Re: [PATCH for-4.19? v5 07/10] xen: Make the maximum number of altp2m views configurable for x86

[Jan Beulich <jbeulich@xxxxxxxx>]

On 10.06.2024 12:34, Petr Beneš wrote:
> On Mon, Jun 10, 2024 at 12:16 PM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>
>> On 10.06.2024 11:10, Petr Beneš wrote:
>>> On Mon, Jun 10, 2024 at 9:30 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>>>
>>>> On 09.06.2024 01:06, Petr Beneš wrote:
>>>>> On Thu, Jun 6, 2024 at 9:24 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>>>>>> @@ -122,7 +131,12 @@ int p2m_init_altp2m(struct domain *d)
>>>>>>>      struct p2m_domain *hostp2m = p2m_get_hostp2m(d);
>>>>>>>
>>>>>>>      mm_lock_init(&d->arch.altp2m_list_lock);
>>>>>>> -    for ( i = 0; i < MAX_ALTP2M; i++ )
>>>>>>> +    d->arch.altp2m_p2m = xzalloc_array(struct p2m_domain *, 
>>>>>>> d->nr_altp2m);
>>>>>>> +
>>>>>>> +    if ( !d->arch.altp2m_p2m )
>>>>>>> +        return -ENOMEM;
>>>>>>
>>>>>> This isn't really needed, is it? Both ...
>>>>>>
>>>>>>> +    for ( i = 0; i < d->nr_altp2m; i++ )
>>>>>>
>>>>>> ... this and ...
>>>>>>
>>>>>>>      {
>>>>>>>          d->arch.altp2m_p2m[i] = p2m = p2m_init_one(d);
>>>>>>>          if ( p2m == NULL )
>>>>>>> @@ -143,7 +157,10 @@ void p2m_teardown_altp2m(struct domain *d)
>>>>>>>      unsigned int i;
>>>>>>>      struct p2m_domain *p2m;
>>>>>>>
>>>>>>> -    for ( i = 0; i < MAX_ALTP2M; i++ )
>>>>>>> +    if ( !d->arch.altp2m_p2m )
>>>>>>> +        return;
>>>>
>>>> I'm sorry, the question was meant to be on this if() instead.
>>>>
>>>>>>> +    for ( i = 0; i < d->nr_altp2m; i++ )
>>>>>>>      {
>>>>>>>          if ( !d->arch.altp2m_p2m[i] )
>>>>>>>              continue;
>>>>>>> @@ -151,6 +168,8 @@ void p2m_teardown_altp2m(struct domain *d)
>>>>>>>          d->arch.altp2m_p2m[i] = NULL;
>>>>>>>          p2m_free_one(p2m);
>>>>>>>      }
>>>>>>> +
>>>>>>> +    XFREE(d->arch.altp2m_p2m);
>>>>>>>  }
>>>>>>
>>>>>> ... this ought to be fine without?
>>>>>
>>>>> Could you, please, elaborate? I honestly don't know what you mean here
>>>>> (by "this isn't needed").
>>>>
>>>> I hope the above correction is enough?
>>>
>>> I'm sorry, but not really? I feel like I'm blind but I can't see
>>> anything I could remove without causing (or risking) crash.
>>
>> The loop is going to do nothing when d->nr_altp2m == 0, and the XFREE() is
>> going to do nothing when d->arch.altp2m_p2m == NULL. Hence what does the
>> if() guard against? IOW what possible crashes are you seeing that I don't
>> see?
> 
> I see now. I was seeing ghosts - my thinking was that if
> p2m_init_altp2m fails to allocate altp2m_p2m, it will call
> p2m_teardown_altp2m - which, without the if(), would start to iterate
> through an array that is not allocated. It doesn't happen, it just
> returns -ENOMEM.
> 
> So to reiterate:
> 
>     if ( !d->arch.altp2m_p2m )
>         return;
> 
> ... are we talking that this condition inside p2m_teardown_altp2m
> isn't needed?

I'm not sure about "isn't" vs "shouldn't". The call from p2m_final_teardown()
also needs to remain safe to make. Which may require that upon allocation
failure you zap d->nr_altp2m. Or which alternatively may mean that the if()
actually needs to stay.

> Or is there anything else?

There was also the question of whether to guard the allocation, to avoid a
de-generate xmalloc_array() of zero size. Yet in the interest of avoiding
not strictly necessary conditionals, that may well want to remain as is.

Jan