Re: [Xen-devel] [PATCH RFC] x86/altp2m: fix display frozen when switching to a new view early

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 04.10.18 at 17:34, <george.dunlap@xxxxxxxxxx> wrote:
> On 10/04/2018 04:20 PM, Jan Beulich wrote:
>>>>> On 04.10.18 at 16:56, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
>>> The biggest problem here is p2m->logdirty_ranges. This patch will
>>> (justly) not work, because struct rangeset is only forward-declared in
>>> xen/rangeset.h, so an incomplete type here:
>>>
>>> -void p2m_init_altp2m_ept(struct domain *d, unsigned int i)
>>> +int p2m_init_altp2m_ept(struct domain *d, unsigned int i)
>>>  {
>>>      struct p2m_domain *p2m = d->arch.altp2m_p2m[i];
>>>      struct p2m_domain *hostp2m = p2m_get_hostp2m(d);
>>>      struct ept_data *ept;
>>>
>>> +    if ( !p2m->logdirty_ranges )
>>> +        p2m->logdirty_ranges = rangeset_new(d, "log-dirty",
>>> +                                            RANGESETF_prettyprint_hex);
>>> +    if ( !p2m->logdirty_ranges )
>>> +        return -ENOMEM;
>>> +
>>> +    *p2m->logdirty_ranges = *hostp2m->logdirty_ranges;
>>> +
>>>      p2m->ept.ad = hostp2m->ept.ad;
>>> +    p2m->max_mapped_pfn = hostp2m->max_mapped_pfn;
>>> +    p2m->default_access = hostp2m->default_access;
>>> +    p2m->domain = hostp2m->domain;
>>> +
>>> +    p2m->global_logdirty = hostp2m->global_logdirty;
>>>      p2m->min_remapped_gfn = gfn_x(INVALID_GFN);
>>>      p2m->max_remapped_gfn = 0;
>>>      ept = &p2m->ept;
>>>      ept->mfn = pagetable_get_pfn(p2m_get_pagetable(p2m));
>>>      d->arch.altp2m_eptp[i] = ept->eptp;
>>> +
>>> +    return 0;
>>> +}
>>>
>>> But that's not even the biggest problem: even if that would compile, it
>>> would still be wrong, because logdirty_pages has pointers of its own,
>>> which means that two bitwise-copied distinct rangesets can still point
>>> to the same data and thus be vulnerable to race conditions and wanting
>>> synchronization.
> 
> Yes, so "deep copy" means if a structure has pointers, you copy the
> structures pointed to; and if that structure has pointers, you copy
> those, all the way down.  After a deep copy, any operations on the
> structure should be operating on completely separate bits of memory and
> pointers.
> 
>>> Furthermore there's no rangeset_copy() function in sight in rangeset.h
>>> (though there is a rangeset_swap()).
>>>
>>> Would you like me to add a rangeset_copy() function (presumably another
>>> intermediary patch) and proceed in that manner?
>> 
>> Roger recently has posted a patch adding rangeset_merge(), which I think
>> is more general than your rangeset_copy(). That said, I'm in no way
>> convinced copying (and then keeping in sync) the range sets across the
>> altp2m-s is the best approach. It may well be that the optimal solution is
>> somewhere in the middle between sharing everything and copying
>> everything.
> 
> Er, you mean maybe we should share logdirty ranges and copy other
> things?  Or do you actually mean somehow to share bits of the logdirty
> range structure?

The former, of course. I'm sorry for the ambiguity.

> I think we can reasonably start with a simple-and-correct approach, and
> try to come up with an optimization later if we decide it's necessary.
> The two basic simple-but-correct approaches would be:
> 
> 1. Share logdirty_ranges.  This would mean not duplicating the structure
> and keeping it in sync; but it would mean grabbing the main p2m lock on
> every resolv_misconfig().
> 
> 2. Duplicate the structure and keep it in sync.  This  means not
> grabbing the main p2m lock on every resolv_misconfig(); but it does mean
> duplicating memory, as well as grabbing the lock of every altp2m
> structure every time logdirty_ranges changes.
> 
> As I've said before, I think #2 is the most promising, since
> resolv_misconfig will be called (potentially) for each entry in the p2m
> table, but logdirty_ranges only changes once or twice during the entire
> lifetime of the guest.

So perhaps some r/w lock wants to be introduced?

Jan



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