linux-kernel - Re: [PATCH 5/5] arm64: kdump: Don't defer the reservation of crash high memory

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Message-ID: <Yrkbak66vYT55H4x@MiWiFi-R3L-srv>
Date:   Mon, 27 Jun 2022 10:52:26 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Kefeng Wang <wangkefeng.wang@...wei.com>,
        Zhen Lei <thunder.leizhen@...wei.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        devicetree@...r.kernel.org, Dave Young <dyoung@...hat.com>,
        Vivek Goyal <vgoyal@...hat.com>, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        Randy Dunlap <rdunlap@...radead.org>,
        Feng Zhou <zhoufeng.zf@...edance.com>,
        Chen Zhou <dingguo.cz@...group.com>,
        John Donnelly <John.p.donnelly@...cle.com>,
        Dave Kleikamp <dave.kleikamp@...cle.com>,
        liushixin <liushixin2@...wei.com>
Subject: Re: [PATCH 5/5] arm64: kdump: Don't defer the reservation of crash
 high memory

On 06/23/22 at 03:07pm, Catalin Marinas wrote:
> On Wed, Jun 22, 2022 at 04:35:16PM +0800, Baoquan He wrote:
> > On 06/21/22 at 07:04pm, Catalin Marinas wrote:
> > > The problem with splitting is that you can end up with two entries in
> > > the TLB for the same VA->PA mapping (e.g. one for a 4KB page and another
> > > for a 2MB block). In the lucky case, the CPU will trigger a TLB conflict
> > > abort (but can be worse like loss of coherency).
> > 
> > Thanks for this explanation. Is this a drawback of arm64 design? X86
> > code do the same thing w/o issue, is there way to overcome this on
> > arm64 from hardware or software side?
> 
> It is a drawback of the arm64 implementations. Having multiple TLB
> entries for the same VA would need additional logic in hardware to
> detect, so the microarchitects have pushed back. In ARMv8.4, some
> balanced was reached with FEAT_BBM so that the only visible side-effect
> is a potential TLB conflict abort that could be resolved by software.

I see, thx.

> 
> > I ever got a arm64 server with huge memory, w or w/o crashkernel setting 
> > have different bootup time. And the more often TLB miss and flush will
> > cause performance cost. It is really a pity if we have very powerful
> > arm64 cpu and system capacity, but bottlenecked by this drawback.
> 
> Is it only the boot time affected or the runtime performance as well?

Sorry for late reply. What I observerd is the boot time serious latecy
with huge memory. Since the timestamp is not available at that time,
we can't tell the number. I didn't notice the runtime performance.