Re: [PATCH v7 0/3] hw/acpi: Add vmclock device

[Michael S. Tsirkin]

On Thu, Jan 16, 2025 at 01:59:40PM +0000, David Woodhouse wrote:
> (Posting one last time with the header commits split out).
> 
> The vmclock device addresses the problem of live migration with
> precision clocks. The tolerances of a hardware counter (e.g. TSC) are
> typically around ±50PPM. A guest will use NTP/PTP/PPS to discipline that
> counter against an external source of 'real' time, and track the precise
> frequency of the counter as it changes with environmental conditions.
> 
> When a guest is live migrated, anything it knows about the frequency of
> the underlying counter becomes invalid. It may move from a host where
> the counter running at -50PPM of its nominal frequency, to a host where
> it runs at +50PPM. There will also be a step change in the value of the
> counter, as the correctness of its absolute value at migration is
> limited by the accuracy of the source and destination host's time
> synchronization.
> 
> The device exposes a shared memory region to guests, which can be mapped
> all the way to userspace. In the first phase, this merely advertises a
> 'disruption_marker', which indicates that the guest should throw away any
> NTP synchronization it thinks it has, and start again.
> 
> Because the region can be exposed all the way to userspace, applications
> can still use time from a fast vDSO 'system call', and check the
> disruption marker to be sure that their timestamp is indeed truthful.
> 
> The structure also allows for the precise time, as known by the host, to
> be exposed directly to guests so that they don't have to wait for NTP to
> resync from scratch.
> 
> The values and fields are based on the nascent virtio-rtc specification,
> and the intent is that a version (hopefully precisely this version) of
> this structure will be included as an optional part of that spec. In the
> meantime, a simple ACPI device along the lines of VMGENID is perfectly
> sufficient and is compatible with what's being shipped in certain
> commercial hypervisors.
> 
> Linux guest support was merged into the 6.13-rc1 kernel:
> https://git.kernel.org/torvalds/c/205032724226



Reviewed-by: Michael S. Tsirkin <mst@redhat.com>

feel free to merge.

> ---
> v7:
>  • Split update-kernel-headers.sh and the addition of the new header
>    file into separate commits, add MAINTAINERS entry.
> 
> v6:
>  • Rebase for DEFINE_PROP_END_OF_LIST removal and sysemu→system
>    rename.
> 
> v5:
>  • Trivial simplification to AML generation.
>  • Import vmclock-abi.h from Linux now the guest support is merged.
> 
> v4:
>  • Trivial checkpatch fixes and comment improvements.
> 
> v3:
>  • Add comment that vmclock-abi.h will come from the Linux kernel
>    headers once it gets merged there.
> 
> v2:
>  • Change esterror/maxerror fields to nanoseconds.
>  • Change to officially assigned AMZNC10C ACPI HID.
>  • Fix little-endian handling of fields in update.
> 
> David Woodhouse (3):
>       linux-headers: Add vmclock-abi.h
>       linux-headers: Update to Linux 6.13-rc7
>       hw/acpi: Add vmclock device
> 
>  MAINTAINERS                                  |   5 +
>  hw/acpi/Kconfig                              |   5 +
>  hw/acpi/meson.build                          |   1 +
>  hw/acpi/vmclock.c                            | 179 ++++++++++++++++++++++++++
>  hw/i386/Kconfig                              |   1 +
>  hw/i386/acpi-build.c                         |  10 +-
>  include/hw/acpi/vmclock.h                    |  34 +++++
>  include/standard-headers/linux/vmclock-abi.h | 182 
> +++++++++++++++++++++++++++
>  linux-headers/linux/iommufd.h                |  31 +++--
>  linux-headers/linux/stddef.h                 |  13 +-
>  scripts/update-linux-headers.sh              |   1 +
>  11 files changed, 447 insertions(+), 15 deletions(-)
>