| Message ID | 20210511171610.170160-5-bgardon@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Lazily allocate memslot rmaps | expand |
On Tue, May 11, 2021, Ben Gardon wrote: > Add a new lock to protect the arch-specific fields of memslots if they > need to be modified in a kvm->srcu read critical section. A future > commit will use this lock to lazily allocate memslot rmaps for x86. > > Signed-off-by: Ben Gardon <bgardon@google.com> > --- > include/linux/kvm_host.h | 9 +++++++++ > virt/kvm/kvm_main.c | 31 ++++++++++++++++++++++++++----- > 2 files changed, 35 insertions(+), 5 deletions(-) > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h > index 8895b95b6a22..2d5e797fbb08 100644 > --- a/include/linux/kvm_host.h > +++ b/include/linux/kvm_host.h > @@ -472,6 +472,15 @@ struct kvm { > #endif /* KVM_HAVE_MMU_RWLOCK */ > > struct mutex slots_lock; > + > + /* > + * Protects the arch-specific fields of struct kvm_memory_slots in > + * use by the VM. To be used under the slots_lock (above) or in a > + * kvm->srcu read cirtical section where acquiring the slots_lock ^^^^^^^^ critical > + * would lead to deadlock with the synchronize_srcu in > + * install_new_memslots. > + */ > + struct mutex slots_arch_lock; > struct mm_struct *mm; /* userspace tied to this vm */ > struct kvm_memslots __rcu *memslots[KVM_ADDRESS_SPACE_NUM]; > struct kvm_vcpu *vcpus[KVM_MAX_VCPUS];
On Tue, May 11, 2021, Ben Gardon wrote: > Add a new lock to protect the arch-specific fields of memslots if they > need to be modified in a kvm->srcu read critical section. A future > commit will use this lock to lazily allocate memslot rmaps for x86. > > Signed-off-by: Ben Gardon <bgardon@google.com> > --- > include/linux/kvm_host.h | 9 +++++++++ > virt/kvm/kvm_main.c | 31 ++++++++++++++++++++++++++----- > 2 files changed, 35 insertions(+), 5 deletions(-) > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h > index 8895b95b6a22..2d5e797fbb08 100644 > --- a/include/linux/kvm_host.h > +++ b/include/linux/kvm_host.h > @@ -472,6 +472,15 @@ struct kvm { > #endif /* KVM_HAVE_MMU_RWLOCK */ > > struct mutex slots_lock; > + > + /* > + * Protects the arch-specific fields of struct kvm_memory_slots in > + * use by the VM. To be used under the slots_lock (above) or in a > + * kvm->srcu read cirtical section where acquiring the slots_lock > + * would lead to deadlock with the synchronize_srcu in > + * install_new_memslots. > + */ > + struct mutex slots_arch_lock; > struct mm_struct *mm; /* userspace tied to this vm */ > struct kvm_memslots __rcu *memslots[KVM_ADDRESS_SPACE_NUM]; > struct kvm_vcpu *vcpus[KVM_MAX_VCPUS]; > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > index 9e106742b388..5c40d83754b1 100644 > --- a/virt/kvm/kvm_main.c > +++ b/virt/kvm/kvm_main.c > @@ -908,6 +908,7 @@ static struct kvm *kvm_create_vm(unsigned long type) > mutex_init(&kvm->lock); > mutex_init(&kvm->irq_lock); > mutex_init(&kvm->slots_lock); > + mutex_init(&kvm->slots_arch_lock); > INIT_LIST_HEAD(&kvm->devices); > > BUILD_BUG_ON(KVM_MEM_SLOTS_NUM > SHRT_MAX); > @@ -1280,6 +1281,10 @@ static struct kvm_memslots *install_new_memslots(struct kvm *kvm, > slots->generation = gen | KVM_MEMSLOT_GEN_UPDATE_IN_PROGRESS; > > rcu_assign_pointer(kvm->memslots[as_id], slots); > + > + /* Acquired in kvm_set_memslot. */ > + mutex_unlock(&kvm->slots_arch_lock); > + > synchronize_srcu_expedited(&kvm->srcu); > > /* > @@ -1351,6 +1356,9 @@ static int kvm_set_memslot(struct kvm *kvm, > struct kvm_memslots *slots; > int