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

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Message-ID: <20190419112801.GB10324@zn.tnic>
Date:   Fri, 19 Apr 2019 13:28:01 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Baoquan He <bhe@...hat.com>
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 Fri, Apr 19, 2019 at 06:50:14PM +0800, Baoquan He wrote:
> Talked with Kairui privately just now. Seems Junichi's patch need add
> this systab mapping. Since the systab region is not mapped on some
> machines. Those machine don't have this issue because they got systab
> region luckily coverred by 1 GB page mapping in 1st kernel before
> kexec jumping.

You don't have to repeat all I that - I know what the problem is. Read
what I said again: it is too late for 5.1 to do any involved surgery.

> > 2. Then, the fact whether the kernel has been kexec'ed and which
> > addresses it should use early, should all be passed through boot_params
> > which is either setup by kexec(1) or by the first kernel itself, in the
> > kexec_file_load() case.
> 
> Seems no better way to check if it's kexec-ed kernel, except of the
> setup data checking of kexec-ed kernel.

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.

> 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.

-- 
Regards/Gruss,
    Boris.

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