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    SubjectRe: [PATCH 0/2] arm64, kdump: enforce to take 4G as the crashkernel low memory end
    Forgot adding Nicolas when sending patch, add now.

    On 08/28/22 at 08:55am, Baoquan He wrote:
    > Problem:
    > =======
    > On arm64, block and section mapping is supported to build page tables.
    > However, currently it enforces to take base page mapping for the whole
    > linear mapping if CONFIG_ZONE_DMA or CONFIG_ZONE_DMA32 is enabled and
    > crashkernel kernel parameter is set. This will cause longer time of the
    > linear mapping process during bootup and severe performance degradation
    > during running time.
    >
    > Root cause:
    > ==========
    > On arm64, crashkernel reservation relies on knowing the upper limit of
    > low memory zone because it needs to reserve memory in the zone so that
    > devices' DMA addressing in kdump kernel can be satisfied. However, the
    > limit on arm64 is variant. And the upper limit can only be decided late
    > till bootmem_init() is called.
    >
    > And we need to map the crashkernel region with base page granularity when
    > doing linear mapping, because kdump needs to protect the crashkernel region
    > via set_memory_valid(,0) after kdump kernel loading. However, arm64 doesn't
    > support well on splitting the built block or section mapping due to some
    > cpu reststriction [1]. And unfortunately, the linear mapping is done before
    > bootmem_init().
    >
    > To resolve the above conflict on arm64, the compromise is enforcing to
    > take base page mapping for the entire linear mapping if crashkernel is
    > set, and CONFIG_ZONE_DMA or CONFIG_ZONE_DMA32 is enabed. Hence
    > performance is sacrificed.
    >
    > Solution:
    > =========
    > To fix the problem, we should always take 4G as the crashkernel low
    > memory end in case CONFIG_ZONE_DMA or CONFIG_ZONE_DMA32 is enabled.
    > With this, we don't need to defer the crashkernel reservation till
    > bootmem_init() is called to set the arm64_dma_phys_limit. As long as
    > memblock init is done, we can conclude what is the upper limit of low
    > memory zone.
    >
    > 1) both CONFIG_ZONE_DMA or CONFIG_ZONE_DMA32 are disabled or memblock_start_of_DRAM() > 4G
    > limit = PHYS_ADDR_MAX+1 (Corner cases)
    > 2) CONFIG_ZONE_DMA or CONFIG_ZONE_DMA32 are enabled:
    > limit = 4G (generic case)
    >
    > Justification:
    > ==============
    > In fact, kdump kernel doesn't need to cover all peripherals'
    > addressing bits. Only device taken as dump target need be taken care of
    > and its addressing bits need be satified. Currently, there are two kinds
    > of dumping, dumped to local storage disk or dumped through network card to
    > remove storage server. It means only storage disk or netowrk card taken
    > as dump target need be consider if their addressing bits are satisfied.
    > For saving memory, we usually generate kdump specific initramfs including
    > necessary kernel modules for dump target devices. All other unnecessary
    > kernel modules are excluded and their correspondent devices won't be
    > initialized during kdump kernel bootup.
    >
    > So far, only Raspberry Pi 4 has some peripherals whcih can only address
    > 30 bits memory range as reported in [2]. Devices on all other arm64 systems
    > can address 32bits memory range.
    >
    > So by enforcing to take 4G as the crashkernel low memory end, the only
    > risk is if RPi4 owns storage disk or network card which can't address
    > 32bits memory range because they could be set as dump target. Even if
    > RPi4 truly has storage devices or network card which can only address 30
    > bits memory range, it should be a corner case. We can document it since
    > crashkernel is more taken as a feature on server. Besides, RPi4 still can
    > use crashkernel=xM@yM to sepcify a location for 32bits addressing if it
    > really has that kind of storage device or network card and kdump is expected.
    >
    > [1]
    > https://lore.kernel.org/all/YrIIJkhKWSuAqkCx@arm.com/T/#u
    >
    > [2]
    > [PATCH v6 0/4] Raspberry Pi 4 DMA addressing support
    > https://lore.kernel.org/linux-arm-kernel/20190911182546.17094-1-nsaenzjulienne@suse.de/T/
    >
    >
    > ======
    > Question to Nicolas:
    >
    > Hi Nicolas,
    >
    > In cover letter of [2] patchset, you told RPi4 has peripherals which
    > can only address 30bits range. In below sentence, do you mean "the PCIe,
    > V3D, GENET" can't address 32bit range, or they have wider view of
    > address space the same as 40-bit DMA channels? I am confused about that.
    >
    > And the storage device or network card on RPi4 can address 32bit range
    > or 32bit range, do we have document or do you happen to know that?
    >
    > """
    > The new Raspberry Pi 4 has up to 4GB of memory but most peripherals can
    > only address the first GB: their DMA address range is
    > 0xc0000000-0xfc000000 which is aliased to the first GB of physical
    > memory 0x00000000-0x3c000000. Note that only some peripherals have these
    > limitations: the PCIe, V3D, GENET, and 40-bit DMA channels have a wider
    > view of the address space by virtue of being hooked up trough a second
    > interconnect.
    > """
    >
    >
    > Baoquan He (2):
    > arm64, kdump: enforce to take 4G as the crashkernel low memory end
    > arm64: remove unneed defer_reserve_crashkernel() and crash_mem_map
    >
    > arch/arm64/include/asm/memory.h | 5 ----
    > arch/arm64/mm/init.c | 24 ++++++++-------
    > arch/arm64/mm/mmu.c | 53 ++++++++++++++-------------------
    > 3 files changed, 36 insertions(+), 46 deletions(-)
    >
    >
    > base-commit: 10d4879f9ef01cc6190fafe4257d06f375bab92c
    > --
    > 2.34.1
    >

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