[PATCH] mshv: Add support for a new parent partition configuration

[PATCH] mshv: Add support for a new parent partition configuration

[Nuno Das Neves]

Detect booting as an "L1VH" partition. This is a new scenario very
similar to root partition where the mshv_root driver can be used to
create and manage guest partitions.

It mostly works the same as root partition, but there are some
differences in how various features are handled. hv_l1vh_partition()
is introduced to handle these cases. Add hv_parent_partition()
which returns true for either case, replacing some hv_root_partition()
checks.

Signed-off-by: Nuno Das Neves <nunodasneves@linux.microsoft.com>
---
 drivers/hv/hv_common.c         | 20 ++++++++++++--------
 drivers/hv/mshv_root_main.c    | 22 ++++++++++++++--------
 include/asm-generic/mshyperv.h | 11 +++++++++++
 3 files changed, 37 insertions(+), 16 deletions(-)

diff --git a/drivers/hv/hv_common.c b/drivers/hv/hv_common.c
index cbe4a954ad46..a6839593ca31 100644
--- a/drivers/hv/hv_common.c
+++ b/drivers/hv/hv_common.c
@@ -357,7 +357,7 @@ int __init hv_common_init(void)
 	hyperv_pcpu_arg = alloc_percpu(void  *);
 	BUG_ON(!hyperv_pcpu_arg);
 
-	if (hv_root_partition()) {
+	if (hv_parent_partition()) {
 		hv_synic_eventring_tail = alloc_percpu(u8 *);
 		BUG_ON(!hv_synic_eventring_tail);
 	}
@@ -506,7 +506,7 @@ int hv_common_cpu_init(unsigned int cpu)
 	if (msr_vp_index > hv_max_vp_index)
 		hv_max_vp_index = msr_vp_index;
 
-	if (hv_root_partition()) {
+	if (hv_parent_partition()) {
 		synic_eventring_tail = (u8 **)this_cpu_ptr(hv_synic_eventring_tail);
 		*synic_eventring_tail = kcalloc(HV_SYNIC_SINT_COUNT,
 						sizeof(u8), flags);
@@ -532,7 +532,7 @@ int hv_common_cpu_die(unsigned int cpu)
 	 * originally allocated memory is reused in hv_common_cpu_init().
 	 */
 
-	if (hv_root_partition()) {
+	if (hv_parent_partition()) {
 		synic_eventring_tail = this_cpu_ptr(hv_synic_eventring_tail);
 		kfree(*synic_eventring_tail);
 		*synic_eventring_tail = NULL;
@@ -703,13 +703,17 @@ void hv_identify_partition_type(void)
 	 * the root partition setting if also a Confidential VM.
 	 */
 	if ((ms_hyperv.priv_high & HV_CREATE_PARTITIONS) &&
-	    (ms_hyperv.priv_high & HV_CPU_MANAGEMENT) &&
 	    !(ms_hyperv.priv_high & HV_ISOLATION)) {
-		pr_info("Hyper-V: running as root partition\n");
-		if (IS_ENABLED(CONFIG_MSHV_ROOT))
-			hv_curr_partition_type = HV_PARTITION_TYPE_ROOT;
-		else
+
+		if (!IS_ENABLED(CONFIG_MSHV_ROOT)) {
 			pr_crit("Hyper-V: CONFIG_MSHV_ROOT not enabled!\n");
+		} else if (ms_hyperv.priv_high & HV_CPU_MANAGEMENT) {
+			pr_info("Hyper-V: running as root partition\n");
+			hv_curr_partition_type = HV_PARTITION_TYPE_ROOT;
+		} else {
+			pr_info("Hyper-V: running as L1VH partition\n");
+			hv_curr_partition_type = HV_PARTITION_TYPE_L1VH;
+		}
 	}
 }
 
diff --git a/drivers/hv/mshv_root_main.c b/drivers/hv/mshv_root_main.c
index aca3331ad516..7c710703cd96 100644
--- a/drivers/hv/mshv_root_main.c
+++ b/drivers/hv/mshv_root_main.c
@@ -37,12 +37,6 @@ MODULE_AUTHOR("Microsoft");
 MODULE_LICENSE("GPL");
 MODULE_DESCRIPTION("Microsoft Hyper-V root partition VMM interface /dev/mshv");
 
