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    SubjectRe: [PATCH 3/3] ARM: allow kernel to be loaded in middle of phymem
    On Wed, 22 Jan 2014, Wang Nan wrote:

    > This patch allows the kernel to be loaded at the middle of kernel awared
    > physical memory. Before this patch, users must use mem= or device tree to cheat
    > kernel about the start address of physical memory.
    >
    > This feature is useful in some special cases, for example, building a crash
    > dump kernel. Without it, kernel command line, atag and devicetree must be
    > adjusted carefully, sometimes is impossible.

    With CONFIG_PATCH_PHYS_VIRT the value for PHYS_OFFSET is determined
    dynamically by rounding down the kernel image start address to the
    previous 16MB boundary. In the case of a crash kernel, this might be
    cleaner to simply readjust __pv_phys_offset during early boot and call
    fixup_pv_table(), and then reserve away the memory from the previous
    kernel. That will let you access that memory directly (with gdb for
    example) and no pointer address translation will be required.


    > Signed-off-by: Wang Nan <wangnan0@huawei.com>
    > Cc: <stable@vger.kernel.org> # 3.4+
    > Cc: Eric Biederman <ebiederm@xmission.com>
    > Cc: Russell King <rmk+kernel@arm.linux.org.uk>
    > Cc: Andrew Morton <akpm@linux-foundation.org>
    > Cc: Geng Hui <hui.geng@huawei.com>
    > ---
    > arch/arm/mm/init.c | 21 ++++++++++++++++++++-
    > arch/arm/mm/mmu.c | 13 +++++++++++++
    > mm/page_alloc.c | 7 +++++--
    > 3 files changed, 38 insertions(+), 3 deletions(-)
    >
    > diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
    > index 3e8f106..4952726 100644
    > --- a/arch/arm/mm/init.c
    > +++ b/arch/arm/mm/init.c
    > @@ -334,9 +334,28 @@ void __init arm_memblock_init(struct meminfo *mi,
    > {
    > int i;
    >
    > - for (i = 0; i < mi->nr_banks; i++)
    > + for (i = 0; i < mi->nr_banks; i++) {
    > memblock_add(mi->bank[i].start, mi->bank[i].size);
    >
    > + /*
    > + * In some special case, for example, building a crushdump
    > + * kernel, we want the kernel to be loaded in the middle of
    > + * physical memory. In such case, the physical memory before
    > + * PHYS_OFFSET is awkward: it can't get directly mapped
    > + * (because its address will be smaller than PAGE_OFFSET,
    > + * disturbs user address space) also can't be mapped as
    > + * HighMem. We reserve such pages here. The only way to access
    > + * those pages is ioremap.
    > + */
    > + if (mi->bank[i].start < PHYS_OFFSET) {
    > + unsigned long reserv_size = PHYS_OFFSET -
    > + mi->bank[i].start;
    > + if (reserv_size > mi->bank[i].size)
    > + reserv_size = mi->bank[i].size;
    > + memblock_reserve(mi->bank[i].start, reserv_size);
    > + }
    > + }
    > +
    > /* Register the kernel text, kernel data and initrd with memblock. */
    > #ifdef CONFIG_XIP_KERNEL
    > memblock_reserve(__pa(_sdata), _end - _sdata);
    > diff --git a/arch/arm/mm/mmu.c b/arch/arm/mm/mmu.c
    > index 580ef2d..2a17c24 100644
    > --- a/arch/arm/mm/mmu.c
    > +++ b/arch/arm/mm/mmu.c
    > @@ -1308,6 +1308,19 @@ static void __init map_lowmem(void)
    > if (start >= end)
    > break;
    >
    > + /*
    > + * If this memblock contain memory before PAGE_OFFSET, memory
    > + * before PAGE_OFFSET should't get directly mapped, see code
    > + * in create_mapping(). However, memory after PAGE_OFFSET is
    > + * occupyed by kernel and still need to be mapped.
    > + */
    > + if (__phys_to_virt(start) < PAGE_OFFSET) {
    > + if (__phys_to_virt(end) > PAGE_OFFSET)
    > + start = __virt_to_phys(PAGE_OFFSET);
    > + else
    > + break;
    > + }
    > +
    > map.pfn = __phys_to_pfn(start);
    > map.virtual = __phys_to_virt(start);
    > map.length = end - start;
    > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
    > index 5248fe0..d2959e3 100644
    > --- a/mm/page_alloc.c
    > +++ b/mm/page_alloc.c
    > @@ -4840,10 +4840,13 @@ static void __init_refok alloc_node_mem_map(struct pglist_data *pgdat)
    > */
    > if (pgdat == NODE_DATA(0)) {
    > mem_map = NODE_DATA(0)->node_mem_map;
    > -#ifdef CONFIG_HAVE_MEMBLOCK_NODE_MAP
    > + /*
    > + * In case of CONFIG_HAVE_MEMBLOCK_NODE_MAP or when kernel
    > + * loaded at the middle of physical memory, mem_map should
    > + * be adjusted.
    > + */
    > if (page_to_pfn(mem_map) != pgdat->node_start_pfn)
    > mem_map -= (pgdat->node_start_pfn - ARCH_PFN_OFFSET);
    > -#endif /* CONFIG_HAVE_MEMBLOCK_NODE_MAP */
    > }
    > #endif
    > #endif /* CONFIG_FLAT_NODE_MEM_MAP */
    > --
    > 1.8.4
    >
    > --
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