Hi Marc,
On 11/7/22 7:33 PM, Marc Zyngier wrote:
On Mon, 07 Nov 2022 10:45:34 +0000,
Gavin Shan <gshan@xxxxxxxxxx> wrote:
On 11/5/22 7:40 AM, Gavin Shan wrote:
ARM64 needs to dirty memory outside of a VCPU context when VGIC/ITS is
enabled. It's conflicting with that ring-based dirty page tracking always
requires a running VCPU context.
Introduce a new flavor of dirty ring that requires the use of both VCPU
dirty rings and a dirty bitmap. The expectation is that for non-VCPU
sources of dirty memory (such as the VGIC/ITS on arm64), KVM writes to
the dirty bitmap. Userspace should scan the dirty bitmap before migrating
the VM to the target.
Use an additional capability to advertise this behavior. The newly added
capability (KVM_CAP_DIRTY_LOG_RING_WITH_BITMAP) can't be enabled before
KVM_CAP_DIRTY_LOG_RING_ACQ_REL on ARM64. In this way, the newly added
capability is treated as an extension of KVM_CAP_DIRTY_LOG_RING_ACQ_REL.
Suggested-by: Marc Zyngier <maz@xxxxxxxxxx>
Suggested-by: Peter Xu <peterx@xxxxxxxxxx>
Co-developed-by: Oliver Upton <oliver.upton@xxxxxxxxx>
Signed-off-by: Oliver Upton <oliver.upton@xxxxxxxxx>
Signed-off-by: Gavin Shan <gshan@xxxxxxxxxx>
Acked-by: Peter Xu <peterx@xxxxxxxxxx>
---
Documentation/virt/kvm/api.rst | 33 ++++++++++++++++++-----
include/linux/kvm_dirty_ring.h | 7 +++++
include/linux/kvm_host.h | 1 +
include/uapi/linux/kvm.h | 1 +
virt/kvm/Kconfig | 8 ++++++
virt/kvm/dirty_ring.c | 10 +++++++
virt/kvm/kvm_main.c | 49 +++++++++++++++++++++++++++-------
7 files changed, 93 insertions(+), 16 deletions(-)
diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
[...]
In order to speed up the review and reduce unnecessary respins. After
collecting comments on PATCH[v8 3/7] from Marc and Peter, I would change
above description as below. Could you please confirm it looks good to you?
In the 4th paragraph, the words starting from "Collecting the dirty bitmap..."
to the end, was previously suggested by Oliver, even Marc suggested to avoid
mentioning "migration".
After enabling the dirty rings, the userspace needs to detect the
capability of KVM_CAP_DIRTY_LOG_RING_WITH_BITMAP to see whether the ring
structures need to be backed by per-slot bitmaps. With this capability
s/need/can/. If there was a *need*, it should happen automatically
without user intervention.
Ok. s/need to/can in next revision :)
advertised, it means the architecture can dirty guest pages without
vcpu/ring context, so that some of the dirty information will still be
maintained in the bitmap structure. KVM_CAP_DIRTY_LOG_RING_WITH_BITMAP
can't be enabled if the capability of KVM_CAP_DIRTY_LOG_RING_ACQ_REL
hasn't been enabled, or any memslot has been existing.
Note that the bitmap here is only a backup of the ring structure. The
use of the ring and bitmap combination is only beneficial if there is
only a very small amount of memory that is dirtied out of vcpu/ring
context. Otherwise, the stand-alone per-slot bitmap mechanism needs to
be considered.
To collect dirty bits in the backup bitmap, userspace can use the same
KVM_GET_DIRTY_LOG ioctl. KVM_CLEAR_DIRTY_LOG isn't needed as long as all
the generation of the dirty bits is done in a single pass. Collecting
the dirty bitmap should be the very last thing that the VMM does before
transmitting state to the target VM. VMM needs to ensure that the dirty
state is final and avoid missing dirty pages from another ioctl ordered
after the bitmap collection.
I would replace "transmitting state to the target VM" with
"considering the state as complete", as I still object to casting this
API into the migration mold. People use this stuff more far more than
migration (checkpointing, for example).
Fair enough. I will change accordingly in next revision.
NOTE: One example of using the backup bitmap is saving arm64 vgic/its
tables through KVM_DEV_ARM_{VGIC_GRP_CTRL, ITS_SAVE_TABLES} command on
KVM device "kvm-arm-vgic-its" during VM's migration.
Same remark about migration.
Ok. I will change this paragraph as below in next revision, to avoid mentioning
"migration".
NOTE: One example of using the backup bitmap is saving arm64 vgic/its
tables through KVM_DEV_ARM_{VGIC_GRP_CTRL, ITS_SAVE_TABLES} command on
KVM device "kvm-arm-vgic-its" when dirty ring is enabled.
Thanks,
Gavin
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