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From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Ben Gardon <bgardon@google.com>
Cc: kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
	Peter Feiner <pfeiner@google.com>,
	Peter Shier <pshier@google.com>,
	Junaid Shahid <junaids@google.com>,
	Jim Mattson <jmattson@google.com>
Subject: Re: [RFC PATCH 13/28] kvm: mmu: Add an iterator for concurrent paging structure walks
Date: Wed, 18 Dec 2019 11:14:55 -0800	[thread overview]
Message-ID: <20191218191455.GD25201@linux.intel.com> (raw)
In-Reply-To: <CANgfPd_9KpwOuk1pQ7jzhmFksE-FBaFMPP-yhmG1yu9txUBi3Q@mail.gmail.com>

On Wed, Dec 18, 2019 at 10:25:45AM -0800, Ben Gardon wrote:
> On Mon, Dec 2, 2019 at 6:15 PM Sean Christopherson
> <sean.j.christopherson@intel.com> wrote:
> >
> > > +static bool direct_walk_iterator_next_pte(struct direct_walk_iterator *iter)
> > > +{
> > > +     /*
> > > +      * This iterator could be iterating over a large number of PTEs, such
> > > +      * that if this thread did not yield, it would cause scheduler\
> > > +      * problems. To avoid this, yield if needed. Note the check on
> > > +      * MMU_LOCK_MAY_RESCHED in direct_walk_iterator_cond_resched. This
> > > +      * iterator will not yield unless that flag is set in its lock_mode.
> > > +      */
> > > +     direct_walk_iterator_cond_resched(iter);
> >
> > This looks very fragile, e.g. one of the future patches even has to avoid
> > problems with this code by limiting the number of PTEs it processes.
>
> With this, functions either need to limit the number of PTEs they
> process or pass the MMU_LOCK_MAY_RESCHED to the iterator. It would
> probably be safer to invert the flag and make it
> MMU_LOCK_MAY_NOT_RESCHED for functions that can self-regulate the
> number of PTEs they process or have weird synchronization
> requirements. For example, the page fault handler can't reschedule and
> we know it won't process many entries, so we could pass
> MMU_LOCK_MAY_NOT_RESCHED in there.

That doesn't address the underlying fragility of the iterator, i.e. relying
on callers to self-regulate.  Especially since the threshold is completely
arbitrary, e.g. in zap_direct_gfn_range(), what's to say PDPE and lower is
always safe, e.g. if should_resched() becomes true at the very start of the
walk?

The direct comparison to zap_direct_gfn_range() is slot_handle_level_range(),
which supports rescheduling regardless of what function is being invoked.
What prevents the TDP iterator from doing the same?  E.g. what's the worst
case scenario if a reschedule pops up at an inopportune time?

  reply	other threads:[~2019-12-18 19:14 UTC|newest]

