[PATCH 3/3] KVM: Aggressively drop and reacquire mmu_lock during CLEAR_DIRTY_LOG

[David Matlack <dmatlack@google.com>]

Drop and reacquire the mmu_lock during CLEAR_DIRTY_LOG to avoid blocking
other threads from acquiring the mmu_lock (e.g. vCPUs taking page
faults).

It should be safe to drop and reacquire the mmu_lock from a correctness
standpoint. slots_lock already ensures that only one thread in KVM is
processing a GET/CLEAR_DIRTY_LOG ioctl. And KVM already has to be
prepared to handle vCPUs updating the dirty log concurrent with
CLEAR_DIRTY_LOG, hence the atomic_long_fetch_andnot(). So holding the
mmu_lock across loop iterations is entirely unnecessary. It only needs
to be acquired when calling in the arch-specific code to modify page
tables.

This change eliminates drops in guest performance during the live
migration of a 160 vCPU VM that we've observed while userspace is
issuing CLEAR ioctls (tested with 1GiB and 8GiB CLEARs). Userspace could
issue finer-grained CLEAR ioctls, which would also reduce contention on
the mmu_lock, but doing so will increase the rate of remote TLB
flushing, so there is a limit. And there's really no reason to punt this
problem to userspace.  KVM can just drop and reacquire the lock more
frequently to avoid holding it for too long.

Signed-off-by: David Matlack <dmatlack@google.com>
---
 virt/kvm/kvm_main.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 486800a7024b..afa61a2309d2 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2297,7 +2297,6 @@ static int kvm_clear_dirty_log_protect(struct kvm *kvm,
 	if (copy_from_user(dirty_bitmap_buffer, log->dirty_bitmap, n))
 		return -EFAULT;
 
-	KVM_MMU_LOCK(kvm);
 	for (offset = log->first_page, i = offset / BITS_PER_LONG,
 		 n = DIV_ROUND_UP(log->num_pages, BITS_PER_LONG); n--;
 	     i++, offset += BITS_PER_LONG) {
@@ -2316,11 +2315,12 @@ static int kvm_clear_dirty_log_protect(struct kvm *kvm,
 		*/
 		if (mask) {
 			flush = true;
+			KVM_MMU_LOCK(kvm);
 			kvm_arch_mmu_enable_log_dirty_pt_masked(kvm, memslot,
 								offset, mask);
+			KVM_MMU_UNLOCK(kvm);
 		}
 	}
-	KVM_MMU_UNLOCK(kvm);
 
 	if (flush)
 		kvm_flush_remote_tlbs_memslot(kvm, memslot);
-- 
2.42.0.820.g83a721a137-goog