linux-kernel - Re: [RFC PATCH] kexec, x86/boot: map systab region in identity mapping before accessing it

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Message-ID: <20190419114421.GH11060@MiWiFi-R3L-srv>
Date:   Fri, 19 Apr 2019 19:44:21 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Kairui Song <kasong@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Junichi Nomura <j-nomura@...jp.nec.com>,
        Dave Young <dyoung@...hat.com>,
        Chao Fan <fanc.fnst@...fujitsu.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "kexec@...ts.infradead.org" <kexec@...ts.infradead.org>
Subject: Re: [RFC PATCH] kexec, x86/boot: map systab region in identity
 mapping before accessing it

On 04/19/19 at 01:28pm, Borislav Petkov wrote:
> Why does that "seem" so?
> 
> Read again what I said: "should all be passed through boot_params".
> Which means, boot_params should be extended with a field of a flag to
> say: "this is a kexec'ed kernel".
> 
> If it "seems" then it should be made to not "seem" but to work properly.

No objection to extending with fields of a flag to mark kexec'ed kernel.
Or kdump kernel either. We now check kdump kernel by /proc/vmcore.

> 
> > Yeah, adding the systab mapping looks good. Kairui put it in
> > decompressing stage just because he wants to cover the case in which the
> > old kernel kexec jumping to 2nd kernel. Now it seems not very
> > reasonable, we also have the new kernel kexec jumping to old 2nd kernel.
> 
> I don't think we can guarantee kexec between old<->new kernel to always
> work. Otherwise, we can forget all development and improvements of new
> kernel.

I personally agree with this. Very earlier, we tried to remove the
896 MB limitation of crashkernel=xM, to extend it to 4G or the whole
RAM, but rejcted by linus since he worried it could break the old kernel
kdumping. I may not remember well.