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 9:46 AM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 17.06.2020 16:49, Tamas K Lengyel wrote:
> > On Wed, Jun 17, 2020 at 8:24 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>
> >> On 17.06.2020 15:43, Tamas K Lengyel wrote:
> >>> 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.
> >>
> >> 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.
>
> I disagree (because such missing unshare requests could be put
> elsewhere), but let me ask another question then: Why is it that
> vmx_load_pdptrs() needs to unshare? The function only reads from
> the page. Even if the page's content changed later on, we wouldn't
> care, as there's no coherence of the PDPTRs once loaded.

Ah, I see what you mean. It really just looks like it reads from the
page so P2M_ALLOC would suffice. Let me verify.

Tamas