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Subject: Re: [virtio-comment] Re: [PATCH v1 3/8] device-context: Define the device context fields for device migration

On 10/9/2023 10:30 PM, Parav Pandit wrote:

From: Zhu, Lingshan <lingshan.zhu@intel.com>
Sent: Monday, October 9, 2023 4:04 PM

On 10/8/2023 7:41 PM, Michael S. Tsirkin wrote:
On Sun, Oct 08, 2023 at 02:25:50PM +0300, Parav Pandit wrote:
Define the device context and its fields for purpose of device
migration. The device context is read and written by the owner driver
on source and destination hypervisor respectively.

Device context fields will experience a rapid growth post this
initial version to cover many details of the device.

Fixes: https://github.com/oasis-tcs/virtio-spec/issues/176
Signed-off-by: Parav Pandit <parav@nvidia.com>
Signed-off-by: Satananda Burla <sburla@marvell.com>
- enrich device context to cover feature bits, device configuration
- corrected alignment of device context fields
   content.tex        |   1 +
   device-context.tex | 142
   2 files changed, 143 insertions(+)
   create mode 100644 device-context.tex

diff --git a/content.tex b/content.tex index 0a62dce..2698931 100644
--- a/content.tex
+++ b/content.tex
@@ -503,6 +503,7 @@ \section{Exporting Objects}\label{sec:Basic Facilities
of a Virtio Device / Expo
   UUIDs as specified by \hyperref[intro:rfc4122]{[RFC4122]}.


   \chapter{General Initialization And Device
Operation}\label{sec:General Initialization And Device Operation}

diff --git a/device-context.tex b/device-context.tex new file mode
100644 index 0000000..5611382
--- /dev/null
+++ b/device-context.tex
@@ -0,0 +1,142 @@
+\section{Device Context}\label{sec:Basic Facilities of a Virtio
+Device / Device Context}
+The device context holds the information that a owner driver can use
+to setup a member device and resume its operation. The device
+context of a member device is read or written by the owner driver
+using administration commands.
+struct virtio_dev_ctx_field_tlv {
+        le32 type;
+        le32 reserved;
+        le64 length;
+        u8 value[];
+struct virtio_dev_ctx {
+        le32 field_count;
+        struct virtio_dev_ctx_field_tlv fields[]; };
so this still doesn't work for nested
In one use case of nesting, that we came across is:
there is large host_VM which is hosting another guest_VMs.
In such case, the owner PF is passthrough to this host_VM and current proposed scheme continue to function for nesting as well for nested guest_VMs.
The system admin can choose only passthrough some of the devices for nested guests, so passthrough the PF to L1 guest is not a good idea, because there can be
many devices still work for the host or L1.

In second use case, where one want to bind only one member device to one VM,
I think same plumbing can be extended to have another VF, to take the role of migration device instead of owner device.

I donât see a good way to passthrough and also do in-band migration without lot of device specific trap and emulation.
I also donât know the cpu performance numbers with 3 levels of nested page table translation which to my understanding cannot be accelerated by the current cpu.
host_PA->L1_QEMU_VA->L1_Guest_PA->L1_QEMU_VA->L2_Guest_PA and so on, there can be performance overhead, but can be done.

So admin vq migration still don't work for nested, this is surely a blocker.
Do you know how does it work for Intel x86_64?
Can it do > 2 level of nested page tables? If no, what is the perf characteristics to expect?
of course that can be done, Page table is not a problem, there are soft mmu emulation and viommu, through performance overhead.

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