On Thu, Jan 29, 2026 at 11:09:46AM -0800, Stanislav Kinsburskii wrote:
> On Thu, Jan 29, 2026 at 05:47:02PM +0000, Michael Kelley wrote:
> > From: Stanislav Kinsburskii <[email protected]> Sent:
> > Wednesday, January 21, 2026 2:36 PM
> > >
> > > From: Andreea Pintilie <[email protected]>
> > >
> > > Query the hypervisor for integrated scheduler support and use it if
> > > configured.
> > >
> > > Microsoft Hypervisor originally provided two schedulers: root and core.
> > > The
> > > root scheduler allows the root partition to schedule guest vCPUs across
> > > physical cores, supporting both time slicing and CPU affinity (e.g., via
> > > cgroups). In contrast, the core scheduler delegates vCPU-to-physical-core
> > > scheduling entirely to the hypervisor.
> > >
> > > Direct virtualization introduces a new privileged guest partition type -
> > > L1
> > > Virtual Host (L1VH) — which can create child partitions from its own
> > > resources. These child partitions are effectively siblings, scheduled by
> > > the hypervisor's core scheduler. This prevents the L1VH parent from
> > > setting
> > > affinity or time slicing for its own processes or guest VPs. While
> > > cgroups,
> > > CFS, and cpuset controllers can still be used, their effectiveness is
> > > unpredictable, as the core scheduler swaps vCPUs according to its own
> > > logic
> > > (typically round-robin across all allocated physical CPUs). As a result,
> > > the system may appear to "steal" time from the L1VH and its children.
> > >
> > > To address this, Microsoft Hypervisor introduces the integrated scheduler.
> > This the s allows an L1VH partition to schedule its own vCPUs and those
> > of its
> > > guests across its "physical" cores, effectively emulating root scheduler
> > > behavior within the L1VH, while retaining core scheduler behavior for the
> > > rest of the system.
> > >
> > > The integrated scheduler is controlled by the root partition and gated by
> > > the vmm_enable_integrated_scheduler capability bit. If set, the hypervisor
> > > supports the integrated scheduler. The L1VH partition must then check if
> > > it
> > > is enabled by querying the corresponding extended partition property. If
> > > this property is true, the L1VH partition must use the root scheduler
> > > logic; otherwise, it must use the core scheduler.
> > >
> > > Signed-off-by: Andreea Pintilie <[email protected]>
> > > Signed-off-by: Stanislav Kinsburskii <[email protected]>
> > > ---
> > > drivers/hv/mshv_root_main.c | 79
> > > +++++++++++++++++++++++++++++--------------
> > > include/hyperv/hvhdk_mini.h | 6 +++
> > > 2 files changed, 58 insertions(+), 27 deletions(-)
> > >
>
> <snip>
>
> > > static int __init mshv_root_partition_init(struct device *dev)
> > > {
> > > int err;
> > >
> > > - err = root_scheduler_init(dev);
> > > - if (err)
> > > - return err;
> > > -
> > > err = register_reboot_notifier(&mshv_reboot_nb);
> > > if (err)
> > > - goto root_sched_deinit;
> > > + return err;
> > >
> > > return 0;
> >
> > This code is now:
> >
> > if (err)
> > return err;
> > return 0;
> >
> > which can be simplified to just:
> >
> > return err;
> >
> > Or drop the local variable 'err' and simplify the entire function to:
> >
> > return register_reboot_notifier(&mshv_reboot_nb);
> >
> > There's a tangential question here: Why is this reboot notifier
> > needed in the first place? All it does is remove the cpuhp state
> > that allocates/frees the per-cpu root_scheduler_input and
> > root_scheduler_output pages. Removing the state will free
> > the pages, but if Linux is rebooting, why bother?
> >
>
> This was originally done to support kexec.
> Here is the original commit message:
>
> mshv: perform synic cleanup during kexec
>
> Register a reboot notifier that performs synic cleanup when a kexec
> is in progress.
>
> One notable issue this commit fixes is one where after a kexec, virtio
> devices are not functional. Linux root partition receives MMIO doorbell
> events in the ring buffer in the SIRB synic page. The hypervisor maintains
> a head pointer where it writes new events into the ring buffer. The root
> partition maintains a tail pointer to read events from the buffer.
>
> Upon kexec reboot, all root data structures are re-initialized and thus
> the
> tail pointer gets reset to zero. The hypervisor on the other hand still
> retains the pre-kexec head pointer which could be non-zero. This means
> that
> when the hypervisor writes new events to the ring buffer, the root
> partition looks at the wrong place and doesn't find any events. So, future
> doorbell events never get delivered. As a result, virtqueue kicks never
> get
> delivered to the host.
>
> When the SIRB page is disabled the hypervisor resets the head pointer.
>
> > > -root_sched_deinit:
> > > - root_scheduler_deinit();
> > > - return err;
> > > }
> > >
> > > -static void mshv_init_vmm_caps(struct device *dev)
> > > +static int mshv_init_vmm_caps(struct device *dev)
> > > {
> > > - /*
> > > - * This can only fail here if HVCALL_GET_PARTITION_PROPERTY_EX or
> > > - * HV_PARTITION_PROPERTY_VMM_CAPABILITIES are not supported. In that
> > > - * case it's valid to proceed as if all vmm_caps are disabled (zero).
