Re: [Xen-devel] [PATCH V3 2/3] x86/mm: allocate logdirty_ranges for altp2ms

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 30.10.18 at 17:32, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
> On 10/30/18 6:22 PM, Jan Beulich wrote:
>>>>> On 29.10.18 at 13:40, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
>>> This patch is a pre-requisite for the one fixing VGA logdirty
>>> freezes when using altp2m. It only concerns itself with the
>>> ranges allocation / deallocation / initialization part.
>> 
>> But while looking (briefly only for now) over patch 3 I couldn't
>> see any sync-ing of the log-dirty ranges there either. Doesn't
>> this need doing either there or here, if you go the copy route?
> 
> The syncing is done in patch 3, in p2m_change_entry_type_global(),
> p2m_memory_type_changed(), p2m_change_type_range(), where now all active
> p2ms (hostp2m + altp2ms) receive the same changes.
> 
> On allocating the logdirty_ranges for a new altp2m, the hostp2m (which
> is sync with all others) is copied over, and then whatever logdirty
> ranges the hypervisor is using for reading should be fine.
> 
> That was the thinking at least, am I missing something?

Ah, yes, I was misremembering how the log-dirty rangesets
get maintained. Sorry.

>>> @@ -2271,6 +2297,7 @@ void p2m_flush_altp2m(struct domain *d)
>>>      {
>>>          p2m_flush_table(d->arch.altp2m_p2m[i]);
>>>          /* Uninit and reinit ept to force TLB shootdown */
>>> +        p2m_free_logdirty(d->arch.altp2m_p2m[i]);
>>>          ept_p2m_uninit(d->arch.altp2m_p2m[i]);
>>>          ept_p2m_init(d->arch.altp2m_p2m[i]);
>>>          d->arch.altp2m_eptp[i] = mfn_x(INVALID_MFN);
>>> @@ -2341,6 +2385,7 @@ int p2m_destroy_altp2m_by_id(struct domain *d, 
>>> unsigned int idx)
>>>          {
>>>              p2m_flush_table(d->arch.altp2m_p2m[idx]);
>>>              /* Uninit and reinit ept to force TLB shootdown */
>>> +            p2m_free_logdirty(d->arch.altp2m_p2m[idx]);
>>>              ept_p2m_uninit(d->arch.altp2m_p2m[idx]);
>>>              ept_p2m_init(d->arch.altp2m_p2m[idx]);
>>>              d->arch.altp2m_eptp[idx] = mfn_x(INVALID_MFN);
>>> @@ -2471,6 +2516,7 @@ static void p2m_reset_altp2m(struct p2m_domain *p2m)
>>>  {
>>>      p2m_flush_table(p2m);
>>>      /* Uninit and reinit ept to force TLB shootdown */
>>> +    p2m_free_logdirty(p2m);
>>>      ept_p2m_uninit(p2m);
>>>      ept_p2m_init(p2m);
>>>      p2m->min_remapped_gfn = gfn_x(INVALID_GFN);
>> 
>> For one these look all pretty similar, so I wonder why there's
>> no helper function. But that's not something you need to change.
>> Yet why are you freeing the log-dirty ranges here? These aren't
>> full cleanup paths afaict.
> 
> After the first iteration Andrew has suggested that I should free in
> ept_p2m_uninit(p2m):
> 
> https://lists.xenproject.org/archives/html/xen-devel/2018-10/msg01430.html 
> 
> at which point I did so for V2, where you've suggested that that
> function should be static and live in p2m.c. Which led to V3 where
> basically I've plopped this function right before ept_p2m_uninit(p2m)
> wherever it was called to match the V2 behaviour.

I don't care much about the history of from where exactly to
do the cleanup you do here; I'd like to understand _why_ this
is needed, when this not a p2m destruction path. IOW why is
the original rangeset structure no longer needed? Wouldn't it
be re-allocated anyway later on, in which case you could just
empty it (if that's indeed needed at these points)?

Jan



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