Re: [PATCH for-4.22 v2] xen/x86: Change stub page allocation/free

[Jan Beulich <jbeulich@xxxxxxxx>]

On 11.06.2026 16:21, Roger Pau Monne wrote:
> @@ -641,41 +642,62 @@ static int do_boot_cpu(int apicid, int cpu)
>      return rc;
>  }
>  
> -#define STUB_BUF_CPU_OFFS(cpu) (((cpu) & (STUBS_PER_PAGE - 1)) * 
> STUB_BUF_SIZE)
> +/* Dynamically allocated, indexed by CPU.  Store physical address of stubs. 
> */
> +static paddr_t *__ro_after_init stubs;
>  
> -unsigned long alloc_stub_page(unsigned int cpu, unsigned long *mfn)
> +static bool assign_stub_page(unsigned int cpu)
>  {
>      unsigned long stub_va;
> -    struct page_info *pg;
> +    paddr_t addr = stubs[cpu];
>  
> -    BUILD_BUG_ON(STUBS_PER_PAGE & (STUBS_PER_PAGE - 1));
> -
> -    if ( *mfn )
> -        pg = mfn_to_page(_mfn(*mfn));
> -    else
> +    if ( addr == INVALID_PADDR )
>      {
> -        nodeid_t node = cpu_to_node(cpu);
> -        unsigned int memflags = node != NUMA_NO_NODE ? MEMF_node(node) : 0;
> +        nodeid_t nid = cpu_to_node(cpu);
>  
> -        pg = alloc_domheap_page(NULL, memflags);
> -        if ( !pg )
> -            return 0;
> +        /*
> +         * Attempt to use the same page as the previous CPU if possible,
> +         * otherwise allocate a new one.
> +         */
> +        if ( cpu && nid == cpu_to_node(cpu - 1) &&
> +             PAGE_OFFSET(stubs[cpu - 1] + STUB_BUF_SIZE) )
> +            addr = stubs[cpu - 1] + STUB_BUF_SIZE;

Isn't this path also going to be taken if stubs[cpu - 1] is still INVALID_PADDR?
That may be a purely hypothetical case right now, but imo would better be
covered right away.

Jan