a strong compelling reason.
> Which I don't see ATM.

OK.

> 
> Thx.
> 

-- 
Regards,
  Zhen Lei