linux-kernel - Re: [PATCH v5 2/2] RISC-V: Setup initial page tables in two stages

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Date:   Wed, 10 Jul 2019 17:05:19 -0700 (PDT)
From:   Paul Walmsley <paul.walmsley@...ive.com>
To:     Anup Patel <Anup.Patel@....com>
cc:     Palmer Dabbelt <palmer@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Atish Patra <Atish.Patra@....com>,
        Christoph Hellwig <hch@...radead.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 2/2] RISC-V: Setup initial page tables in two stages

On Fri, 7 Jun 2019, Anup Patel wrote:

> Currently, the setup_vm() does initial page table setup in one-shot
> very early before enabling MMU. Due to this, the setup_vm() has to map
> all possible kernel virtual addresses since it does not know size and
> location of RAM. This means we have kernel mappings for non-existent
> RAM and any buggy driver (or kernel) code doing out-of-bound access
> to RAM will not fault and cause underterministic behaviour.
> 
> Further, the setup_vm() creates PMD mappings (i.e. 2M mappings) for
> RV64 systems. This means for PAGE_OFFSET=0xffffffe000000000 (i.e.
> MAXPHYSMEM_128GB=y), the setup_vm() will require 129 pages (i.e.
> 516 KB) of memory for initial page tables which is never freed. The
> memory required for initial page tables will further increase if
> we chose a lower value of PAGE_OFFSET (e.g. 0xffffff0000000000)
> 
> This patch implements two-staged initial page table setup, as follows:
> 1. Early (i.e. setup_vm()): This stage maps kernel image and DTB in
> a early page table (i.e. early_pg_dir). The early_pg_dir will be used
> only by boot HART so it can be freed as-part of init memory free-up.
> 2. Final (i.e. setup_vm_final()): This stage maps all possible RAM
> banks in the final page table (i.e. swapper_pg_dir). The boot HART
> will start using swapper_pg_dir at the end of setup_vm_final(). All
> non-boot HARTs directly use the swapper_pg_dir created by boot HART.
> 
> We have following advantages with this new approach:
> 1. Kernel mappings for non-existent RAM don't exists anymore.
> 2. Memory consumed by initial page tables is now indpendent of the
> chosen PAGE_OFFSET.
> 3. Memory consumed by initial page tables on RV64 system is 2 pages
> (i.e. 8 KB) which has significantly reduced and these pages will be
> freed as-part of the init memory free-up.
> 
> The patch also provides a foundation for implementing strict kernel
> mappings where we protect kernel text and rodata using PTE permissions.
> 
> Suggested-by: Mike Rapoport <rppt@...ux.ibm.com>
> Signed-off-by: Anup Patel <anup.patel@....com>

Thanks, updated to apply and to fix a checkpatch warning, and queued.  

This may not make it in for v5.3-rc1; if not, we'll submit it later.


- Paul