Re: [Patch] KVM: x86/mmu: Make optimized __handle_changed_spte() for clear dirty log

Sean Christopherson
2023-01-26T01:49:27Z

On Wed, Jan 25, 2023, Ben Gardon wrote:
> On Wed, Jan 25, 2023 at 2:00 PM David Matlack <dmatlack@google.com> wrote:
> >
> > On Wed, Jan 25, 2023 at 01:38:57PM -0800, Vipin Sharma wrote:
> > > Use a tone down version of __handle_changed_spte() when clearing dirty
> > > log. Remove checks which will not be needed when dirty logs are cleared.
> > >
> > > This change shows ~13% improvement in clear dirty log calls in
> > > dirty_log_perf_test
> > >
> > > Before tone down version:
> > > Clear dirty log over 3 iterations took 10.006764203s. (Avg 3.335588067s/iteration)
> > >
> > > After tone down version:
> > > Clear dirty log over 3 iterations took 8.686433554s. (Avg 2.895477851s/iteration)
> > >
> > > Signed-off-by: Vipin Sharma <vipinsh@google.com>
> > > ---
> > > arch/x86/kvm/mmu/tdp_mmu.c | 20 +++++++++++++++++++-
> > > 1 file changed, 19 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > > index bba33aea0fb0..ca21b33c4386 100644
> > > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > > @@ -504,6 +504,19 @@ static void handle_removed_pt(struct kvm *kvm, tdp_ptep_t pt, bool shared)
> > > call_rcu(&sp->rcu_head, tdp_mmu_free_sp_rcu_callback);
> > > }
> > >
> > > +static void handle_changed_spte_clear_dirty_log(int as_id, gfn_t gfn,
> > > + u64 old_spte, u64 new_spte,
> > > + int level)
> > > +{
> > > + if (old_spte == new_spte)
> > > + return;
> > > +
> > > + trace_kvm_tdp_mmu_spte_changed(as_id, gfn, level, old_spte, new_spte);
> > > +
> > > + if (is_dirty_spte(old_spte) && !is_dirty_spte(new_spte))
> > > + kvm_set_pfn_dirty(spte_to_pfn(old_spte));
> > > +}
> > > +
> > > /**
> > > * __handle_changed_spte - handle bookkeeping associated with an SPTE change
> > > * @kvm: kvm instance
> > > @@ -736,7 +749,12 @@ static u64 __tdp_mmu_set_spte(struct kvm *kvm, int as_id, tdp_ptep_t sptep,
> > >
> > > old_spte = kvm_tdp_mmu_write_spte(sptep, old_spte, new_spte, level);
> > >
> > > - __handle_changed_spte(kvm, as_id, gfn, old_spte, new_spte, level, false);
> > > + if (record_dirty_log)
> > > + __handle_changed_spte(kvm, as_id, gfn, old_spte, new_spte,
> > > + level, false);
> > > + else
> > > + handle_changed_spte_clear_dirty_log(as_id, gfn, old_spte,
> > > + new_spte, level);
> >
> > I find it very non-intuitive to tie this behavior to !record_dirty_log,
> > even though it happens to work. It's also risky if any future calls are
> > added that pass record_dirty_log=false but change other bits in the
> > SPTE.
> >
> > I wonder if we could get the same effect by optimizing
> > __handle_changed_spte() to check for a cleared D-bit *first* and if
> > that's the only diff between the old and new SPTE, bail immediately
> > rather than doing all the other checks.
>
> I would also prefer that course. One big lesson I took when building
> the TDP MMU was the value of having all the changed PTE handling go
> through one function. That's not always the right choice but the
> Shadow MMU has a crazy spaghetti code of different functions which
> handle different parts of responding to a PTE change and it makes the
> code very hard to read. I agree this path is worth optimizing, but the
> more we can keep the code together, the better.

Hrm, not sure I agree on that last point. I find record_dirty_log and record_acc_track
to be eye sores and terribly confusing (I always forget when they're true/false).
I think we would end up with cleaner code overall if we special case clearing A/D
bits (or their write-protected/access-protect counterparts).

handle_changed_spte_dirty_log() takes effect if and only a SPTE becomes newly
writable, and that should never happen when aging gfns (record_acc_track=%false),
i.e. neither the "age gfns" nor the "clear dirty" paths need to call
handle_changed_spte_dirty_log(), which means @record_dirty_log is superfluous.

Similarly, clearing dirty bits should never clear the accessed bit, nor should it
toggle PRESENT or change the PFN, i.e. neither path needs to call
handle_changed_spte_acc_track(), which means @record_acc_track is superfluous too.

If we jettison those, then AFAICT the only remaining heuristic is that
tdp_mmu_set_spte_atomic() doesn't update the dirty bitmaps (the comment about
that behavior is unhelpful and doesn't explain _why_). That heuristic is easy to
handled by looking at @shared.

Looking through all the other stuff in __handle_changed_spte()...

