On 10.06.2025 15:05, 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, 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 `__next_node':
> /build/xen/./include/xen/nodemask.h:202: undefined reference to
> `page_is_ram_type'
> riscv64-linux-gnu-ld: prelink.o: in function `get_free_buddy':
> /build/xen/common/page_alloc.c:881: undefined reference to
> `page_is_ram_type'
> riscv64-linux-gnu-ld: prelink.o: in function `alloc_heap_pages':
> /build/xen/common/page_alloc.c:1043: undefined reference to
> `page_get_owner_and_reference'
> riscv64-linux-gnu-ld: /build/xen/common/page_alloc.c:1098: undefined
> reference to `page_is_ram_type'
> riscv64-linux-gnu-ld: prelink.o: in function `ns16550_interrupt':
> /build/xen/drivers/char/ns16550.c:205: undefined reference to `get_page'
> riscv64-linux-gnu-ld: ./.xen-syms.0: hidden symbol
> `page_get_owner_and_reference' isn't defined
> riscv64-linux-gnu-ld: final link failed: bad value
> make[2]: *** [arch/riscv/Makefile:35: 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.
> - put_page() and put_page_nr() to release page references and free the page
> when the count drops to zero.
> For put_page_nr(), code related to static memory configuration is wrapped
> with CONFIG_STATIC_MEMORY, as this configuration has not yet been moved to
> common code. Therefore, PGC_static and free_domstatic_page() are not
> introduced for RISC-V. However, since this configuration could be useful
> in the future, the relevant code is retained and conditionally compiled.
> - A stub for page_is_ram_type() that currently always returns 0 and asserts
> unreachable, as RAM type checking is not yet implemented.
How does this end up working when common code references the function?
> @@ -288,8 +289,12 @@ static inline bool arch_mfns_in_directmap(unsigned long
> mfn, unsigned long nr)
> #define page_get_owner(p) (p)->v.inuse.domain
> #define page_set_owner(p, d) ((p)->v.inuse.domain = (d))
>
> -/* TODO: implement */
> -#define mfn_valid(mfn) ({ (void)(mfn); 0; })
> +extern unsigned long start_page;
> +
> +#define mfn_valid(mfn) ({ \
> + unsigned long mfn__ = mfn_x(mfn); \
> + likely((mfn__ >= start_page) && (mfn__ < max_page)); \
> +})
I don't think you should try to be clever and avoid using __mfn_valid() here,
at least not without an easily identifiable TODO. Surely you've seen that both
Arm and x86 use it.
Also, according to all I know, likely() doesn't work very well when used like
this, except for architectures supporting conditionally executed insns (like
Arm32 or IA-64, i.e. beyond conditional branches). I.e. if you want to use
likely() here, I think you need two of them.
> @@ -525,6 +520,8 @@ static void __init setup_directmap_mappings(unsigned long
> base_mfn,
> #error setup_{directmap,frametable}_mapping() should be implemented for RV_32
> #endif
>
> +unsigned long __read_mostly start_page;
Memory hotplug question again: __read_mostly or __ro_after_init?
> @@ -613,3 +612,91 @@ void __iomem *ioremap(paddr_t pa, size_t len)
> {
> return ioremap_attr(pa, len, PAGE_HYPERVISOR_NOCACHE);
> }
> +
> +int page_is_ram_type(unsigned long mfn, unsigned long mem_type)
> +{
> + ASSERT_UNREACHABLE();
> +
> + return 0;
> +}
> +
> +static struct domain *page_get_owner_and_nr_reference(struct page_info *page,
> + unsigned long nr)
> +{
> + unsigned long x, y = page->count_info;
> + struct domain *owner;
> +
> + /* Restrict nr to avoid "double" overflow */
> + if ( nr >= PGC_count_mask )
> + {
> + ASSERT_UNREACHABLE();
> + return NULL;
> + }
I question the validity of this, already in the Arm original: I can't spot
how the caller guarantees to stay below that limit. Without such an
(attempted) guarantee, ASSERT_UNREACHABLE() is wrong to use. All I can see
is process_shm_node() incrementing shmem_extra[].nr_shm_borrowers, without
any limit check.
> + do {
> + x = y;
> + /*
> + * Count == 0: Page is not allocated, so we cannot take a reference.
> + * Count == -1: Reference count would wrap, which is invalid.
> + */
May I once again ask that you look carefully at comments (as much as at code)
you copy. Clearly this comment wasn't properly updated when the bumping by 1
was changed to bumping by nr.
> + if ( unlikely(((x + nr) & PGC_count_mask) <= nr) )
> + return NULL;
> + }
> + while ( (y = cmpxchg(&page->count_info, x, x + nr)) != x );
> +
> + owner = page_get_owner(page);
> + ASSERT(owner);
> +
> + return owner;
> +}
> +
> +struct domain *page_get_owner_and_reference(struct page_info *page)
> +{
> + return page_get_owner_and_nr_reference(page, 1);
> +}
> +
> +void put_page_nr(struct page_info *page, unsigned long nr)
> +{
> + unsigned long nx, x, y = page->count_info;
> +
> + do {
> + ASSERT((y & PGC_count_mask) >= nr);
> + x = y;
> + nx = x - nr;
> + }
> + while ( unlikely((y = cmpxchg(&page->count_info, x, nx)) != x) );
> +
> + if ( unlikely((nx & PGC_count_mask) == 0) )
> + {
> +#ifdef CONFIG_STATIC_MEMORY
> + if ( unlikely(nx & PGC_static) )
> + free_domstatic_page(page);
> + else
> +#endif
Such #ifdef-ed-out code is liable to go stale. Minimally use IS_ENABLED().
Even better would imo be if you introduced a "stub" PGC_static, resolving
to 0 (i.e. for now unconditionally).
Jan
