Message-ID:
 <SN6PR02MB415767BB59E00442812F47B5D49EA@SN6PR02MB4157.namprd02.prod.outlook.com>
Date: Thu, 29 Jan 2026 17:47:02 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com>,
	"kys@...rosoft.com" <kys@...rosoft.com>, "haiyangz@...rosoft.com"
	<haiyangz@...rosoft.com>, "wei.liu@...nel.org" <wei.liu@...nel.org>,
	"decui@...rosoft.com" <decui@...rosoft.com>, "longli@...rosoft.com"
	<longli@...rosoft.com>
CC: "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 2/2] mshv: Add support for integrated scheduler

From: Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com> Sent: Wednesday, January 21, 2026 2:36 PM
> 
> From: Andreea Pintilie <anpintil@...rosoft.com>
> 
> 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 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 <anpintil@...rosoft.com>
> Signed-off-by: Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com>
> ---
>  drivers/hv/mshv_root_main.c |   79 +++++++++++++++++++++++++++++--------------
>  include/hyperv/hvhdk_mini.h |    6 +++
>  2 files changed, 58 insertions(+), 27 deletions(-)
> 
> diff --git a/drivers/hv/mshv_root_main.c b/drivers/hv/mshv_root_main.c
> index 1134a82c7881..7a36297feea7 100644
> --- a/drivers/hv/mshv_root_main.c
> +++ b/drivers/hv/mshv_root_main.c
> @@ -2053,6 +2053,32 @@ static const char *scheduler_type_to_string(enum hv_scheduler_type type)
>  	};
>  }
> 
> +static int __init l1vh_retrive_scheduler_type(enum hv_scheduler_type *out)
> +{
> +	size_t root_sched_enabled;
> +	int ret;
> +
> +	*out = HV_SCHEDULER_TYPE_CORE_SMT;
> +
> +	if (!mshv_root.vmm_caps.vmm_enable_integrated_scheduler)
> +		return 0;
> +
> +	ret = hv_call_get_partition_property_ex(HV_PARTITION_ID_SELF,
> +					 	HV_PARTITION_PROPERTY_INTEGRATED_SCHEDULER_ENABLED,
> +						0, &root_sched_enabled,
> +						sizeof(root_sched_enabled));

hv_call_get_partition_property_ex() makes a hypercall, and then copies
back the number of bytes indicated by the 4th argument above; i.e.,
sizeof(root_sched_enabled). But using the size of a Linux type (size_t) to
control how much data is copied back from a hypercall seems inappropriate.
There should be a hypervisor-defined size that is copied back, or at worst,
an exactly specified Linux size like u64. By comparison, the use of
hv_call_get_partition_property_ex() in mshv_init_vmm_caps() copies back
sizeof(struct hv_partition_property_vmm_capabilities) bytes, which comes
from hvhdk_mini.h, so that's good.

The naming of root_sched_enabled is a bit of a cognitive dissonance with
getting the INTEGRATED_SCHEDULER_ENABLED property. I'd suggest the
local variable should be named "integrated_sched_enabled". Code in this
function then makes the decision that if the integrated scheduler is enabled,
L1VH partitions should be using the root scheduler (which is what the
commit message describes).

> +	if (ret)
> +		return ret;
> +
> +	if (root_sched_enabled)
> +		*out = HV_SCHEDULER_TYPE_ROOT;
> +
> +	pr_debug("%s: integrated scheduler property read: ret=%d value=%lu\n",
> +		 __func__, ret, root_sched_enabled);
> +
> +	return 0;
> +}
> +
>  /* TODO move this to hv_common.c when needed outside */
>  static int __init hv_retrieve_scheduler_type(enum hv_scheduler_type *out)
>  {
> @@ -2085,13 +2111,12 @@ static int __init hv_retrieve_scheduler_type(enum hv_scheduler_type *out)
>  /* Retrieve and stash the supported scheduler type */
>  static int __init mshv_retrieve_scheduler_type(struct device *dev)
>  {
> -	int ret = 0;
> +	int ret;
> 
>  	if (hv_l1vh_partition())
> -		hv_scheduler_type = HV_SCHEDULER_TYPE_CORE_SMT;
> +		ret = l1vh_retrive_scheduler_type(&hv_scheduler_type);
>  	else
>  		ret = hv_retrieve_scheduler_type(&hv_scheduler_type);
> -
>  	if (ret)
>  		return ret;
> 
> @@ -2211,42 +2236,35 @@ struct notifier_block mshv_reboot_nb = {
>  static void mshv_root_partition_exit(void)
>  {
>  	unregister_reboot_notifier(&mshv_reboot_nb);
> -	root_scheduler_deinit();
>  }
> 
>  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?

> -
> -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?

> 
>  	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;
>  	};
> 
> 

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