Re: [PATCH v4 16/18] xen/riscv: implement mfn_valid() and page reference, ownership handling helpers

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.09.2025 23:55, Oleksii Kurochko wrote:
> Implement the mfn_valid() macro to verify whether a given MFN is valid by
> checking that it falls within the range [start_page, max_page).
> These bounds are initialized based on the start and end addresses of RAM.
> 
> As part of this patch, start_page is introduced and initialized with the
> PFN of the first RAM page.
> Also, initialize pdx_group_valid() by calling set_pdx_range() when
> memory banks are being mapped.
> 
> Also, after providing a non-stub implementation of the mfn_valid() macro,
> the following compilation errors started to occur:
>   riscv64-linux-gnu-ld: prelink.o: in function `alloc_heap_pages':
>   /build/xen/common/page_alloc.c:1054: undefined reference to 
> `page_is_offlinable'
>   riscv64-linux-gnu-ld: /build/xen/common/page_alloc.c:1035: undefined 
> reference to `page_is_offlinable'
>   riscv64-linux-gnu-ld: prelink.o: in function `reserve_offlined_page':
>   /build/xen/common/page_alloc.c:1151: undefined reference to 
> `page_is_offlinable'
>   riscv64-linux-gnu-ld: ./.xen-syms.0: hidden symbol `page_is_offlinable' 
> isn't defined
>   riscv64-linux-gnu-ld: final link failed: bad value
>   make[2]: *** [arch/riscv/Makefile:28: xen-syms] Error 1
> 
> To resolve these errors, the following functions have also been introduced,
> based on their Arm counterparts:
> - page_get_owner_and_reference() and its variant to safely acquire a
>   reference to a page and retrieve its owner.
> - Implement page_is_offlinable() to return false for RISC-V.
> 
> Signed-off-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>

Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
with two cosmetic adjustments:

> @@ -642,3 +648,30 @@ void put_page(struct page_info *page)
>              free_domheap_page(page);
>      }
>  }
> +
> +bool page_is_offlinable(mfn_t mfn)
> +{
> +    return false;
> +}

I think this wants to move elsewhere, or ...

> +struct domain *page_get_owner_and_reference(struct page_info *page)

... this wants to move up, such that the "get" and "put" logic are next
to each other.

> +{
> +    unsigned long x, y = page->count_info;
> +    struct domain *owner;
> +
> +    do {
> +        x = y;
> +        /*
> +         * Count ==  0: Page is not allocated, so we cannot take a reference.
> +         * Count == -1: Reference count would wrap, which is invalid.
> +         */
> +        if ( unlikely(((x + 1) & PGC_count_mask) <= 1) )
> +            return NULL;
> +    }
> +    while ( (y = cmpxchg(&page->count_info, x, x + 1)) != x );

This again wants the figure brace placement corrected.

Jan