I'm 100% comfortable skipping these sanity checks:

if (was_leaf && is_leaf && pfn_changed)
BUG();

if (is_leaf)
check_spte_writable_invariants(new_spte);

if (!was_present && !is_present)
WARN_ON(!MMIO && !REMOVED);

And none of these are relevant (again assuming we don't have an egregious bug)
except for the kvm_set_pfn_dirty() case, which is trivial to handle.

if (is_leaf != was_leaf)
kvm_update_page_stats(kvm, level, is_leaf ? 1 : -1);

if (was_leaf && is_dirty_spte(old_spte) &&
(!is_present || !is_dirty_spte(new_spte) || pfn_changed))
kvm_set_pfn_dirty(spte_to_pfn(old_spte));

/*
* Recursively handle child PTs if the change removed a subtree from
* the paging structure. Note the WARN on the PFN changing without the
* SPTE being converted to a hugepage (leaf) or being zapped. Shadow
* pages are kernel allocations and should never be migrated.
*/
if (was_present && !was_leaf &&
(is_leaf || !is_present || WARN_ON_ONCE(pfn_changed)))
handle_removed_pt(kvm, spte_to_child_pt(old_spte, level), shared);

So unless I'm missing something, I think we can end up with the below. Compile
tested only...

---
arch/x86/kvm/mmu/tdp_mmu.c | 92 ++++++++++----------------------------
1 file changed, 24 insertions(+), 68 deletions(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index bba33aea0fb0..2f78ca43a276 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -505,7 +505,7 @@ static void handle_removed_pt(struct kvm *kvm, tdp_ptep_t pt, bool shared)
}

/**
- * __handle_changed_spte - handle bookkeeping associated with an SPTE change
+ * handle_changed_spte - handle bookkeeping associated with an SPTE change
* @kvm: kvm instance
* @as_id: the address space of the paging structure the SPTE was a part of
* @gfn: the base GFN that was mapped by the SPTE
@@ -519,9 +519,9 @@ static void handle_removed_pt(struct kvm *kvm, tdp_ptep_t pt, bool shared)
* Handle bookkeeping that might result from the modification of a SPTE.
* This function must be called for all TDP SPTE modifications.
*/
-static void __handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
- u64 old_spte, u64 new_spte, int level,
- bool shared)
+static void handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
+ u64 old_spte, u64 new_spte, int level,
+ bool shared)
{
bool was_present = is_shadow_present_pte(old_spte);
bool is_present = is_shadow_present_pte(new_spte);
@@ -605,23 +605,18 @@ static void __handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
if (was_present && !was_leaf &&
(is_leaf || !is_present || WARN_ON_ONCE(pfn_changed)))
handle_removed_pt(kvm, spte_to_child_pt(old_spte, level), shared);
-}

-static void handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
- u64 old_spte, u64 new_spte, int level,
- bool shared)
-{
- __handle_changed_spte(kvm, as_id, gfn, old_spte, new_spte, level,
- shared);
handle_changed_spte_acc_track(old_spte, new_spte, level);
- handle_changed_spte_dirty_log(kvm, as_id, gfn, old_spte,
- new_spte, level);
+
+ /* COMMENT GOES HERE. */
+ if (!shared)
+ handle_changed_spte_dirty_log(kvm, as_id, gfn, old_spte,
+ new_spte, level);
}

/*
* tdp_mmu_set_spte_atomic - Set a TDP MMU SPTE atomically
- * and handle the associated bookkeeping. Do not mark the page dirty
- * in KVM's dirty bitmaps.
+ * and handle the associated bookkeeping.
*
* If setting the SPTE fails because it has changed, iter->old_spte will be
* refreshed to the current value of the spte.
@@ -658,9 +653,8 @@ static inline int tdp_mmu_set_spte_atomic(struct kvm *kvm,
if (!try_cmpxchg64(sptep, &iter->old_spte, new_spte))
return -EBUSY;

- __handle_changed_spte(kvm, iter->as_id, iter->gfn, iter->old_spte,
- new_spte, iter->level, true);
- handle_changed_spte_acc_track(iter->old_spte, new_spte, iter->level);
+ handle_changed_spte(kvm, iter->as_id, iter->gfn, iter->old_spte,
+ new_spte, iter->level, true);

return 0;
}
@@ -705,23 +699,12 @@ static inline int tdp_mmu_zap_spte_atomic(struct kvm *kvm,
* @new_spte: The new value that will be set for the SPTE
* @gfn: The base GFN that was (or will be) mapped by the SPTE
* @level: The level _containing_ the SPTE (its parent PT's level)
- * @record_acc_track: Notify the MM subsystem of changes to the accessed state
- * of the page. Should be set unless handling an MMU
- * notifier for access tracking. Leaving record_acc_track
- * unset in that case prevents page accesses from being
- * double counted.
- * @record_dirty_log: Record the page as dirty in the dirty bitmap if
- * appropriate for the change being made. Should be set
- * unless performing certain dirty logging operations.
- * Leaving record_dirty_log unset in that case prevents page
- * writes from being double counted.
*
* Returns the old SPTE value, which _may_ be different than @old_spte if the
* SPTE had voldatile bits.
*/
static u64 __tdp_mmu_set_spte(struct kvm *kvm, int as_id, tdp_ptep_t sptep,
- u64 old_spte, u64 new_spte, gfn_t gfn, int level,
- bool record_acc_track, bool record_dirty_log)
+ u64 old_spte, u64 new_spte, gfn_t gfn, int level)
{
lockdep_assert_held_write(&kvm->mmu_lock);

