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Date:   Thu, 5 May 2022 15:20:18 +0100
From:   Catalin Marinas <catalin.marinas@arm.com>
To:     Baoquan He <bhe@redhat.com>
Cc:     "Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
        linux-kernel@vger.kernel.org, Dave Young <dyoung@redhat.com>,
        Vivek Goyal <vgoyal@redhat.com>,
        Eric Biederman <ebiederm@xmission.com>,
        kexec@lists.infradead.org, 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>,
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        Kefeng Wang <wangkefeng.wang@huawei.com>,
        Chen Zhou <dingguo.cz@antgroup.com>,
        John Donnelly <John.p.donnelly@oracle.com>,
        Dave Kleikamp <dave.kleikamp@oracle.com>
Subject: Re: [PATCH v22 5/9] arm64: kdump: Reimplement crashkernel=X
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On Thu, May 05, 2022 at 11:00:19AM +0800, Baoquan He wrote:
> On 05/03/22 at 11:00pm, Catalin Marinas wrote:
> > So, to recap, IIUC you are fine with:
> > 
> > 	crashkernel=Y		- allocate within ZONE_DMA with fallback
> > 				  above with a default in ZONE_DMA (like
> > 				  x86, 256M or swiotlb size)
> 
>         Ack to this one.
> 
> 
> > 	crashkernel=Y,high	- allocate from above ZONE_DMA
> 
>         Not exactly. If there's only ZONE_DMA, crashkernel,high will
>         be reserved in ZONE_DMA, and crashkernel,low will be ignored.
>         Other than this, ack.

Yes, that's fine.

> > 	crashkernel=Y,low	- allocate within ZONE_DMA
> 
>         Ack to this one.
> > 
> > 'crashkernel' overrides the high and low while the latter two can be
> > passed independently.
> 
>         crashkernel=,high can be passed independently, then a crashkernel=,low
>         is needed implicitly. If people don't want crashkernel=,low
>         explicitly, crashkernel=0,low need be specified.

I find this complicating the interface. I don't know the background to
the x86 implementation but we diverge already on arm64 since we talk
about ZONE_DMA rather than 4G limit (though for most platforms these
would be the same).

I guess we could restate the difference between crashkernel= and
crashkernel=,high as the hint to go for allocation above ZONE_DMA first.

>         An independent crashkernel=,low makes no sense. Crashkernel=,low
>         should be paird with crashkernel=,high.

You could argue that crashkernel=,low gives the current crashkernel=
behaviour, i.e. either all within ZONE_DMA or fail to allocate. So it
may have some value on its own.

>         My personal opinion according to the existed senmantics on x86.
>         Otherwise, the guidance of crashkernel= |,high|,low reservation
>         will be complicated to write.

It's more that I find the current semantics unnecessarily confusing. But
even reading the x86_64 text it's not that clear. For example the
default low allocation for crashkernel= and crashkernel=,high is only
mentioned in the crashkernel=,low description.

-- 
Catalin