linux-kernel - Re: [PATCH v3] riscv: Use PUD/P4D/PGD pages for the linear mapping

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Date:   Thu, 22 Dec 2022 23:21:06 +0000
From:   Conor Dooley <conor@...nel.org>
To:     Alexandre Ghiti <alexghiti@...osinc.com>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>, Guo Ren <guoren@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH v3] riscv: Use PUD/P4D/PGD pages for the linear mapping

On Tue, Dec 13, 2022 at 07:02:04AM +0100, Alexandre Ghiti wrote:
> During the early page table creation, we used to set the mapping for
> PAGE_OFFSET to the kernel load address: but the kernel load address is
> always offseted by PMD_SIZE which makes it impossible to use PUD/P4D/PGD
> pages as this physical address is not aligned on PUD/P4D/PGD size (whereas
> PAGE_OFFSET is).
> 
> But actually we don't have to establish this mapping (ie set va_pa_offset)
> that early in the boot process because:
> 
> - first, setup_vm installs a temporary kernel mapping and among other
>   things, discovers the system memory,
> - then, setup_vm_final creates the final kernel mapping and takes
>   advantage of the discovered system memory to create the linear
>   mapping.
> 
> During the first phase, we don't know the start of the system memory and
> then until the second phase is finished, we can't use the linear mapping at
> all and phys_to_virt/virt_to_phys translations must not be used because it
> would result in a different translation from the 'real' one once the final
> mapping is installed.
> 
> So here we simply delay the initialization of va_pa_offset to after the
> system memory discovery. But to make sure noone uses the linear mapping
> before, we add some guard in the DEBUG_VIRTUAL config.
> 
> Finally we can use PUD/P4D/PGD hugepages when possible, which will result
> in a better TLB utilization.
> 
> Note that we rely on the firmware to protect itself using PMP.
> 
> Signed-off-by: Alexandre Ghiti <alexghiti@...osinc.com>
> ---
> 
> v3:
> - Change the comment about initrd_start VA conversion so that it fits
>   ARM64 and RISCV64 (and others in the future if needed), as suggested
>   by Rob
> 
> v2:
> - Add a comment on why RISCV64 does not need to set initrd_start/end that
>   early in the boot process, as asked by Rob
> 
> Note that this patch is rebased on top of:
> [PATCH v1 1/1] riscv: mm: call best_map_size many times during linear-mapping

Hey Alex, unfortunately I could not get this to apply either (I tried a
riscv/for-next & Linus' tree).
The above patch should be in both, so idk:
git am -3 v3_20221213_alexghiti_riscv_use_pud_p4d_pgd_pages_for_the_linear_mapping.mbx
Applying: riscv: Use PUD/P4D/PGD pages for the linear mapping
error: sha1 information is lacking or useless (arch/riscv/mm/init.c).
error: could not build fake ancestor
Patch failed at 0001 riscv: Use PUD/P4D/PGD pages for the linear mapping

I assume it'll need a rebase after -rc1?

It's really nice to see someone looking at this early code, from the
bit of digging around that I've done chasing bugs it feels like it needs
it- so thanks!


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