Message-ID: <aYIny_VDVn8w9-E7@anirudh-surface.localdomain>
Date: Tue, 3 Feb 2026 16:52:27 +0000
From: Anirudh Rayabharam <anirudh@...rudhrb.com>
To: Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com>
Cc: kys@...rosoft.com, haiyangz@...rosoft.com, wei.liu@...nel.org,
	decui@...rosoft.com, longli@...rosoft.com,
	linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mshv: Add support for integrated scheduler

On Mon, Feb 02, 2026 at 11:24:23AM -0800, Stanislav Kinsburskii wrote:
> On Fri, Jan 30, 2026 at 05:47:48PM +0000, Anirudh Rayabharam wrote:
> > On Fri, Jan 30, 2026 at 04:04:14PM +0000, Stanislav Kinsburskii wrote:
> > > 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
> > 
> > How could an L1VH partition schedule its own vCPUs?
> > 
> 
> By the mean of the integrated scheduler. Or,  from another perspective,
> the same way like any other root partition does: by placing load on a
> particular code and halting when there is nothing to do.

Maybe it's the terminology that's throwing me off. And L1VH partition
would schedule *guest* vCPUs right? And not its own? Hypervisor will
schedule the vCPUs belonging to an L1VH.

> 
> > > 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. This requirement makes
> > > reading VMM capabilities in L1VH partition a requirement too.
> > > 
> > > Signed-off-by: Andreea Pintilie <anpintil@...rosoft.com>
> > > Signed-off-by: Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com>
> > > ---
> > >  drivers/hv/mshv_root_main.c |   85 +++++++++++++++++++++++++++----------------
> > >  include/hyperv/hvhdk_mini.h |    7 +++-
> > >  2 files changed, 59 insertions(+), 33 deletions(-)
> > > 
> > > diff --git a/drivers/hv/mshv_root_main.c b/drivers/hv/mshv_root_main.c
> > > index 1134a82c7881..6a6bf641b352 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)
> > 
> > typo: retrieve*
> > 
> 
> Thanks, will fix.
> 
> > > +{
> > > +	u64 integrated_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, &integrated_sched_enabled,
> > > +						sizeof(integrated_sched_enabled));
> > > +	if (ret)
> > > +		return ret;
> > > +
> > > +	if (integrated_sched_enabled)
> > > +		*out = HV_SCHEDULER_TYPE_ROOT;
> > > +
> > > +	pr_debug("%s: integrated scheduler property read: ret=%d value=%llu\n",
> > > +		 __func__, ret, integrated_sched_enabled);
> > 
> > ret is always 0 here, right? We don't need to bother printing then.
> > 
> 
> Oh yes, good point. Will fix.
> 
> > > +
> > > +	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 +2236,29 @@ 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 0;
> > > -
> > > -root_sched_deinit:
> > > -	root_scheduler_deinit();
> > > -	return err;
> > > +	return register_reboot_notifier(&mshv_reboot_nb);
> > >  }
> > >  
> > > -static void mshv_init_vmm_caps(struct device *dev)
> > > +static int __init 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 && hv_l1vh_partition()) {
> > > +		dev_err(dev, "Failed to get VMM capabilities: %d\n", ret);
> > > +		return ret;
> > 
> > I don't think we need to fail here. If there are not VMM caps available,
> > that means integrated scheduler is not supported by the hypervisor, so
> > fall back to core scheduler.
> > 
> 
> I believe we discussed this in a personal conversation earlier.
> Let me know, is we need to discuss it further.

So if we want to ensure that we proceed only if vmm caps are available,
we don't need the "&& hv_l1vh_partition()" part?

Thanks,
Anirudh.

> 
> Thanks,
> Stanislav
> 
> > Thanks,
> > Anirudh
> > 
> > > +	}
> > >  
> > >  	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 +2304,10 @@ 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 +2317,13 @@ 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 +2332,8 @@ 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 +2352,7 @@ 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 41a29bf8ec14..c0300910808b 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	59
> > > +#define HV_PARTITION_VMM_CAPABILITIES_RESERVED_BITFIELD_COUNT	57
> > >  
> > >  struct hv_partition_property_vmm_capabilities {
> > >  	u16 bank_count;
> > > @@ -119,6 +122,8 @@ struct hv_partition_property_vmm_capabilities {
> > >  			u64 reservedbit3: 1;
> > >  #endif
> > >  			u64 assignable_synthetic_proc_features: 1;
> > > +			u64 reservedbit5: 1;
> > > +			u64 vmm_enable_integrated_scheduler : 1;
> > >  			u64 reserved0: HV_PARTITION_VMM_CAPABILITIES_RESERVED_BITFIELD_COUNT;
> > >  		} __packed;
> > >  	};
> > > 
> > > 

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