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

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

On Wed, Jun 17, 2020 at 7:36 AM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 17.06.2020 15:31, Tamas K Lengyel wrote:
> > On Wed, Jun 17, 2020 at 7:28 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>
> >> On 17.06.2020 15:21, Tamas K Lengyel wrote:
> >>> On Wed, Jun 17, 2020 at 7:04 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>>>
> >>>> On 17.06.2020 15:00, Tamas K Lengyel wrote:
> >>>>> On Wed, Jun 17, 2020 at 3:59 AM Jan Beulich <jbeulich@suse.com> 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.

Tamas