linux-kernel - Re: [PATCH 0/2] PUD/PGDIR entries for linear mapping

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Date:   Mon, 29 Jun 2020 10:46:47 -0400
From:   Alex Ghiti <alex@...ti.fr>
To:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Anup Patel <anup@...infault.org>,
        Atish Patra <Atish.Patra@....com>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] PUD/PGDIR entries for linear mapping

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