Page Table A&D bits scheme

[Alexandre Ghiti]

Hi Theo,

On 05/01/2024 09:27, Theo Z wrote:
> According to Privileged: 4.3.1 (p81)
>
> "Two schemes to manage the A and D bits are permitted:"
>
> * When a virtual page is accessed and the A bit is clear, or is 
> written and the D bit is clear, a
> page-fault exception is raised.


At the moment, this is the only scheme that the kernel supports.


>
> * When a virtual page is accessed and the A bit is clear, or is 
> written and the D bit is clear, the
> implementation sets the corresponding bit(s) in the PTE


This will be supported by the kernel when svadu support is merged in the 
kernel 
(https://lore.kernel.org/linux-riscv/20231102120129.11261-1-yongxuan.wang@sifive.com/) 
and in openSBI 
(https://lists.infradead.org/pipermail/opensbi/2023-October/005768.html), 
but it depends on openSBI FWFT extension.


>
> Which implementation does Linux is expected to work best with?
>

svadu will avoid a page fault so this will be the most performant 
implementation.


> (Unix kernel xv6 works with 2nd scheme only)
>
> Best regards,
>
> Theo Z
>

Thanks,

Alex


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