r; > > + /* Released in install_new_memslots. */ This needs a much more comprehensive comment, either here or above the declaration of slots_arch_lock. I can't find anything that explicitly states the the lock must be held from the time the previous memslots are duplicated/copied until the new memslots are set. Without that information, the rules/expecations are not clear. > + mutex_lock(&kvm->slots_arch_lock); > + > slots = kvm_dup_memslots(__kvm_memslots(kvm, as_id), change); > if (!slots) > return -ENOMEM; Fails to drop slots_arch_lock. > @@ -1364,10 +1372,9 @@ static int kvm_set_memslot(struct kvm *kvm, > slot->flags |= KVM_MEMSLOT_INVALID; > > /* > - * We can re-use the old memslots, the only difference from the > - * newly installed memslots is the invalid flag, which will get > - * dropped by update_memslots anyway. We'll also revert to the > - * old memslots if preparing the new memory region fails. > + * We can re-use the memory from the old memslots. > + * It will be overwritten with a copy of the new memslots > + * after reacquiring the slots_arch_lock below. > */ > slots = install_new_memslots(kvm, as_id, slots); > > @@ -1379,6 +1386,17 @@ static int kvm_set_memslot(struct kvm *kvm, > * - kvm_is_visible_gfn (mmu_check_root) > */ > kvm_arch_flush_shadow_memslot(kvm, slot); > + > + /* Released in install_new_memslots. */ > + mutex_lock(&kvm->slots_arch_lock); > + > + /* > + * The arch-specific fields of the memslots could have changed > + * between releasing the slots_arch_lock in > + * install_new_memslots and here, so get a fresh copy of the > + * slots. > + */ > + kvm_copy_memslots(__kvm_memslots(kvm, as_id), slots); Ow. This is feedback for a previous patch, but kvm_copy_memslots() absolutely needs to swap the order of params to match memcpy(), i.e. @to is first, @from is second. > } > > r = kvm_arch_prepare_memory_region(kvm, new, mem, change); > @@ -1394,8 +1412,11 @@ static int kvm_set_memslot(struct kvm *kvm, > return 0; > > out_slots: > - if (change == KVM_MR_DELETE || change == KVM_MR_MOVE) > + if (change == KVM_MR_DELETE || change == KVM_MR_MOVE) { > + slot = id_to_memslot(slots, old->id); > + slot->flags &= ~KVM_MEMSLOT_INVALID; > slots = install_new_memslots(kvm, as_id, slots); > + } Fails to drop slots_arch_lock if kvm_arch_prepare_memory_region() fails for anything other than DELETE and MOVE. I really, really don't like dropping the lock in install_new_memslots(). Unlocking bugs aside, IMO it makes it quite difficult to understand exactly what slots_arch_lock protects. Unfortunately I'm just whining at this point since I don't have a better idea :-( > kvfree(slots); > return r; > } > -- > 2.31.1.607.g51e8a6a459-goog >
On 11/05/21 21:21, Sean Christopherson wrote: >> + /* Released in install_new_memslots. */ > > This needs a much more comprehensive comment, either here or above the declaration > of slots_arch_lock. I can't find anything that explicitly states the the lock > must be held from the time the previous memslots are duplicated/copied until the > new memslots are set. Without that information, the rules/expecations are not > clear. Indeed, this needs basically a description of the races that can happen, as you explained them in the v1 review. > Unfortunately I'm just whining at this point since I > don't have a better idea Yeah, the synchronize_srcu call in install_new_memslots throws a wrench in most alternatives. Paolo
