[PATCH 2/2] KVM: mmu/x86: optimize zapping by retaining non-leaf SPTEs and avoid rcu stall

[Mingwei Zhang]

TDP MMU SPTE zapping process currently uses two levels of iterations. The
first level iteration happens at the for loop within the zap_gfn_range()
with the purpose of calibrating the accurate range for zapping. The second
level itreration start at tdp_mmu_set_spte{,_atomic}() that tears down the
whole paging structures (leaf and non-leaf SPTEs) within the range. The
former iteration is yield safe, while the second one is not.

In many cases, zapping SPTE process could be optimized since the non-leaf
SPTEs could most likely be retained for the next allocation. On the other
hand, for large scale SPTE zapping scenarios, we may end up zapping too
many SPTEs and use excessive CPU time that causes the RCU stall warning.

The follow selftest reproduces the warning:

	(env: kvm.tdp_mmu=Y)
	./dirty_log_perf_test -v 64 -b 8G

Optimize the zapping process by skipping all SPTEs above a certain level in
the first iteration. This allows us to control the granularity of the
actual zapping and invoke tdp_mmu_iter_cond_resched() on time. In addition,
we would retain some of the non-leaf SPTEs to accelerate next allocation.

For the selection of the `certain level`, we choose the PG_LEVEL_1G because
it is currently the largest page size supported and it natually fits the
scenario of splitting large pages.

For `zap_all` case (usually) at VM teardown time, we use a two-phase
mechanism: the 1st phase zaps all SPTEs at PG_LEVEL_1G level and 2nd phase
zaps everything else. This is achieved by the helper function
__zap_gfn_range().

Cc: Sean Christopherson <seanjc@xxxxxxxxxx>
Cc: Ben Gardon <bgardon@xxxxxxxxxx>
Cc: David Matlack <dmatlack@xxxxxxxxxx>

Signed-off-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
---
 arch/x86/kvm/mmu/tdp_mmu.c | 57 ++++++++++++++++++++++++++------------
 1 file changed, 40 insertions(+), 17 deletions(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 89d16bb104de..3fadc51c004a 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -697,24 +697,16 @@ static inline bool tdp_mmu_iter_cond_resched(struct kvm *kvm,
  * account for the possibility that other threads are modifying the paging
  * structures concurrently. If shared is false, this thread should hold the
  * MMU lock in write mode.
+ *
+ * If zap_all is true, eliminate all the paging structures that contains the
+ * SPTEs.
  */
-static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
-			  gfn_t start, gfn_t end, bool can_yield, bool flush,
-			  bool shared)
+static bool __zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
+			    gfn_t start, gfn_t end, bool can_yield, bool flush,
+			    bool shared, bool zap_all)
 {
-	gfn_t max_gfn_host = 1ULL << (shadow_phys_bits - PAGE_SHIFT);
-	bool zap_all = (start == 0 && end >= max_gfn_host);
 	struct tdp_iter iter;
 
-	/*
-	 * Bound the walk at host.MAXPHYADDR, guest accesses beyond that will
-	 * hit a #PF(RSVD) and never get to an EPT Violation/Misconfig / #NPF,
-	 * and so KVM will never install a SPTE for such addresses.
-	 */
-	end = min(end, max_gfn_host);
-
-	kvm_lockdep_assert_mmu_lock_held(kvm, shared);
-
 	rcu_read_lock();
 
 	tdp_root_for_each_pte(iter, root, start, end) {
@@ -725,17 +717,24 @@ static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
 			continue;
 		}
 
-		if (!is_shadow_present_pte(iter.old_spte))
+		/*
+		 * In zap_all case, ignore the checking of present since we have
+		 * to zap everything.
+		 */
+		if (!zap_all && !is_shadow_present_pte(iter.old_spte))
 			continue;
 
 		/*
 		 * If this is a non-last-level SPTE that covers a larger range
 		 * than should be zapped, continue, and zap the mappings at a
-		 * lower level, except when zapping all SPTEs.
+		 * lower level. Actual zapping started at proper granularity
+		 * that is not so large as to cause a soft lockup when handling
+		 * the changed pte (which does not yield).
 		 */
 		if (!zap_all &&
 		    (iter.gfn < start ||
-		     iter.gfn + KVM_PAGES_PER_HPAGE(iter.level) > end) &&
+		     iter.gfn + KVM_PAGES_PER_HPAGE(iter.level) > end ||
+		     iter.level > PG_LEVEL_1G) &&
 		    !is_last_spte(iter.old_spte, iter.level))
 			continue;
 
@@ -756,6 +755,30 @@ static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
 	return flush;
 }
 
+static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
+			  gfn_t start, gfn_t end, bool can_yield, bool flush,
+			  bool shared)
+{
+	gfn_t max_gfn_host = 1ULL << (shadow_phys_bits - PAGE_SHIFT);
+	bool zap_all = (start == 0 && end >= max_gfn_host);
+
+	/*
+	 * Bound the walk at host.MAXPHYADDR, guest accesses beyond that will
+	 * hit a #PF(RSVD) and never get to an EPT Violation/Misconfig / #NPF,
+	 * and so KVM will never install a SPTE for such addresses.
+	 */
+	end = min(end, max_gfn_host);
+
+	kvm_lockdep_assert_mmu_lock_held(kvm, shared);
+
+	flush = __zap_gfn_range(kvm, root, start, end, can_yield, flush, shared,
+				false);
+	if (zap_all)
+		flush = __zap_gfn_range(kvm, root, start, end, can_yield, flush,
+					shared, true);
+	return flush;
+}
+
 /*
  * Tears down the mappings for the range of gfns, [start, end), and frees the
  * non-root pages mapping GFNs strictly within that range. Returns true if
-- 
2.34.0.rc2.393.gf8c9666880-goog