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

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 10/04/2018 04:45 PM, Jan Beulich wrote:
>>>> 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?

There will also be locking order issues to consider if we do that.

What's your main reason for not wanting #2?

 -George

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