-/* TODO move this to mshyperv.h when needed outside driver */
-static inline bool hv_parent_partition(void)
-{
-	return hv_root_partition();
-}
-
 /* TODO move this to another file when debugfs code is added */
 enum hv_stats_vp_counters {			/* HV_THREAD_COUNTER */
 #if defined(CONFIG_X86)
@@ -2190,6 +2184,15 @@ struct notifier_block mshv_reboot_nb = {
 	.notifier_call = mshv_reboot_notify,
 };
 
+static int __init mshv_l1vh_partition_init(struct device *dev)
+{
+	hv_scheduler_type = HV_SCHEDULER_TYPE_CORE_SMT;
+	dev_info(dev, "Hypervisor using %s\n",
+		 scheduler_type_to_string(hv_scheduler_type));
+
+	return 0;
+}
+
 static void mshv_root_partition_exit(void)
 {
 	unregister_reboot_notifier(&mshv_reboot_nb);
@@ -2224,7 +2227,7 @@ static int __init mshv_parent_partition_init(void)
 	struct device *dev;
 	union hv_hypervisor_version_info version_info;
 
-	if (!hv_root_partition() || is_kdump_kernel())
+	if (!hv_parent_partition() || is_kdump_kernel())
 		return -ENODEV;
 
 	if (hv_get_hypervisor_version(&version_info))
@@ -2261,7 +2264,10 @@ static int __init mshv_parent_partition_init(void)
 
 	mshv_cpuhp_online = ret;
 
-	ret = mshv_root_partition_init(dev);
+	if (hv_root_partition())
+		ret = mshv_root_partition_init(dev);
+	else
+		ret = mshv_l1vh_partition_init(dev);
 	if (ret)
 		goto remove_cpu_state;
 
diff --git a/include/asm-generic/mshyperv.h b/include/asm-generic/mshyperv.h
index dbbacd47ca35..f0f0eacb2eef 100644
--- a/include/asm-generic/mshyperv.h
+++ b/include/asm-generic/mshyperv.h
@@ -31,6 +31,7 @@
 enum hv_partition_type {
 	HV_PARTITION_TYPE_GUEST,
 	HV_PARTITION_TYPE_ROOT,
+	HV_PARTITION_TYPE_L1VH,
 };
 
 struct ms_hyperv_info {
@@ -457,12 +458,22 @@ static inline bool hv_root_partition(void)
 {
 	return hv_curr_partition_type == HV_PARTITION_TYPE_ROOT;
 }
+static inline bool hv_l1vh_partition(void)
+{
+	return hv_curr_partition_type == HV_PARTITION_TYPE_L1VH;
+}
+static inline bool hv_parent_partition(void)
+{
+	return hv_root_partition() || hv_l1vh_partition();
+}
 int hv_call_deposit_pages(int node, u64 partition_id, u32 num_pages);
 int hv_call_add_logical_proc(int node, u32 lp_index, u32 acpi_id);
 int hv_call_create_vp(int node, u64 partition_id, u32 vp_index, u32 flags);
 
 #else /* CONFIG_MSHV_ROOT */
 static inline bool hv_root_partition(void) { return false; }
+static inline bool hv_l1vh_partition(void) { return false; }
+static inline bool hv_parent_partition(void) { return false; }
 static inline int hv_call_deposit_pages(int node, u64 partition_id, u32 num_pages)
 {
 	return -EOPNOTSUPP;
-- 
2.34.1

Re: [PATCH] mshv: Add support for a new parent partition configuration

[Easwar Hariharan]

On 8/19/2025 12:29 AM, Nuno Das Neves wrote:
> Detect booting as an "L1VH" partition. This is a new scenario very
> similar to root partition where the mshv_root driver can be used to
> create and manage guest partitions.
> 
> It mostly works the same as root partition, but there are some
> differences in how various features are handled. hv_l1vh_partition()
> is introduced to handle these cases. Add hv_parent_partition()
> which returns true for either case, replacing some hv_root_partition()
> checks.
> 
> Signed-off-by: Nuno Das Neves <nunodasneves@linux.microsoft.com>

Seems the plurality of subject prefixes for drivers/hv files has been "Drivers: hv"
so far, including the 2 commits for drivers/hv/mshv*. Are you planning to change
the standard for mshv driver going forward?

