Date:   Sat, 27 Oct 2018 18:02:41 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Bhupesh Sharma <bhsharma@...hat.com>
Cc:     linux-kernel@...r.kernel.org, bhupesh.linux@...il.com,
        Boris Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kazuhito Hagio <k-hagio@...jp.nec.com>,
        Dave Anderson <anderson@...hat.com>,
        James Morse <james.morse@....com>,
        Omar Sandoval <osandov@...com>, x86@...nel.org,
        kexec@...ts.infradead.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] x86_64, vmcoreinfo: Append 'page_offset_base' to
 vmcoreinfo

Hi Bhupesh,

Sorry for top posting. Because I don't know which line at below I should
add comment into.

So could you plese tell what problem you have met in user space tools?
Which user space tool is broken so that we need export page_offset_base
to vmcoreinfo?

Sorry I didn't get what problem this patch is trying to fix from the
patch log.

About this, I have replied to you in
lkml.kernel.org/r/20181025063446.GD2120@...iFi-R3L-srv
You might miss it. 

About this exporting, I ever posted patch to upstream and we have had
discussion, please check 
https://lore.kernel.org/patchwork/patch/723472/

In makedumpfile and crash, we have had a clear method to analyze and
deduce it from kcore or vmcore.

Thanks
Baoquan

On 10/27/18 at 04:13am, Bhupesh Sharma wrote:
> Since commit 23c85094fe1895caefdd
> ["proc/kcore: add vmcoreinfo note to /proc/kcore"]), '/proc/kcore'
> contains a new PT_NOTE which carries the VMCOREINFO information.
> 
> If the same is available, one can use it in user-land to
> retrieve machine specific symbols or strings being appended to the
> vmcoreinfo even for live-debugging of the primary kernel as a
> standard interface exposed by kernel for sharing machine specific
> details with the user-land.
> 
> In the past I had a discussion with James, where he suggested this
> approach (please see [0]) and I really liked the idea. Since then I
> have been working on unifying the implementations of
> (atleast) the commonly used user-space utilities that provide
> live-debugging capabilities (tools like 'makedumpfile' and
> 'crash-utility', see [1] for details of these tools).
> 
> For the same, when live debugging on x86_64 machines, user-space
> tools currently rely on different mechanisms to determine
> the 'page_offset_base' value (i.e. start of direct mapping of all
> physical memory). One of the approach used by 'makedumpfile'
> user-space tool for e.g. is to calculate the same from the last
> PT_LOAD available in '/proc/kcore', which can be flaky as and when
> new sections (for e.g. KCORE_REMAP which was added
> to recent kernels) are added to kcore.
> 
> For other architectures like arm64, I have already proposed using
> the vmcoreinfo note (in '/proc/kcore') in the user-space utilities to
> determine machine specific details like VA_BITS, PAGE_OFFSET,
> kasrl_offset() (see [2] for details), for which different user-space
> tools earlier used different (and at times flaky) approaches like:
> 
> - Reading kernel CONFIGs from user-space and determining CONFIG values
>    like VA_BITS from there.
>  - Reading symbols from '/proc/kallsyms' and determining their values
>    via '/dev/mem' interface.
>  - Reading symbols from 'vmlinux' and determing their values from
>    reading memory.
> 
> This patch allows appending 'page_offset_base' for x86_64 platforms
> to vmcoreinfo, so that user-space tools can use the same as a standard
> interface to determine the start of direct mapping of all physical
> memory.
> 
> Testing:
> -------
>  - I tested this patch (rebased on 'linux-next') on a x86_64 machine
>    using the modified 'makedumpfile' user-space code (see [3] for my
>    github tree which contains the same) for determining how many pages
>    are dumpable when different dump_level is specified (which is
>    one use-case of live-debugging via 'makedumpfile').
>  - I tested both the KASLR and non-KASLR boot cases with this patch.
>  - Here is one sample log (for KASLR boot case) on my x86_64 machine:
> 
>    < snip..>
>    The kernel doesn't support mmap(),read() will be used instead.
> 
>    TYPE		PAGES			EXCLUDABLE	DESCRIPTION
>    ----------------------------------------------------------------------
>    ZERO		21299           	yes		Pages filled
>    with zero
>    NON_PRI_CACHE	91785           	yes		Cache
>    pages without private flag
>    PRI_CACHE	1               	yes		Cache pages with
>    private flag
>    USER		14057           	yes		User process
>    pages
>    FREE		740346          	yes		Free pages
>    KERN_DATA	58152           	no		Dumpable kernel
>    data
> 
>    page size:		4096
>    Total pages on system:	925640
>    Total size on system:	3791421440       Byte
> 
> I understand that there might be some reservations about exporting
> such machine-specific details in the vmcoreinfo, but to unify
> the implementations across user-land and archs, perhaps this would be
> good starting point to start a discussion.
> 
> [0]. https://www.mail-archive.com/kexec@lists.infradead.org/msg20300.html
> [1]. MAN pages -> MAKEDUMPFILE(8) and CRASH(8)
> [2]. https://www.spinics.net/lists/kexec/msg21608.html
>      http://lists.infradead.org/pipermail/kexec/2018-October/021725.html
> [3]. https://github.com/bhupesh-sharma/makedumpfile/tree/add-page-offset-base-to-vmcore-v1
> 
> Cc: Boris Petkov <bp@...en8.de>
> Cc: Baoquan He <bhe@...hat.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Kazuhito Hagio <k-hagio@...jp.nec.com>
> Cc: Dave Anderson <anderson@...hat.com>
> Cc: James Morse <james.morse@....com>
> Cc: Omar Sandoval <osandov@...com>
> Cc: x86@...nel.org
> Cc: kexec@...ts.infradead.org
> Cc: linux-arm-kernel@...ts.infradead.org
> Signed-off-by: Bhupesh Sharma <bhsharma@...hat.com>
> ---
>  arch/x86/kernel/machine_kexec_64.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
> index 4c8acdfdc5a7..834ccefef867 100644
> --- a/arch/x86/kernel/machine_kexec_64.c
> +++ b/arch/x86/kernel/machine_kexec_64.c
> @@ -356,6 +356,7 @@ void arch_crash_save_vmcoreinfo(void)
>  	VMCOREINFO_SYMBOL(init_top_pgt);
>  	vmcoreinfo_append_str("NUMBER(pgtable_l5_enabled)=%d\n",
>  			pgtable_l5_enabled());
> +	VMCOREINFO_NUMBER(page_offset_base);
>  
>  #ifdef CONFIG_NUMA
>  	VMCOREINFO_SYMBOL(node_data);
> -- 
> 2.7.4
> 

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