On 10/15/24 5:22 AM, Salil Mehta wrote:
Certain CPU architecture specifications [1][2][3] prohibit changes to CPU
presence after the kernel has booted. This limitation exists because many system
initializations rely on the exact CPU count at boot time and do not expect it to
change later. For example, components like interrupt controllers, which are
closely tied to CPUs, or various per-CPU features, may not support configuration
changes once the kernel has been initialized. This presents a challenge for
virtualization features such as vCPU hotplug.
To address this issue, introduce an `is_enabled` state in the `AcpiCpuStatus`,
which reflects whether a vCPU has been hot-plugged or hot-unplugged in QEMU,
marking it as (un)available in the Guest Kernel. The `is_present` state should
be set based on the `acpi_persistent` flag. In cases where unplugged vCPUs need
to be deliberately simulated in the ACPI to maintain a persistent view of vCPUs,
this flag ensures the guest kernel continues to see those vCPUs.
Additionally, introduce an `acpi_persistent` property that can be used to
initialize the ACPI vCPU presence state accordingly. Architectures requiring
ACPI to expose a persistent view of vCPUs can override its default value. Refer
to the patch-set implelenting vCPU hotplug support for ARM for more details on
its usage.
References:
[1] KVMForum 2023 Presentation: Challenges Revisited in Supporting Virt CPU
Hotplug on
architectures that don’t Support CPU Hotplug (like ARM64)
a. Kernel Link:
https://kvm-forum.qemu.org/2023/KVM-forum-cpu-hotplug_7OJ1YyJ.pdf
b. Qemu Link:
https://kvm-forum.qemu.org/2023/Challenges_Revisited_in_Supporting_Virt_CPU_Hotplug_-__ii0iNb3.pdf
[2] KVMForum 2020 Presentation: Challenges in Supporting Virtual CPU Hotplug on
SoC Based Systems (like ARM64)
Link: https://kvmforum2020.sched.com/event/eE4m
[3] Check comment 5 in the bugzilla entry
Link: https://bugzilla.tianocore.org/show_bug.cgi?id=4481#c5
Signed-off-by: Salil Mehta <salil.me...@huawei.com>
---
cpu-target.c | 1 +
hw/acpi/cpu.c | 35 ++++++++++++++++++++++++++++++++++-
include/hw/acpi/cpu.h | 21 +++++++++++++++++++++
include/hw/core/cpu.h | 21 +++++++++++++++++++++
4 files changed, 77 insertions(+), 1 deletion(-)
[...]
+/**
+ * acpi_persistent_cpu:
+ * @cpu: The vCPU to check
+ *
+ * Checks if the vCPU state should always be reflected as *present* via ACPI
+ * to the Guest. By default, this is False on all architectures and has to be
+ * explicity set during initialization.
^^^^^^^^^
explicitly
+ *
+ * Returns: True if it is ACPI 'persistent' CPU
+ *
+ */
+static inline bool acpi_persistent_cpu(CPUState *cpu)
+{
+ /*
+ * returns if 'Presence' of the vCPU is persistent and should be simulated
+ * via ACPI even after vCPUs have been unplugged in QOM
+ */
+ return cpu && cpu->acpi_persistent;
+}
#endif
Thanks,
Gavin