It'd be good to have a small blurb about what an L1VH partition is in the commit
message, as well as what guides which hv_root_partition() call gets replaced with
hv_parent_partition().

<snip>

Thanks,
Easwar (he/him)

Re: [PATCH] mshv: Add support for a new parent partition configuration

[Wei Liu]

On Tue, Aug 19, 2025 at 12:29:19AM -0700, Nuno Das Neves wrote:
> Detect booting as an "L1VH" partition. This is a new scenario very
> similar to root partition where the mshv_root driver can be used to
> create and manage guest partitions.
> 
> It mostly works the same as root partition, but there are some
> differences in how various features are handled. hv_l1vh_partition()
> is introduced to handle these cases. Add hv_parent_partition()
> which returns true for either case, replacing some hv_root_partition()
> checks.
> 
> Signed-off-by: Nuno Das Neves <nunodasneves@linux.microsoft.com>

Acked-by: Wei Liu <wei.liu@kernel.org>

Re: [PATCH] mshv: Add support for a new parent partition configuration

[Wei Liu]

On Tue, Aug 19, 2025 at 09:17:27AM -0700, Easwar Hariharan wrote:
> On 8/19/2025 12:29 AM, Nuno Das Neves wrote:
> > Detect booting as an "L1VH" partition. This is a new scenario very
> > similar to root partition where the mshv_root driver can be used to
> > create and manage guest partitions.
> > 
> > It mostly works the same as root partition, but there are some
> > differences in how various features are handled. hv_l1vh_partition()
> > is introduced to handle these cases. Add hv_parent_partition()
> > which returns true for either case, replacing some hv_root_partition()
> > checks.
> > 
> > Signed-off-by: Nuno Das Neves <nunodasneves@linux.microsoft.com>
> 
> Seems the plurality of subject prefixes for drivers/hv files has been "Drivers: hv"
> so far, including the 2 commits for drivers/hv/mshv*. Are you planning to change
> the standard for mshv driver going forward?

IMO it is okay to have a different prefix to call out the commits that
are relevant to the mshv driver. This eases the burden for anyone
porting the changes to a different kernel tree. We may also touch many
different parts of the codebase (though not in this particular patch).

Wei

Re: [PATCH] mshv: Add support for a new parent partition configuration

[Easwar Hariharan]

On 8/19/2025 10:22 AM, Wei Liu wrote:
> On Tue, Aug 19, 2025 at 09:17:27AM -0700, Easwar Hariharan wrote:
>> On 8/19/2025 12:29 AM, Nuno Das Neves wrote:
>>> Detect booting as an "L1VH" partition. This is a new scenario very
>>> similar to root partition where the mshv_root driver can be used to
>>> create and manage guest partitions.
>>>
>>> It mostly works the same as root partition, but there are some
>>> differences in how various features are handled. hv_l1vh_partition()
>>> is introduced to handle these cases. Add hv_parent_partition()
>>> which returns true for either case, replacing some hv_root_partition()
>>> checks.
>>>
>>> Signed-off-by: Nuno Das Neves <nunodasneves@linux.microsoft.com>
>>
>> Seems the plurality of subject prefixes for drivers/hv files has been "Drivers: hv"
>> so far, including the 2 commits for drivers/hv/mshv*. Are you planning to change
>> the standard for mshv driver going forward?
> 
> IMO it is okay to have a different prefix to call out the commits that
> are relevant to the mshv driver. This eases the burden for anyone
> porting the changes to a different kernel tree. We may also touch many
> different parts of the codebase (though not in this particular patch).
> 
> Wei

It's fine either way for me, I was trying to understand if it was an oversight or
intentional.

