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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.06.2020 15:21, 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.

Jan