[PATCH] arm64/mm: Introduce a variable to hold base address of linear region

Jin, Yanjiang yanjiang.jin at hxt-semitech.com
Tue Jun 19 02:57:03 PDT 2018



> -----Original Message-----
> From: Will Deacon [mailto:will.deacon at arm.com]
> Sent: 2018年6月19日 17:41
> To: Jin, Yanjiang <yanjiang.jin at hxt-semitech.com>
> Cc: James Morse <james.morse at arm.com>; Bhupesh Sharma
> <bhsharma at redhat.com>; Mark Rutland <mark.rutland at arm.com>; Ard
> Biesheuvel <ard.biesheuvel at linaro.org>; Catalin Marinas
> <catalin.marinas at arm.com>; Kexec Mailing List <kexec at lists.infradead.org>;
> AKASHI Takahiro <takahiro.akashi at linaro.org>; Bhupesh SHARMA
> <bhupesh.linux at gmail.com>; linux-arm-kernel <linux-arm-
> kernel at lists.infradead.org>
> Subject: Re: [PATCH] arm64/mm: Introduce a variable to hold base address of
> linear region
>
> On Tue, Jun 19, 2018 at 09:34:56AM +0000, Jin, Yanjiang wrote:
> > > On Tue, Jun 19, 2018 at 03:02:15AM +0000, Jin, Yanjiang wrote:
> > > > > You seem to be using this for user-space phys_to_virt() based on
> > > > > values found in /proc/iomem. This should give you what you want,
> > > > > and isolate your user-space from the kernel's unexpected naming of
> variables.
> > > >
> > > > I don't know could I simplify this problem?
> > > > Let's ignore what memstart_addr represents here, we just want to
> > > > implement
> > > > phys_to_virt() in an userspace applications(kexec-tools or others).
> > > >
> > > > ARM64 Kernel has a below definition:
> > > >
> > > > #define __phys_to_virt(x)       ((unsigned long)((x) - PHYS_OFFSET) |
> > > PAGE_OFFSET)
> > > >
> > > > So userspace app must know PHYS_OFFSET(equal to memstart_addr now).
> > > > Seems this is very simple, but memstart_addr has gone through
> > > > several operations in arm64_memblock_init() depends on different
> > > > Kernel configurations, so userspace app needs to know many
> > > > additional definitions as
> > > following:
> > > >
> > > > memblock_start_of_DRAM(),  (ifdef CONFIG_SPARSEMEM_VMEMMAP),
> > > > ARM64_MEMSTART_SHIFT,  SECTION_SIZE_BITS,  PAGE_OFFSET,
> > > > memblock_end_of_DRAM(), IS_ENABLED(CONFIG_RANDOMIZE_BASE),
> > > > memstart_offset_seed.
> > > >
> > > > It is hard to know all above in kexec-tools now. Originally I
> > > > planned to read memstart_addr's value from "/dev/mem", but someone
> > > > thought not all Kernels enable "/dev/mem", we'd better find a more
> > > > generic approach. So we want to get some suggestions from ARM kernel
> community.
> > > > Can we export this variable in Kernel side through sysconf() or
> > > > other similar methods? Or someone can provide an effect way to get
> > > > memstart_addr's value?
> > >
> > > I thought the suggestion from James was to expose this via an ELF
> > > NOTE in kcore and vmcore (or in the header directly if that's possible, but I'm
> not sure about it)?
> >
> > Thanks for your reply firstly. But same as DEVMEM, kcore is not a
> > must-have, so we can't depend on it.
>
> Neither is KEXEC. We can select PROC_KCORE from KEXEC if it helps.
>
> > On the other hand, phys_to_virt() is called during generating vmcore
> > in Kexec-tools, vmcore also can't help this issue.
>
> I don't understand this part. If you have the vmcore in your hand, why can't you
> grok the pv offset from the note and use that in phys_to_virt()?

It is a chicken-and-egg issue.
phys_to virt() is for crashdump setup. To generate vmcore, we must call phys_to_virt(). At this point, no vmcore exists.

Yanjiang
>
> > Unfortunately, not all platforms support analyzing Kernel config in
> > userspace application, so Kexec-tools can't know some key kernel options.
> > If not so, we can simulate the whole arm64_memblock_init()  progress
> > in kexec-tools.
>
> I don't understand what the kernel config has to do with kexec tools.

I mean that if we can know kernel .config in all circumstances, we can calculate memstart_addr  as below in Kexec-tools:


        memstart_addr = round_down(memblock_start_of_DRAM(),
                                   ARM64_MEMSTART_ALIGN);


#if defined(CONFIG_SPARSEMEM_VMEMMAP) && ARM64_MEMSTART_SHIFT < SECTION_SIZE_BITS
#define ARM64_MEMSTART_ALIGN    (1UL << SECTION_SIZE_BITS)
......
#endif

#define ARM64_MEMSTART_SHIFT            PMD_SHIFT

#if CONFIG_PGTABLE_LEVELS > 2
#define PMD_SHIFT               ARM64_HW_PGTABLE_LEVEL_SHIFT(2)
...........
#endif

Yanjiang
>
> Will



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