Messages in this thread | | | Subject | Re: [PATCH 0/2] PUD/PGDIR entries for linear mapping | From | Alex Ghiti <> | Date | Mon, 29 Jun 2020 10:46:47 -0400 |
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Le 6/3/20 à 11:36 AM, Alexandre Ghiti a écrit : > This small patchset intends to use PUD/PGDIR entries for linear mapping > in order to better utilize TLB. > > At the moment, only PMD entries can be used since on common platforms > (qemu/unleashed), the kernel is loaded at DRAM + 2MB which dealigns virtual > and physical addresses and then prevents the use of PUD/PGDIR entries. > So the kernel must be able to get those 2MB for PAGE_OFFSET to map the > beginning of the DRAM: this is achieved in patch 1. > > But furthermore, at the moment, the firmware (opensbi) explicitly asks the > kernel not to map the region it occupies, which is on those common > platforms at the very beginning of the DRAM and then it also dealigns > virtual and physical addresses. I proposed a patch here: > > https://github.com/riscv/opensbi/pull/167 > > that removes this 'constraint' but *not* all the time as it offers some > kind of protection in case PMP is not available. So sometimes, we may > have a part of the memory below the kernel that is removed creating a > misalignment between virtual and physical addresses. So for performance > reasons, we must at least make sure that PMD entries can be used: that > is guaranteed by patch 1 too. > > Finally the second patch simply improves best_map_size so that whenever > possible, PUD/PGDIR entries are used. > > Below is the kernel page table without this patch on a 6G platform: > > ---[ Linear mapping ]--- > 0xffffc00000000000-0xffffc00176e00000 0x0000000080200000 5998M PMD D A . . . W R V > > And with this patchset + opensbi patch: > > ---[ Linear mapping ]--- > 0xffffc00000000000-0xffffc00140000000 0x0000000080000000 5G PUD D A . . . W R V > 0xffffc00140000000-0xffffc00177000000 0x00000001c0000000 880M PMD D A . . . W R V > > Alexandre Ghiti (2): > riscv: Get memory below load_pa while ensuring linear mapping is PMD > aligned > riscv: Use PUD/PGDIR entries for linear mapping when possible > > arch/riscv/include/asm/page.h | 8 ++++ > arch/riscv/mm/init.c | 69 +++++++++++++++++++++++++++++------ > 2 files changed, 65 insertions(+), 12 deletions(-) >
The way to handle the remapping of the first 2MB is incorrect: Atish has issues while using an initrd because the initrd_start variable is defined using __va between setup_vm and setup_vm_final and then its value is inconsistent after setup_vm_final since virtual addressing was modified with the remapping of the first 2MB.
I will come with another solution to this problem since the way I handle it for now is not correct.
Thanks,
Alex
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