- Easwar (he/him)

RE: [PATCH] mshv: Add support for a new parent partition configuration

[Michael Kelley]

From: Nuno Das Neves <nunodasneves@linux.microsoft.com> Sent: Tuesday, August 19, 2025 12:29 AM
> 
> Detect booting as an "L1VH" partition. This is a new scenario very
> similar to root partition where the mshv_root driver can be used to
> create and manage guest partitions.
> 
> It mostly works the same as root partition, but there are some
> differences in how various features are handled. hv_l1vh_partition()
> is introduced to handle these cases. Add hv_parent_partition()
> which returns true for either case, replacing some hv_root_partition()
> checks.
> 
> Signed-off-by: Nuno Das Neves <nunodasneves@linux.microsoft.com>
> ---
>  drivers/hv/hv_common.c         | 20 ++++++++++++--------
>  drivers/hv/mshv_root_main.c    | 22 ++++++++++++++--------
>  include/asm-generic/mshyperv.h | 11 +++++++++++
>  3 files changed, 37 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/hv/hv_common.c b/drivers/hv/hv_common.c
> index cbe4a954ad46..a6839593ca31 100644
> --- a/drivers/hv/hv_common.c
> +++ b/drivers/hv/hv_common.c
> @@ -357,7 +357,7 @@ int __init hv_common_init(void)
>  	hyperv_pcpu_arg = alloc_percpu(void  *);
>  	BUG_ON(!hyperv_pcpu_arg);
> 
> -	if (hv_root_partition()) {
> +	if (hv_parent_partition()) {
>  		hv_synic_eventring_tail = alloc_percpu(u8 *);
>  		BUG_ON(!hv_synic_eventring_tail);
>  	}
> @@ -506,7 +506,7 @@ int hv_common_cpu_init(unsigned int cpu)
>  	if (msr_vp_index > hv_max_vp_index)
>  		hv_max_vp_index = msr_vp_index;
> 
> -	if (hv_root_partition()) {
> +	if (hv_parent_partition()) {
>  		synic_eventring_tail = (u8 **)this_cpu_ptr(hv_synic_eventring_tail);
>  		*synic_eventring_tail = kcalloc(HV_SYNIC_SINT_COUNT,
>  						sizeof(u8), flags);
> @@ -532,7 +532,7 @@ int hv_common_cpu_die(unsigned int cpu)
>  	 * originally allocated memory is reused in hv_common_cpu_init().
>  	 */
> 
> -	if (hv_root_partition()) {
> +	if (hv_parent_partition()) {
>  		synic_eventring_tail = this_cpu_ptr(hv_synic_eventring_tail);
>  		kfree(*synic_eventring_tail);
>  		*synic_eventring_tail = NULL;
> @@ -703,13 +703,17 @@ void hv_identify_partition_type(void)
>  	 * the root partition setting if also a Confidential VM.
>  	 */
>  	if ((ms_hyperv.priv_high & HV_CREATE_PARTITIONS) &&
> -	    (ms_hyperv.priv_high & HV_CPU_MANAGEMENT) &&
>  	    !(ms_hyperv.priv_high & HV_ISOLATION)) {
> -		pr_info("Hyper-V: running as root partition\n");
> -		if (IS_ENABLED(CONFIG_MSHV_ROOT))
> -			hv_curr_partition_type = HV_PARTITION_TYPE_ROOT;
> -		else
> +
> +		if (!IS_ENABLED(CONFIG_MSHV_ROOT)) {
>  			pr_crit("Hyper-V: CONFIG_MSHV_ROOT not enabled!\n");
> +		} else if (ms_hyperv.priv_high & HV_CPU_MANAGEMENT) {
> +			pr_info("Hyper-V: running as root partition\n");
> +			hv_curr_partition_type = HV_PARTITION_TYPE_ROOT;
> +		} else {
> +			pr_info("Hyper-V: running as L1VH partition\n");
> +			hv_curr_partition_type = HV_PARTITION_TYPE_L1VH;
> +		}
>  	}
>  }
> 
> diff --git a/drivers/hv/mshv_root_main.c b/drivers/hv/mshv_root_main.c
> index aca3331ad516..7c710703cd96 100644
> --- a/drivers/hv/mshv_root_main.c
> +++ b/drivers/hv/mshv_root_main.c
> @@ -37,12 +37,6 @@ MODULE_AUTHOR("Microsoft");
>  MODULE_LICENSE("GPL");
>  MODULE_DESCRIPTION("Microsoft Hyper-V root partition VMM interface /dev/mshv");
> 
> -/* TODO move this to mshyperv.h when needed outside driver */
> -static inline bool hv_parent_partition(void)
> -{
> -	return hv_root_partition();
> -}
> -
>  /* TODO move this to another file when debugfs code is added */
>  enum hv_stats_vp_counters {			/* HV_THREAD_COUNTER */
>  #if defined(CONFIG_X86)
> @@ -2190,6 +2184,15 @@ struct notifier_block mshv_reboot_nb = {
>  	.notifier_call = mshv_reboot_notify,
>  };
> 
> +static int __init mshv_l1vh_partition_init(struct device *dev)
> +{
> +	hv_scheduler_type = HV_SCHEDULER_TYPE_CORE_SMT;
> +	dev_info(dev, "Hypervisor using %s\n",
> +		 scheduler_type_to_string(hv_scheduler_type));
> +
> +	return 0;
> +}