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index 8895b95b6a22..2d5e797fbb08 100644 --- a/include/linux/kvm_host.h +++ b/include/linux/kvm_host.h @@ -472,6 +472,15 @@ struct kvm { #endif /* KVM_HAVE_MMU_RWLOCK */ struct mutex slots_lock; + + /* + * Protects the arch-specific fields of struct kvm_memory_slots in + * use by the VM. To be used under the slots_lock (above) or in a + * kvm->srcu read cirtical section where acquiring the slots_lock + * would lead to deadlock with the synchronize_srcu in + * install_new_memslots. + */ + struct mutex slots_arch_lock; struct mm_struct *mm; /* userspace tied to this vm */ struct kvm_memslots __rcu *memslots[KVM_ADDRESS_SPACE_NUM]; struct kvm_vcpu *vcpus[KVM_MAX_VCPUS]; diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index 9e106742b388..5c40d83754b1 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -908,6 +908,7 @@ static struct kvm *kvm_create_vm(unsigned long type) mutex_init(&kvm->lock); mutex_init(&kvm->irq_lock); mutex_init(&kvm->slots_lock); + mutex_init(&kvm->slots_arch_lock); INIT_LIST_HEAD(&kvm->devices); BUILD_BUG_ON(KVM_MEM_SLOTS_NUM > SHRT_MAX); @@ -1280,6 +1281,10 @@ static struct kvm_memslots *install_new_memslots(struct kvm *kvm, slots->generation = gen | KVM_MEMSLOT_GEN_UPDATE_IN_PROGRESS; rcu_assign_pointer(kvm->memslots[as_id], slots); + + /* Acquired in kvm_set_memslot. */ + mutex_unlock(&kvm->slots_arch_lock); + synchronize_srcu_expedited(&kvm->srcu); /* @@ -1351,6 +1356,9 @@ static int kvm_set_memslot(struct kvm *kvm, struct kvm_memslots *slots; int r; + /* Released in install_new_memslots. */ + mutex_lock(&kvm->slots_arch_lock); + slots = kvm_dup_memslots(__kvm_memslots(kvm, as_id), change); if (!slots) return -ENOMEM; @@ -1364,10 +1372,9 @@ static int kvm_set_memslot(struct kvm *kvm, slot->flags |= KVM_MEMSLOT_INVALID; /* - * We can re-use the old memslots, the only difference from the - * newly installed memslots is the invalid flag, which will get - * dropped by update_memslots anyway. We'll also revert to the - * old memslots if preparing the new memory region fails. + * We can re-use the memory from the old memslots. + * It will be overwritten with a copy of the new memslots + * after reacquiring the slots_arch_lock below. */ slots = install_new_memslots(kvm, as_id, slots); @@ -1379,6 +1386,17 @@ static int kvm_set_memslot(struct kvm *kvm, * - kvm_is_visible_gfn (mmu_check_root) */ kvm_arch_flush_shadow_memslot(kvm, slot); + + /* Released in install_new_memslots. */ + mutex_lock(&kvm->slots_arch_lock); + + /* + * The arch-specific fields of the memslots could have changed + * between releasing the slots_arch_lock in + * install_new_memslots and here, so get a fresh copy of the + * slots. + */ + kvm_copy_memslots(__kvm_memslots(kvm, as_id), slots); } r = kvm_arch_prepare_memory_region(kvm, new, mem, change); @@ -1394,8 +1412,11 @@ static int kvm_set_memslot(struct kvm *kvm, return 0; out_slots: - if (change == KVM_MR_DELETE || change == KVM_MR_MOVE) + if (change == KVM_MR_DELETE || change == KVM_MR_MOVE) { + slot = id_to_memslot(slots, old->id); + slot->flags &= ~KVM_MEMSLOT_INVALID; slots = install_new_memslots(kvm, as_id, slots); + } kvfree(slots); return r; }
Add a new lock to protect the arch-specific fields of memslots if they need to be modified in a kvm->srcu read critical section. A future commit will use this lock to lazily allocate memslot rmaps for x86. Signed-off-by: Ben Gardon <bgardon@google.com> --- include/linux/kvm_host.h | 9 +++++++++ virt/kvm/kvm_main.c | 31 ++++++++++++++++++++++++++----- 2 files changed, 35 insertions(+), 5 deletions(-)