On 27.01.2022 16:04, Jan Beulich wrote:
> On 04.01.2022 11:57, Jan Beulich wrote:
>> When not holding the PoD lock across the entire region covering P2M
>> update and stats update, the entry count should indicate too large a
>> value in preference to a too small one, to avoid functions bailing early
>> when they find the count is zero. Hence increments should happen ahead
>> of P2M updates, while decrements should happen only after. Deal with the
>> one place where this hasn't been the case yet.
>>
>> Signed-off-by: Jan Beulich <jbeul...@suse.com>
>> ---
>> v2: Add comments.
>
> Ping?
Similarly here: Another 4 weeks have passed ...
Thanks for feedback either way, Jan
>> ---
>> While it might be possible to hold the PoD lock over the entire
>> operation, I didn't want to chance introducing a lock order violation on
>> a perhaps rarely taken code path.
>>
>> --- a/xen/arch/x86/mm/p2m-pod.c
>> +++ b/xen/arch/x86/mm/p2m-pod.c
>> @@ -1342,19 +1342,22 @@ mark_populate_on_demand(struct domain *d
>> }
>> }
>>
>> + /*
>> + * Without holding the PoD lock across the entire operation, bump the
>> + * entry count up front assuming success of p2m_set_entry(), undoing the
>> + * bump as necessary upon failure. Bumping only upon success would risk
>> + * code elsewhere observing entry count being zero despite there
>> actually
>> + * still being PoD entries.
>> + */
>> + pod_lock(p2m);
>> + p2m->pod.entry_count += (1UL << order) - pod_count;
>> + pod_unlock(p2m);
>> +
>> /* Now, actually do the two-way mapping */
>> rc = p2m_set_entry(p2m, gfn, INVALID_MFN, order,
>> p2m_populate_on_demand, p2m->default_access);
>> if ( rc == 0 )
>> - {
>> - pod_lock(p2m);
>> - p2m->pod.entry_count += 1UL << order;
>> - p2m->pod.entry_count -= pod_count;
>> - BUG_ON(p2m->pod.entry_count < 0);
>> - pod_unlock(p2m);
>> -
>> ioreq_request_mapcache_invalidate(d);
>> - }
>> else if ( order )
>> {
>> /*
>> @@ -1366,6 +1369,14 @@ mark_populate_on_demand(struct domain *d
>> d, gfn_l, order, rc);
>> domain_crash(d);
>> }
>> + else if ( !pod_count )
>> + {
>> + /* Undo earlier increment; see comment above. */
>> + pod_lock(p2m);
>> + BUG_ON(!p2m->pod.entry_count);
>> + --p2m->pod.entry_count;
>> + pod_unlock(p2m);
>> + }
>>
>> out:
>> gfn_unlock(p2m, gfn, order);
>>
>>
>
