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 04:20 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?

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.

 -George

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