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 7:04 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". 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? If it only
held the gfn_lock/p2m_lock then we would have the lock order violation
down the road.

Tamas