I'm a bit late reviewing this patch, but I have a suggestion. With this
function added, setting hv_scheduler_type and outputting the message
is now in two places:  here and in mshv_retrieve_scheduler_type().

Instead, check for root vs. L1VH in mshv_retrieve_scheduler_type(),
and either call hv_retrieve_scheduler_type(), or force to
HV_SCHEDULER_TYPE_CORE_SMT.  Then the rest of the code in
mshv_retrieve_scheduler_type(), including outputting the message,
just works. That puts all the logic about the scheduler type in one
place.

Then move the call to mshv_retrieve_scheduler_type() directly
into mshv_parent_partition_init() just before

	if (hv_root_partition())
		ret = mshv_root_partition_init(dev);

mshv_l1vh_partition_init() is no longer needed.

Independent of the suggestion, everything else looks good.

Reviewed-by: Michael Kelley <mhklinux@outlook.com>

> +
>  static void mshv_root_partition_exit(void)
>  {
>  	unregister_reboot_notifier(&mshv_reboot_nb);
> @@ -2224,7 +2227,7 @@ static int __init mshv_parent_partition_init(void)
>  	struct device *dev;
>  	union hv_hypervisor_version_info version_info;
> 
> -	if (!hv_root_partition() || is_kdump_kernel())
> +	if (!hv_parent_partition() || is_kdump_kernel())
>  		return -ENODEV;
> 
>  	if (hv_get_hypervisor_version(&version_info))
> @@ -2261,7 +2264,10 @@ static int __init mshv_parent_partition_init(void)
> 
>  	mshv_cpuhp_online = ret;
> 
> -	ret = mshv_root_partition_init(dev);
> +	if (hv_root_partition())
> +		ret = mshv_root_partition_init(dev);
> +	else
> +		ret = mshv_l1vh_partition_init(dev);
>  	if (ret)
>  		goto remove_cpu_state;
> 
> diff --git a/include/asm-generic/mshyperv.h b/include/asm-generic/mshyperv.h
> index dbbacd47ca35..f0f0eacb2eef 100644
> --- a/include/asm-generic/mshyperv.h
> +++ b/include/asm-generic/mshyperv.h
> @@ -31,6 +31,7 @@
>  enum hv_partition_type {
>  	HV_PARTITION_TYPE_GUEST,
>  	HV_PARTITION_TYPE_ROOT,
> +	HV_PARTITION_TYPE_L1VH,
>  };
> 
>  struct ms_hyperv_info {
> @@ -457,12 +458,22 @@ static inline bool hv_root_partition(void)
>  {
>  	return hv_curr_partition_type == HV_PARTITION_TYPE_ROOT;
>  }
> +static inline bool hv_l1vh_partition(void)
> +{
> +	return hv_curr_partition_type == HV_PARTITION_TYPE_L1VH;
> +}
> +static inline bool hv_parent_partition(void)
> +{
> +	return hv_root_partition() || hv_l1vh_partition();
> +}
>  int hv_call_deposit_pages(int node, u64 partition_id, u32 num_pages);
>  int hv_call_add_logical_proc(int node, u32 lp_index, u32 acpi_id);
>  int hv_call_create_vp(int node, u64 partition_id, u32 vp_index, u32 flags);
> 
>  #else /* CONFIG_MSHV_ROOT */
>  static inline bool hv_root_partition(void) { return false; }
> +static inline bool hv_l1vh_partition(void) { return false; }
> +static inline bool hv_parent_partition(void) { return false; }
>  static inline int hv_call_deposit_pages(int node, u64 partition_id, u32 num_pages)
>  {
>  	return -EOPNOTSUPP;
> --
> 2.34.1