> > > - */
> > > - if (hv_call_get_partition_property_ex(HV_PARTITION_ID_SELF,
> > > -
> > > HV_PARTITION_PROPERTY_VMM_CAPABILITIES,
> > > - 0, &mshv_root.vmm_caps,
> > > - sizeof(mshv_root.vmm_caps)))
> > > - dev_warn(dev, "Unable to get VMM capabilities\n");
> > > + int ret;
> > > +
> > > + ret = hv_call_get_partition_property_ex(HV_PARTITION_ID_SELF,
> > > +
> > > HV_PARTITION_PROPERTY_VMM_CAPABILITIES,
> > > + 0, &mshv_root.vmm_caps,
> > > + sizeof(mshv_root.vmm_caps));
> > > + if (ret) {
> > > + dev_err(dev, "Failed to get VMM capabilities: %d\n", ret);
> > > + return ret;
> > > + }
> >
> > This is a functional change that isn't mentioned in the commit message.
> > Why is it now appropriate to fail instead of treating the VMM capabilities
> > as all disabled? Presumably there are older versions of the hypervisor that
> > don't support the requirements described in the original comment, but
> > perhaps they are no longer relevant?
> >
>
> To fail is now the only option for the L1VH partition. It must discover
> the scheduler type. Without this information, the partition cannot
> operate. The core scheduler logic will not work with an integrated
> scheduler, and vice versa.
I don't think we need to fail here. If we don't find vmm caps, that
means we are on an older hypervisor that supports l1vh but not
integrated scheduler (yes, such a version exists). In this case since
integrated scheduler is not supported by the hypervisor, the core
scheduler logic will work.
Thanks,
Anirudh.
>
> And yes, older hypervisor versions do not support L1VH.
>
> Thanks,
> Stanislav
>
> > >
> > > dev_dbg(dev, "vmm_caps = %#llx\n", mshv_root.vmm_caps.as_uint64[0]);
> > > +
> > > + return 0;
> > > }
> > >
> > > static int __init mshv_parent_partition_init(void)
> > > @@ -2292,6 +2310,10 @@ static int __init mshv_parent_partition_init(void)
> > >
> > > mshv_cpuhp_online = ret;
> > >
> > > + ret = mshv_init_vmm_caps(dev);
> > > + if (ret)
> > > + goto remove_cpu_state;
> > > +
> > > ret = mshv_retrieve_scheduler_type(dev);
> > > if (ret)
> > > goto remove_cpu_state;
> > > @@ -2301,11 +2323,13 @@ static int __init mshv_parent_partition_init(void)
> > > if (ret)
> > > goto remove_cpu_state;
> > >
> > > - mshv_init_vmm_caps(dev);
> > > + ret = root_scheduler_init(dev);
> > > + if (ret)
> > > + goto exit_partition;
> > >
> > > ret = mshv_irqfd_wq_init();
> > > if (ret)
> > > - goto exit_partition;
> > > + goto deinit_root_scheduler;
> > >
> > > spin_lock_init(&mshv_root.pt_ht_lock);
> > > hash_init(mshv_root.pt_htable);
> > > @@ -2314,6 +2338,8 @@ static int __init mshv_parent_partition_init(void)
> > >
> > > return 0;
> > >
> > > +deinit_root_scheduler:
> > > + root_scheduler_deinit();
> > > exit_partition:
> > > if (hv_root_partition())
> > > mshv_root_partition_exit();
> > > @@ -2332,6 +2358,7 @@ static void __exit mshv_parent_partition_exit(void)
> > > mshv_port_table_fini();
> > > misc_deregister(&mshv_dev);
> > > mshv_irqfd_wq_cleanup();
> > > + root_scheduler_deinit();
> > > if (hv_root_partition())
> > > mshv_root_partition_exit();
> > > cpuhp_remove_state(mshv_cpuhp_online);
> > > diff --git a/include/hyperv/hvhdk_mini.h b/include/hyperv/hvhdk_mini.h
> > > index aa03616f965b..0f7178fa88a8 100644
> > > --- a/include/hyperv/hvhdk_mini.h
> > > +++ b/include/hyperv/hvhdk_mini.h
> > > @@ -87,6 +87,9 @@ enum hv_partition_property_code {
> > > HV_PARTITION_PROPERTY_PRIVILEGE_FLAGS = 0x00010000,
> > > HV_PARTITION_PROPERTY_SYNTHETIC_PROC_FEATURES = 0x00010001,
> > >
> > > + /* Integrated scheduling properties */
> > > + HV_PARTITION_PROPERTY_INTEGRATED_SCHEDULER_ENABLED = 0x00020005,
> > > +
> > > /* Resource properties */
> > > HV_PARTITION_PROPERTY_GPA_PAGE_ACCESS_TRACKING = 0x00050005,
> > > HV_PARTITION_PROPERTY_UNIMPLEMENTED_MSR_ACTION = 0x00050017,
> > > @@ -102,7 +105,7 @@ enum hv_partition_property_code {
> > > };
> > >
> > > #define HV_PARTITION_VMM_CAPABILITIES_BANK_COUNT 1
> > > -#define HV_PARTITION_VMM_CAPABILITIES_RESERVED_BITFIELD_COUNT 58
> > > +#define HV_PARTITION_VMM_CAPABILITIES_RESERVED_BITFIELD_COUNT 57
> > >
> > > struct hv_partition_property_vmm_capabilities {
> > > u16 bank_count;
> > > @@ -120,6 +123,7 @@ struct hv_partition_property_vmm_capabilities {
> > > #endif
> > > u64 assignable_synthetic_proc_features: 1;
> > > u64 tag_hv_message_from_child: 1;
> > > + u64 vmm_enable_integrated_scheduler : 1;
> > > u64 reserved0:
> > > HV_PARTITION_VMM_CAPABILITIES_RESERVED_BITFIELD_COUNT;
> > > } __packed;
> > > };
> > >
> > >
> >
