Hi Gavin,
On Wed, 28 Sep 2022 00:47:43 +0100,
Gavin Shan <[email protected]> wrote:
> I have rough idea as below. It's appreciated if you can comment before I'm
> going a head for the prototype. The overall idea is to introduce another
> dirty ring for KVM (kvm-dirty-ring). It's updated and visited separately
> to dirty ring for vcpu (vcpu-dirty-ring).
>
> - When the various VGIC/ITS table base addresses are specified,
> kvm-dirty-ring
> entries are added to mark those pages as 'always-dirty'. In
> mark_page_dirty_in_slot(),
> those 'always-dirty' pages will be skipped, no entries pushed to
> vcpu-dirty-ring.
>
> - Similar to vcpu-dirty-ring, kvm-dirty-ring is accessed from userspace
> through
> mmap(kvm->fd). However, there won't have similar reset interface. It
> means
> 'struct kvm_dirty_gfn::flags' won't track any information as we do for
> vcpu-dirty-ring. In this regard, kvm-dirty-ring is purely shared buffer
> to
> advertise 'always-dirty' pages from host to userspace.
> - For QEMU, shutdown/suspend/resume cases won't be concerning
> us any more. The
> only concerned case is migration. When the migration is about to
> complete,
> kvm-dirty-ring entries are fetched and the dirty bits are updated to
> global
> dirty page bitmap and RAMBlock's dirty page bitmap. For this, I'm still
> reading
> the code to find the best spot to do it.
I think it makes a lot of sense to have a way to log writes that are
not generated by a vpcu, such as the GIC and maybe other things in the
future, such as DMA traffic (some SMMUs are able to track dirty pages
as well).
However, I don't really see the point in inventing a new mechanism for
that. Why don't we simply allow non-vpcu dirty pages to be tracked in
the dirty *bitmap*?
>From a kernel perspective, this is dead easy:
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 5b064dbadaf4..ae9138f29d51 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3305,7 +3305,7 @@ void mark_page_dirty_in_slot(struct kvm *kvm,
struct kvm_vcpu *vcpu = kvm_get_running_vcpu();
#ifdef CONFIG_HAVE_KVM_DIRTY_RING
- if (WARN_ON_ONCE(!vcpu) || WARN_ON_ONCE(vcpu->kvm != kvm))
+ if (WARN_ON_ONCE(vcpu && vcpu->kvm != kvm))
return;
#endif
@@ -3313,10 +3313,11 @@ void mark_page_dirty_in_slot(struct kvm *kvm,
unsigned long rel_gfn = gfn - memslot->base_gfn;
u32 slot = (memslot->as_id << 16) | memslot->id;
- if (kvm->dirty_ring_size)
+ if (vpcu && kvm->dirty_ring_size)
kvm_dirty_ring_push(&vcpu->dirty_ring,
slot, rel_gfn);
- else
+ /* non-vpcu dirtying ends up in the global bitmap */
+ if (!vcpu && memslot->dirty_bitmap)
set_bit_le(rel_gfn, memslot->dirty_bitmap);
}
}
though I'm sure there is a few more things to it.
To me, this is just a relaxation of an arbitrary limitation, as the
current assumption that only vcpus can dirty memory doesn't hold at
all.
Thanks,
M.
--
Without deviation from the norm, progress is not possible.
_______________________________________________
kvmarm mailing list
[email protected]
https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
