Re: [PATCH v4] riscv: Use PUD/P4D/PGD pages for the linear mapping

[Conor Dooley <conor@kernel.org>]

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On Mon, Jan 23, 2023 at 12:28:02PM +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.
> 
> Acked-by: Rob Herring <robh@kernel.org> # DT bits
> Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>

No good on !MMU unfortunately Alex:
../arch/riscv/mm/init.c:222:2: error: use of undeclared identifier 'riscv_pfn_base'
        riscv_pfn_base = PFN_DOWN(phys_ram_base);
        ^

Reproduces with nommu_virt_defconfig.

Thanks,
Conor.

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