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

[Tamas K Lengyel <tamas.k.lengyel@xxxxxxxxx>]

On Wed, Jun 17, 2020 at 8:24 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>
> On 17.06.2020 15:43, Tamas K Lengyel wrote:
> > On Wed, Jun 17, 2020 at 7:36 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
> >>
> >> 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.
> >
> > Umm, I did. The fixups entail populating the EPT from the parent as I
> > described above.
>
> Isn't this taken care of by the new call to get_gfn_type() which you add?
> As said before, I think at the point we want to obtain the PDPTs all
> other adjustments and arrangements should have been done already, by
> higher layers. This code should have no need to do anything beyond a
> simple lookup.

I don't really know what else to say. There are multiple paths leading
to vmx_load_pdptrs, some take the paging_lock while some don't. In
this particular case we can do the fixups earlier as I do in this
patch because there happens to be a lookup before the paging_lock is
taken but in other cases there isn't such a route so removing
P2M_UNSHARE from vmx_load_pdptrs is not an option.

Tamas