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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Jun 12, 2026 at 09:40:03AM +0200, Jan Beulich wrote:
> 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.

Yes, indeed I was assuming that stubs[cpu - 1] would always be
populated, which it's the case currently, but you are right that it's
trivial to also take this scenario into account here.

Thanks, Roger.