Thread overview: 57+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-26 23:17 [RFC PATCH 00/28] kvm: mmu: Rework the x86 TDP direct mapped case Ben Gardon
2019-09-26 23:17 ` [RFC PATCH 01/28] kvm: mmu: Separate generating and setting mmio ptes Ben Gardon
2019-11-27 18:15   ` Sean Christopherson
2019-09-26 23:17 ` [RFC PATCH 02/28] kvm: mmu: Separate pte generation from set_spte Ben Gardon
2019-11-27 18:25   ` Sean Christopherson
2019-09-26 23:17 ` [RFC PATCH 03/28] kvm: mmu: Zero page cache memory at allocation time Ben Gardon
2019-11-27 18:32   ` Sean Christopherson
2019-09-26 23:18 ` [RFC PATCH 04/28] kvm: mmu: Update the lpages stat atomically Ben Gardon
2019-11-27 18:39   ` Sean Christopherson
2019-12-06 20:10     ` Ben Gardon
2019-09-26 23:18 ` [RFC PATCH 05/28] sched: Add cond_resched_rwlock Ben Gardon
2019-11-27 18:42   ` Sean Christopherson
2019-12-06 20:12     ` Ben Gardon
2019-09-26 23:18 ` [RFC PATCH 06/28] kvm: mmu: Replace mmu_lock with a read/write lock Ben Gardon
2019-11-27 18:47   ` Sean Christopherson
2019-12-02 22:45     ` Sean Christopherson
2019-09-26 23:18 ` [RFC PATCH 07/28] kvm: mmu: Add functions for handling changed PTEs Ben Gardon
2019-11-27 19:04   ` Sean Christopherson
2019-09-26 23:18 ` [RFC PATCH 08/28] kvm: mmu: Init / Uninit the direct MMU Ben Gardon
2019-12-02 23:40   ` Sean Christopherson
2019-12-06 20:25     ` Ben Gardon
2019-09-26 23:18 ` [RFC PATCH 09/28] kvm: mmu: Free direct MMU page table memory in an RCU callback Ben Gardon
2019-09-26 23:18 ` [RFC PATCH 10/28] kvm: mmu: Flush TLBs before freeing direct MMU page table memory Ben Gardon
2019-12-02 23:46   ` Sean Christopherson
2019-12-06 20:31     ` Ben Gardon
2019-09-26 23:18 ` [RFC PATCH 11/28] kvm: mmu: Optimize for freeing direct MMU PTs on teardown Ben Gardon
2019-12-02 23:54   ` Sean Christopherson
2019-09-26 23:18 ` [RFC PATCH 12/28] kvm: mmu: Set tlbs_dirty atomically Ben Gardon
2019-12-03  0:13   ` Sean Christopherson
2019-09-26 23:18 ` [RFC PATCH 13/28] kvm: mmu: Add an iterator for concurrent paging structure walks Ben Gardon
2019-12-03  2:15   ` Sean Christopherson
2019-12-18 18:25     ` Ben Gardon
2019-12-18 19:14       ` Sean Christopherson [this message]
2019-09-26 23:18 ` [RFC PATCH 14/28] kvm: mmu: Batch updates to the direct mmu disconnected list Ben Gardon
2019-09-26 23:18 ` [RFC PATCH 15/28] kvm: mmu: Support invalidate_zap_all_pages Ben Gardon
2019-09-26 23:18 ` [RFC PATCH 16/28] kvm: mmu: Add direct MMU page fault handler Ben Gardon
2020-01-08 17:20   ` Peter Xu
2020-01-08 18:15     ` Ben Gardon
2020-01-08 19:00       ` Peter Xu
2019-09-26 23:18 ` [RFC PATCH 17/28] kvm: mmu: Add direct MMU fast " Ben Gardon
2019-09-26 23:18 ` [RFC PATCH 18/28] kvm: mmu: Add an hva range iterator for memslot GFNs Ben Gardon
2019-09-26 23:18 ` [RFC PATCH 19/28] kvm: mmu: Make address space ID a property of memslots Ben Gardon
2019-09-26 23:18 ` [RFC PATCH 20/28] kvm: mmu: Implement the invalidation MMU notifiers for the direct MMU Ben Gardon
2019-09-26 23:18 ` [RFC PATCH 21/28] kvm: mmu: Integrate the direct mmu with the changed pte notifier Ben Gardon
2019-09-26 23:18 ` [RFC PATCH 22/28] kvm: mmu: Implement access tracking for the direct MMU Ben Gardon
2019-09-26 23:18 ` [RFC PATCH 23/28] kvm: mmu: Make mark_page_dirty_in_slot usable from outside kvm_main Ben Gardon
2019-09-26 23:18 ` [RFC PATCH 24/28] kvm: mmu: Support dirty logging in the direct MMU Ben Gardon
2019-09-26 23:18 ` [RFC PATCH 25/28] kvm: mmu: Support kvm_zap_gfn_range " Ben Gardon
2019-09-26 23:18 ` [RFC PATCH 26/28] kvm: mmu: Integrate direct MMU with nesting Ben Gardon
2019-09-26 23:18 ` [RFC PATCH 27/28] kvm: mmu: Lazily allocate rmap when direct MMU is enabled Ben Gardon
2019-09-26 23:18 ` [RFC PATCH 28/28] kvm: mmu: Support MMIO in the direct MMU Ben Gardon
2019-10-17 18:50 ` [RFC PATCH 00/28] kvm: mmu: Rework the x86 TDP direct mapped case Sean Christopherson
2019-10-18 13:42   ` Paolo Bonzini
2019-11-27 19:09 ` Sean Christopherson
2019-12-06 19:55   ` Ben Gardon
2019-12-06 19:57     ` Sean Christopherson
2019-12-06 20:42       ` Ben Gardon

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