KVM: x86/mmu: Always drop mmu_lock to allocate TDP MMU SPs for eager splitting

Always drop mmu_lock to allocate shadow pages in the TDP MMU when doing
eager page splitting. Dropping mmu_lock during eager page splitting is
cheap since KVM does not have to flush remote TLBs, and avoids stalling
vCPU threads that are taking page faults while KVM is eager splitting
under mmu_lock held for write.

This change reduces 20%+ dips in MySQL throughput during live migration
in a 160 vCPU VM while userspace is issuing CLEAR_DIRTY_LOG ioctls
(tested with 1GiB and 8GiB CLEARs). Userspace could issue finer-grained
CLEARs, which would also reduce contention on mmu_lock, but doing so
will increase the rate of remote TLB flushing, since KVM must flush TLBs
before returning from CLEAR_DITY_LOG.

When there isn't contention on mmu_lock[1], this change does not regress
the time it takes to perform eager page splitting (the cost of releasing
and re-acquiring an uncontended lock is minimal on x86).

[1] Tested with dirty_log_perf_test, which does not run vCPUs during
eager page splitting, and with a 16 vCPU VM Live Migration with
manual-protect disabled (where mmu_lock is held for read).

Cc: Bibo Mao <maobibo@loongson.cn>
Cc: Sean Christopherson <seanjc@google.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20240611220512.2426439-2-dmatlack@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/mmu/tdp_mmu.c

@@ -1366,19 +1366,6 @@ static struct kvm_mmu_page *tdp_mmu_alloc_sp_for_split(struct kvm *kvm,
 
 	kvm_lockdep_assert_mmu_lock_held(kvm, shared);
 
-	/*
-	 * Since we are allocating while under the MMU lock we have to be
-	 * careful about GFP flags. Use GFP_NOWAIT to avoid blocking on direct
-	 * reclaim and to avoid making any filesystem callbacks (which can end
-	 * up invoking KVM MMU notifiers, resulting in a deadlock).
-	 *
-	 * If this allocation fails we drop the lock and retry with reclaim
-	 * allowed.
-	 */
-	sp = __tdp_mmu_alloc_sp_for_split(GFP_NOWAIT | __GFP_ACCOUNT);
-	if (sp)
-		return sp;
-
 	rcu_read_unlock();
 
 	if (shared)
@@ -1478,7 +1465,6 @@ static int tdp_mmu_split_huge_pages_root(struct kvm *kvm,
 				break;
 			}
 
-			if (iter.yielded)
 			continue;
 		}