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 >
