Re: [PATCH for-4.14] x86/hap: use get_gfn_type in hap_update_paging_modes

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.06.2020 15:31, Tamas K Lengyel wrote:
> On Wed, Jun 17, 2020 at 7:28 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>
>> On 17.06.2020 15:21, Tamas K Lengyel wrote:
>>> On Wed, Jun 17, 2020 at 7:04 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>>>
>>>> On 17.06.2020 15:00, Tamas K Lengyel wrote:
>>>>> On Wed, Jun 17, 2020 at 3:59 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>>>>> If there are code paths of both kinds, which approach to use in
>>>>>> vmx_load_pdptrs() may need to be chosen based on what
>>>>>> paging_locked_by_me() returns. Or perhaps an unlocked query is
>>>>>> fine in either case?
>>>>>
>>>>> Perhaps adjusting vmx_load_pdptrs to chose the unlocked query would be
>>>>> fine. But at that point what is the reason for having the lock
>>>>> ordering at all? Why not just have a single recursive lock and avoid
>>>>> issues like this altogether?
>>>>
>>>> With just a single lock, contention problems we already know we
>>>> have would be even worse. When the current locking model was
>>>> introduced, there was actually a plan to make gfn_lock() more
>>>> fine-grained (i.e. not simply "de-generate" to p2m_lock()), for
>>>> example.
>>>
>>> Sigh. Well, I've been checking and adjust vmx_load_pdptrs to use an
>>> unlocked query doesn't seem as straightforward because, well, there is
>>> no unlocked version of p2m_get_page_from_gfn which would also do the
>>> "fixups".
>>
>> Which fixups do we need here, in particular? Of course, whenever
>> any fixups get done, the operation can't be lock-less.
>>
>>> What seems redundant to me though is that
>>> hap_update_paging_modes takes both the p2m_lock via get_gfn PLUS the
>>> paging_lock. Does it really need to take the paging_lock?
>>
>> From mm-locks.h's comments:
>>
>>  * For HAP, it protects the NPT/EPT tables and mode changes.
> 
> We do the population of the EPT as part of fork_page() if there was a
> hole in the p2m when the query was issued using P2M_ALLOC (or
> P2M_UNSHARE). I checked and without the paging lock held it throws up
> at hap_alloc's ASSERT.. So yea, currently I don't think we have a
> better route then what I currently sent in.

You didn't answer my question regarding the "fixups" needed, so
for the moment it's not clear to me yet whether indeed there's
no better way.

Jan