On Sat, Nov 05, 2022 at 05:57:33AM +0800, Gavin Shan wrote:
> On 11/5/22 4:12 AM, Oliver Upton wrote:
> > I agree that we should address (1) like this, but in (2) requiring that
> > no memslots were created before enabling the existing capabilities would
> > be a change in ABI. If we can get away with that, great, but otherwise
> > we may need to delete the bitmaps associated with all memslots when the
> > cap is enabled.
> >
>
> I had the assumption QEMU and kvm/selftests are the only consumers to
> use DIRTY_RING. In this case, requiring that no memslots were created
> to enable DIRTY_RING won't break userspace.
> Following your thoughts, the tracked dirty pages in the bitmap also
> need to be synchronized to the per-vcpu-ring before the bitmap can be
> destroyed.
Eh, I don't think we'd need to go that far. No matter what, any dirty
bits that were present in the bitmap could never be read again anyway,
as we reject KVM_GET_DIRTY_LOG if kvm->dirty_ring_size != 0.
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 91cf51a25394..420cc101a16e 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -4588,6 +4588,32 @@ static int kvm_vm_ioctl_enable_cap_generic(struct kvm *kvm,
> > return -EINVAL;
> > return kvm_vm_ioctl_enable_dirty_log_ring(kvm, cap->args[0]);
> > +
> > + case KVM_CAP_DIRTY_LOG_RING_WITH_BITMAP: {
> > + struct kvm_memslots *slots;
> > + int r = -EINVAL;
> > +
> > + if (!IS_ENABLED(CONFIG_HAVE_KVM_DIRTY_RING_WITH_BITMAP) ||
> > + !kvm->dirty_ring_size)
> > + return r;
> > +
> > + mutex_lock(&kvm->slots_lock);
> > +
> > + slots = kvm_memslots(kvm);
> > +
> > + /*
> > + * Avoid a race between memslot creation and enabling the ring +
> > + * bitmap capability to guarantee that no memslots have been
> > + * created without a bitmap.
> > + */
> > + if (kvm_memslots_empty(slots)) {
> > + kvm->dirty_ring_with_bitmap = cap->args[0];
> > + r = 0;
> > + }
> > +
> > + mutex_unlock(&kvm->slots_lock);
> > + return r;
> > + }
> > default:
> > return kvm_vm_ioctl_enable_cap(kvm, cap);
> > }
> >
>
> The proposed changes look good to me. It will be integrated to PATCH[v8 4/9].
> By the way, v8 will be posted shortly.
Excellent, thanks!
--
Best,
Oliver
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