Re: [Xen-devel] [PATCH 02/13] xen: anchor linear p2m list in shared info structure

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 18/02/15 10:42, Juergen Gross wrote:
>
>>>   /* Set up p2m_top to point to the domain-builder provided p2m
>>> pages */
>>> @@ -469,8 +473,10 @@ static pte_t *alloc_p2m_pmd(unsigned long addr,
>>> pte_t *pte_pg)
>>>
>>>           ptechk = lookup_address(vaddr, &level);
>>>           if (ptechk == pte_pg) {
>>> +            HYPERVISOR_shared_info->arch.p2m_generation++;
>>>               set_pmd(pmdp,
>>>                   __pmd(__pa(pte_newpg[i]) | _KERNPG_TABLE));
>>> +            HYPERVISOR_shared_info->arch.p2m_generation++;
>>
>> Do these increments of p2m_generation need to be atomic?
>
> Hmm, they are done under lock. I don't think the compiler is allowed to
> reorder the writes to p2m_generation across set_pmd().

They do need smp_wmb() to guarantee that the increment is visible before
the update occurs, just as the toolstack will need smp_rmb() to read.

They also need to be protected from concurrent update inside the kernel,
for which a lock should appear to suffice.

~Andrew