linux-kernel - Re: [PATCH v19 02/13] x86/setup: Use parse_crashkernel_high

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Date:   Thu, 30 Dec 2021 16:56:00 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     Borislav Petkov <bp@...en8.de>
CC:     Dave Young <dyoung@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...hat.com>, <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>, <linux-kernel@...r.kernel.org>,
        Baoquan He <bhe@...hat.com>, Vivek Goyal <vgoyal@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        <kexec@...ts.infradead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        "Will Deacon" <will@...nel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        <devicetree@...r.kernel.org>, Jonathan Corbet <corbet@....net>,
        <linux-doc@...r.kernel.org>, Randy Dunlap <rdunlap@...radead.org>,
        Feng Zhou <zhoufeng.zf@...edance.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        "Chen Zhou" <dingguo.cz@...group.com>,
        John Donnelly <John.p.donnelly@...cle.com>
Subject: Re: [PATCH v19 02/13] x86/setup: Use parse_crashkernel_high_low() to
 simplify code



On 2021/12/30 10:39, Leizhen (ThunderTown) wrote:
> 
> 
> 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.

In fact, this is how my current function parse_crashkernel_high_low() is
implemented.

+	/* crashkernel=X,high */
+	ret = parse_crashkernel_high(cmdline, 0, high_size, &base);
+	if (ret)			//crashkernel=X,high is not specified
+		return ret;
+
+	if (*high_size <= 0)		//crashkernel=X,high is specified but the value is invalid
+		return -EINVAL;		//Sorry, the type of high_size is "unsigned long long *", so less than zero is impossible
+
+	/* crashkernel=Y,low */
+	ret = parse_crashkernel_low(cmdline, 0, low_size, &base);	//If crashkernel=Y,low is specified, the parsed value is stored in *low_size
+	if (ret)
+		*low_size = -1;		//crashkernel=Y,low is not specified


> 
> 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