[PATCH v2] KVM: Aggressively drop and reacquire mmu_lock during CLEAR_DIRTY_LOG

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Aggressively drop and reacquire mmu_lock during CLEAR_DIRTY_LOG to avoid
blocking other threads (e.g. vCPUs taking page faults) for too long.

Specifically, change kvm_clear_dirty_log_protect() to acquire/release
mmu_lock only when calling kvm_arch_mmu_enable_log_dirty_pt_masked(),
rather than around the entire for loop. This ensures that KVM will only
hold mmu_lock for the time it takes the architecture-specific code to
process up to 64 pages, rather than holding mmu_lock for log->num_pages,
which is controllable by userspace. This also avoids holding mmu_lock
when processing parts of the dirty_bitmap that are zero (i.e. when there
is nothing to clear).

Moving the acquire/release points for mmu_lock should be safe since
dirty_bitmap_buffer is already protected by slots_lock, and dirty_bitmap
is already accessed with atomic_long_fetch_andnot(). And at least on x86
holding mmu_lock doesn't even serialize access to the memslot dirty
bitmap, as vCPUs can call mark_page_dirty_in_slot() without holding

This change eliminates dips in guest performance during live migration
in a 160 vCPU VM when userspace is issuing CLEAR 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. And there's really no reason to punt this
problem to userspace since KVM can just drop and reacquire mmu_lock more

Cc: Marc Zyngier <maz@xxxxxxxxxx>
Cc: Oliver Upton <oliver.upton@xxxxxxxxx>
Cc: Tianrui Zhao <zhaotianrui@xxxxxxxxxxx>
Cc: Bibo Mao <maobibo@xxxxxxxxxxx>
Cc: Huacai Chen <chenhuacai@xxxxxxxxxx>
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Cc: Anup Patel <anup@xxxxxxxxxxxxxx>
Cc: Christian Borntraeger <borntraeger@xxxxxxxxxxxxx>
Cc: Janosch Frank <frankja@xxxxxxxxxxxxx>
Cc: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>
Cc: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: David Matlack <dmatlack@xxxxxxxxxx>
 - Rebase onto kvm/queue [Marc]

v1: https://lore.kernel.org/kvm/20231205181645.482037-1-dmatlack@xxxxxxxxxx/

 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 fb49c2a60200..0a8b25a52c15 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2386,7 +2386,6 @@ 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) {
@@ -2405,11 +2404,12 @@ 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);
 	if (flush)
 		kvm_flush_remote_tlbs_memslot(kvm, memslot);

base-commit: 9bc60f733839ab6fcdde0d0b15cbb486123e6402

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