lkml.org 
[lkml]   [2021]   [Apr]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
SubjectRe: [PATCH 5/6] KVM: x86/mmu: Protect kvm->memslots with a mutex
Date
On 28/04/21 23:46, Ben Gardon wrote:
> On Wed, Apr 28, 2021 at 2:41 PM Paolo Bonzini <pbonzini@redhat.com> wrote:
>>
>> On 28/04/21 22:40, Ben Gardon wrote:
>>> ... However with the locking you propose below, we might still run
>>> into issues on a move or delete, which would mean we'd still need the
>>> separate memory allocation for the rmaps array. Or we do some
>>> shenanigans where we try to copy the rmap pointers from the other set
>>> of memslots.
>>
>> If that's (almost) as easy as passing old to
>> kvm_arch_prepare_memory_region, that would be totally okay.
>
> Unfortunately it's not quite that easy because it's all the slots
> _besides_ the one being modified where we'd need to copy the rmaps.

Ah, now I understand the whole race. And it seems to me that if one
kvm_dup_memslots within the new lock fixed a bug, two kvm_dup_memslots
within the new lock are going to fix two bugs. :)

Seriously: unless I'm missing another case (it's late here...), it's
not ugly and it's still relatively easy to explain.

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 2799c6660cce..48929dd5fb29 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1270,7 +1270,7 @@ static int check_memory_region_flags(const struct kvm_userspace_memory_region *m
return 0;
}

-static struct kvm_memslots *install_new_memslots(struct kvm *kvm,
+static void install_new_memslots(struct kvm *kvm,
int as_id, struct kvm_memslots *slots)
{
struct kvm_memslots *old_memslots = __kvm_memslots(kvm, as_id);
@@ -1280,7 +1280,9 @@ 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);
+ mutex_unlock(&kvm->slots_arch_lock);
synchronize_srcu_expedited(&kvm->srcu);
+ kvfree(old_memslots);

/*
* Increment the new memslot generation a second time, dropping the
@@ -1302,8 +1304,6 @@ static struct kvm_memslots *install_new_memslots(struct kvm *kvm,
kvm_arch_memslots_updated(kvm, gen);

slots->generation = gen;
-
- return old_memslots;
}

/*
@@ -1342,6 +1342,7 @@ static int kvm_set_memslot(struct kvm *kvm,
struct kvm_memslots *slots;
int r;

+ mutex_lock(&kvm->slots_arch_lock);
slots = kvm_dup_memslots(__kvm_memslots(kvm, as_id), change);
if (!slots)
return -ENOMEM;
@@ -1353,14 +1354,7 @@ static int kvm_set_memslot(struct kvm *kvm,
*/
slot = id_to_memslot(slots, old->id);
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.
- */
- slots = install_new_memslots(kvm, as_id, slots);
+ install_new_memslots(kvm, as_id, slots);

/* From this point no new shadow pages pointing to a deleted,
* or moved, memslot will be created.
@@ -1370,6 +1364,9 @@ static int kvm_set_memslot(struct kvm *kvm,
* - kvm_is_visible_gfn (mmu_check_root)
*/
kvm_arch_flush_shadow_memslot(kvm, slot);
+
+ mutex_lock(&kvm->slots_arch_lock);
+ slots = kvm_dup_memslots(__kvm_memslots(kvm, as_id), change);
}

r = kvm_arch_prepare_memory_region(kvm, new, mem, change);
@@ -1377,16 +1374,17 @@ static int kvm_set_memslot(struct kvm *kvm,
goto out_slots;

update_memslots(slots, new, change);
- slots = install_new_memslots(kvm, as_id, slots);
+ install_new_memslots(kvm, as_id, slots);

kvm_arch_commit_memory_region(kvm, mem, old, new, change);
-
- kvfree(slots);
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;
}
One could optimize things a bit by reusing the allocation and only
doing a memcpy from the new memslots array to the old one under the
slots_arch_lock. (Plus the above still lacks a mutex_init and
should be split in two patches, with the mutex going in the second;
but you get the idea and code sometimes is easier than words).

Paolo

\
 
 \ /
  Last update: 2021-04-29 01:43    [W:0.051 / U:0.400 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site