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Message-ID: <aC3oIjkivS2KqKZH@google.com>
Date: Wed, 21 May 2025 07:50:10 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Peter Xu <peterx@...hat.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
Yan Zhao <yan.y.zhao@...el.com>, Maxim Levitsky <mlevitsk@...hat.com>,
Binbin Wu <binbin.wu@...ux.intel.com>, James Houghton <jthoughton@...gle.com>,
Pankaj Gupta <pankaj.gupta@....com>
Subject: Re: [PATCH v3 0/6] KVM: Dirty ring fixes and cleanups
On Tue, May 20, 2025, Peter Xu wrote:
> On Tue, May 20, 2025 at 04:16:00PM -0700, Sean Christopherson wrote:
> > On Tue, May 20, 2025, Peter Xu wrote:
> > > On Fri, May 16, 2025 at 02:35:34PM -0700, Sean Christopherson wrote:
> > > > Sean Christopherson (6):
> > > > KVM: Bound the number of dirty ring entries in a single reset at
> > > > INT_MAX
> > > > KVM: Bail from the dirty ring reset flow if a signal is pending
> > > > KVM: Conditionally reschedule when resetting the dirty ring
> > > > KVM: Check for empty mask of harvested dirty ring entries in caller
> > > > KVM: Use mask of harvested dirty ring entries to coalesce dirty ring
> > > > resets
> > > > KVM: Assert that slots_lock is held when resetting per-vCPU dirty
> > > > rings
> > >
> > > For the last one, I'd think it's majorly because of the memslot accesses
> > > (or CONFIG_LOCKDEP=y should yell already on resets?).
> >
> > No? If KVM only needed to ensure stable memslot accesses, then SRCU would suffice.
> > It sounds like holding slots_lock may have been a somewhat unintentional, but the
> > reason KVM can't switch to SRCU is that doing so would break ordering, not because
> > slots_lock is needed to protect the memslot accesses.
>
> Hmm.. isn't what you said exactly means a "yes"? :)
>
> I mean, I would still expect lockdep to report this ioctl if without the
> slots_lock, please correct me if it's not the case.
Yes, one of slots_lock or SRCU needs to be held.
> And if using RCU is not trivial (or not necessary either), so far the
> slots_lock is still required to make sure the memslot accesses are legal?
I don't follow this part. The intent of the comment is to document why slots_lock
is required, which is exceptional because memslot access for readers are protected
by kvm->srcu. The fact that slots_lock also protects memslots is notable only
because it makes acquiring kvm->srcu superfluous. But grabbing kvm->srcu is still
safe/legal/ok:
diff --git a/virt/kvm/dirty_ring.c b/virt/kvm/dirty_ring.c
index 1ba02a06378c..6bf4f9e2f291 100644
--- a/virt/kvm/dirty_ring.c
+++ b/virt/kvm/dirty_ring.c
@@ -121,18 +121,26 @@ int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring,
u64 cur_offset, next_offset;
unsigned long mask = 0;
struct kvm_dirty_gfn *entry;
+ int idx;
/*
* Ensure concurrent calls to KVM_RESET_DIRTY_RINGS are serialized,
* e.g. so that KVM fully resets all entries processed by a given call
- * before returning to userspace. Holding slots_lock also protects
- * the various memslot accesses.
+ * before returning to userspace.
*/
lockdep_assert_held(&kvm->slots_lock);
+ /*
+ * Holding slots_lock also protects the various memslot accesses, but
+ * acquiring kvm->srcu for read here is still safe, just unnecessary.
+ */
+ idx = srcu_read_lock(&kvm->srcu);
+
while (likely((*nr_entries_reset) < INT_MAX)) {
- if (signal_pending(current))
+ if (signal_pending(current)) {
+ srcu_read_unlock(&kvm->srcu, idx);
return -EINTR;
+ }
entry = &ring->dirty_gfns[ring->reset_index & (ring->size - 1)];
@@ -205,6 +213,8 @@ int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring,
if (mask)
kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);
+ srcu_read_unlock(&kvm->srcu, idx);
+
/*
* The request KVM_REQ_DIRTY_RING_SOFT_FULL will be cleared
* by the VCPU thread next time when it enters the guest.
--
And unless there are other behaviors that are protected by slots_lock (which is
entirely possible), serializing the processing of each ring could be done via a
dedicated (for example only, the dedicated mutex could/should be per-vCPU, not
global).
This diff in particular shows why I ordered and phrased the comment the way I
did. The blurb about protecting memslot accesses is purely a friendly reminder
to readers. The sole reason for an assert and comment is to call out the need
for ordering.
diff --git a/virt/kvm/dirty_ring.c b/virt/kvm/dirty_ring.c
index 1ba02a06378c..92ac82b535fe 100644
--- a/virt/kvm/dirty_ring.c
+++ b/virt/kvm/dirty_ring.c
@@ -102,6 +102,8 @@ static inline bool kvm_dirty_gfn_harvested(struct kvm_dirty_gfn *gfn)
return smp_load_acquire(&gfn->flags) & KVM_DIRTY_GFN_F_RESET;
}
+static DEFINE_MUTEX(per_ring_lock);
+
int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring,
int *nr_entries_reset)
{
@@ -121,18 +123,22 @@ int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring,
u64 cur_offset, next_offset;
unsigned long mask = 0;
struct kvm_dirty_gfn *entry;
+ int idx;
/*
* Ensure concurrent calls to KVM_RESET_DIRTY_RINGS are serialized,
* e.g. so that KVM fully resets all entries processed by a given call
- * before returning to userspace. Holding slots_lock also protects
- * the various memslot accesses.
+ * before returning to userspace.
*/
- lockdep_assert_held(&kvm->slots_lock);
+ guard(mutex)(&per_ring_lock);
+
+ idx = srcu_read_lock(&kvm->srcu);
while (likely((*nr_entries_reset) < INT_MAX)) {
- if (signal_pending(current))
+ if (signal_pending(current)) {
+ srcu_read_unlock(&kvm->srcu, idx);
return -EINTR;
+ }
entry = &ring->dirty_gfns[ring->reset_index & (ring->size - 1)];
@@ -205,6 +211,8 @@ int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring,
if (mask)
kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);
+ srcu_read_unlock(&kvm->srcu, idx);
+
/*
* The request KVM_REQ_DIRTY_RING_SOFT_FULL will be cleared
* by the VCPU thread next time when it enters the guest.
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 571688507204..45729a6f6451 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4908,16 +4908,12 @@ static int kvm_vm_ioctl_reset_dirty_pages(struct kvm *kvm)
if (!kvm->dirty_ring_size)
return -EINVAL;
- mutex_lock(&kvm->slots_lock);
-
kvm_for_each_vcpu(i, vcpu, kvm) {
r = kvm_dirty_ring_reset(vcpu->kvm, &vcpu->dirty_ring, &cleared);
if (r)
break;
}
- mutex_unlock(&kvm->slots_lock);
-
if (cleared)
kvm_flush_remote_tlbs(kvm);
--
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