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

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Date:   Tue, 27 Dec 2022 19:36:03 -0600
From:   Samuel Holland <samuel@...lland.org>
To:     Conor Dooley <conor@...nel.org>,
        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 12/22/22 17:21, Conor Dooley wrote:
> 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

That's because my CONFIG_DEBUG_VIRTUAL fix got applied first. This patch
applies cleanly after:

	git revert -m1 61b2f0bdaa3c7e6956fdac0a7c1e8284b9b81d1d

though you'll get one trivial conflict if you try to undo the revert.

Regards,
Samuel