Re: [PATCH v4 14/18] xen/riscv: Implement superpage splitting for p2m mappings

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.09.2025 23:55, Oleksii Kurochko wrote:
> Add support for down large memory mappings ("superpages") in the RISC-V
> p2m mapping so that smaller, more precise mappings ("finer-grained entries")
> can be inserted into lower levels of the page table hierarchy.
> 
> To implement that the following is done:
> - Introduce p2m_split_superpage(): Recursively shatters a superpage into
>   smaller page table entries down to the target level, preserving original
>   permissions and attributes.
> - p2m_set_entry() updated to invoke superpage splitting when inserting
>   entries at lower levels within a superpage-mapped region.
> 
> This implementation is based on the ARM code, with modifications to the part
> that follows the BBM (break-before-make) approach, some parts are simplified
> as according to RISC-V spec:
>   It is permitted for multiple address-translation cache entries to co-exist
>   for the same address. This represents the fact that in a conventional
>   TLB hierarchy, it is possible for multiple entries to match a single
>   address if, for example, a page is upgraded to a superpage without first
>   clearing the original non-leaf PTE’s valid bit and executing an SFENCE.VMA
>   with rs1=x0, or if multiple TLBs exist in parallel at a given level of the
>   hierarchy. In this case, just as if an SFENCE.VMA is not executed between
>   a write to the memory-management tables and subsequent implicit read of the
>   same address: it is unpredictable whether the old non-leaf PTE or the new
>   leaf PTE is used, but the behavior is otherwise well defined.
> In contrast to the Arm architecture, where BBM is mandatory and failing to
> use it in some cases can lead to CPU instability, RISC-V guarantees
> stability, and the behavior remains safe — though unpredictable in terms of
> which translation will be used.
> 
> Additionally, the page table walk logic has been adjusted, as ARM uses the
> opposite level numbering compared to RISC-V.
> 
> Signed-off-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>

Acked-by: Jan Beulich <jbeulich@xxxxxxxx>