Re: [PATCH 0/2] arm64, kdump: enforce to take 4G as the crashkernel low memory end

[Baoquan He]

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@xxxxxxx/T/#u
> 
> [2]
> [PATCH v6 0/4] Raspberry Pi 4 DMA addressing support
> https://lore.kernel.org/linux-arm-kernel/20190911182546.17094-1-nsaenzjulienne@xxxxxxx/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
>