@@ -736,46 +719,18 @@ static u64 __tdp_mmu_set_spte(struct kvm *kvm, int as_id, tdp_ptep_t sptep,

old_spte = kvm_tdp_mmu_write_spte(sptep, old_spte, new_spte, level);

- __handle_changed_spte(kvm, as_id, gfn, old_spte, new_spte, level, false);
-
- if (record_acc_track)
- handle_changed_spte_acc_track(old_spte, new_spte, level);
- if (record_dirty_log)
- handle_changed_spte_dirty_log(kvm, as_id, gfn, old_spte,
- new_spte, level);
+ handle_changed_spte(kvm, as_id, gfn, old_spte, new_spte, level, false);
return old_spte;
}

-static inline void _tdp_mmu_set_spte(struct kvm *kvm, struct tdp_iter *iter,
- u64 new_spte, bool record_acc_track,
- bool record_dirty_log)
+static inline void tdp_mmu_set_spte(struct kvm *kvm, struct tdp_iter *iter,
+ u64 new_spte)
{
WARN_ON_ONCE(iter->yielded);

iter->old_spte = __tdp_mmu_set_spte(kvm, iter->as_id, iter->sptep,
iter->old_spte, new_spte,
- iter->gfn, iter->level,
- record_acc_track, record_dirty_log);
-}
-
-static inline void tdp_mmu_set_spte(struct kvm *kvm, struct tdp_iter *iter,
- u64 new_spte)
-{
- _tdp_mmu_set_spte(kvm, iter, new_spte, true, true);
-}
-
-static inline void tdp_mmu_set_spte_no_acc_track(struct kvm *kvm,
- struct tdp_iter *iter,
- u64 new_spte)
-{
- _tdp_mmu_set_spte(kvm, iter, new_spte, false, true);
-}
-
-static inline void tdp_mmu_set_spte_no_dirty_log(struct kvm *kvm,
- struct tdp_iter *iter,
- u64 new_spte)
-{
- _tdp_mmu_set_spte(kvm, iter, new_spte, true, false);
+ iter->gfn, iter->level);
}

#define tdp_root_for_each_pte(_iter, _root, _start, _end) \
@@ -925,7 +880,7 @@ bool kvm_tdp_mmu_zap_sp(struct kvm *kvm, struct kvm_mmu_page *sp)
return false;

__tdp_mmu_set_spte(kvm, kvm_mmu_page_as_id(sp), sp->ptep, old_spte, 0,
- sp->gfn, sp->role.level + 1, true, true);
+ sp->gfn, sp->role.level + 1);

return true;
}
@@ -1289,8 +1244,7 @@ static bool age_gfn_range(struct kvm *kvm, struct tdp_iter *iter,
new_spte = mark_spte_for_access_track(new_spte);
}

- tdp_mmu_set_spte_no_acc_track(kvm, iter, new_spte);
-
+ kvm_tdp_mmu_write_spte(iter->sptep, iter->old_spte, new_spte, iter->level);
return true;
}

@@ -1326,7 +1280,7 @@ static bool set_spte_gfn(struct kvm *kvm, struct tdp_iter *iter,
* Note, when changing a read-only SPTE, it's not strictly necessary to
* zero the SPTE before setting the new PFN, but doing so preserves the
* invariant that the PFN of a present * leaf SPTE can never change.
- * See __handle_changed_spte().
+ * See handle_changed_spte().
*/
tdp_mmu_set_spte(kvm, iter, 0);

@@ -1351,7 +1305,7 @@ bool kvm_tdp_mmu_set_spte_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
/*
* No need to handle the remote TLB flush under RCU protection, the
* target SPTE _must_ be a leaf SPTE, i.e. cannot result in freeing a
- * shadow page. See the WARN on pfn_changed in __handle_changed_spte().
+ * shadow page. See the WARN on pfn_changed in handle_changed_spte().
*/
return kvm_tdp_mmu_handle_gfn(kvm, range, set_spte_gfn);
}
@@ -1703,9 +1657,11 @@ static void clear_dirty_pt_masked(struct kvm *kvm, struct kvm_mmu_page *root,
new_spte = iter.old_spte & ~shadow_dirty_mask;
else
continue;
+
+ kvm_set_pfn_dirty(spte_to_pfn(iter.old_spte));
}

- tdp_mmu_set_spte_no_dirty_log(kvm, &iter, new_spte);
+ kvm_tdp_mmu_write_spte(iter.sptep, iter.old_spte, new_spte, iter.level);
}

rcu_read_unlock();

base-commit: f15a87c006901e02727bf8ac75b0251cdf8e0ecc
--