linux-kernel - [PATCH v2] Drivers: hv: ioctl for self targeted passthrough hvcalls

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Message-Id: <20251117095207.113502-1-anirudh@anirudhrb.com>
Date: Mon, 17 Nov 2025 09:52:06 +0000
From: Anirudh Rayabharam <anirudh@...rudhrb.com>
To: "K. Y. Srinivasan" <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	Wei Liu <wei.liu@...nel.org>,
	Dexuan Cui <decui@...rosoft.com>,
	Long Li <longli@...rosoft.com>
Cc: anirudh@...rudhrb.com,
	linux-hyperv@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH v2] Drivers: hv: ioctl for self targeted passthrough hvcalls

From: Anirudh Rayabharam (Microsoft) <anirudh@...rudhrb.com>

Allow MSHV_ROOT_HVCALL IOCTL on the /dev/mshv fd. This IOCTL would
execute a passthrough hypercall targeting the root/parent partition
i.e. HV_PARTITION_ID_SELF.

This will be useful for the VMM to query things like supported
synthetic processor features, supported VMM capabiliites etc.

While at it, add HVCALL_GET_PARTITION_PROPERTY_EX to the allowed list of
passthrough hypercalls.

Signed-off-by: Anirudh Rayabharam (Microsoft) <anirudh@...rudhrb.com>
---

v2: rebased on latest hyperv-next

---
 drivers/hv/mshv_root_main.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/drivers/hv/mshv_root_main.c b/drivers/hv/mshv_root_main.c
index 45c7a5fea1cf..671c4d18520a 100644
--- a/drivers/hv/mshv_root_main.c
+++ b/drivers/hv/mshv_root_main.c
@@ -122,6 +122,7 @@ static struct miscdevice mshv_dev = {
  */
 static u16 mshv_passthru_hvcalls[] = {
 	HVCALL_GET_PARTITION_PROPERTY,
+	HVCALL_GET_PARTITION_PROPERTY_EX,
 	HVCALL_SET_PARTITION_PROPERTY,
 	HVCALL_INSTALL_INTERCEPT,
 	HVCALL_GET_VP_REGISTERS,
@@ -160,6 +161,7 @@ static int mshv_ioctl_passthru_hvcall(struct mshv_partition *partition,
 	void *input_pg = NULL;
 	void *output_pg = NULL;
 	u16 reps_completed;
+	u64 pt_id = partition ? partition->pt_id : HV_PARTITION_ID_SELF;
 
 	if (copy_from_user(&args, user_args, sizeof(args)))
 		return -EFAULT;
@@ -181,7 +183,7 @@ static int mshv_ioctl_passthru_hvcall(struct mshv_partition *partition,
 	is_async = mshv_hvcall_is_async(args.code);
 	if (is_async) {
 		/* async hypercalls can only be called from partition fd */
-		if (!partition_locked)
+		if (!partition || !partition_locked)
 			return -EINVAL;
 		ret = mshv_init_async_handler(partition);
 		if (ret)
@@ -209,7 +211,7 @@ static int mshv_ioctl_passthru_hvcall(struct mshv_partition *partition,
 	 * NOTE: This only works because all the allowed hypercalls' input
 	 * structs begin with a u64 partition_id field.
 	 */
-	*(u64 *)input_pg = partition->pt_id;
+	*(u64 *)input_pg = pt_id;
 
 	reps_completed = 0;
 	do {
@@ -238,7 +240,7 @@ static int mshv_ioctl_passthru_hvcall(struct mshv_partition *partition,
 			ret = hv_result_to_errno(status);
 		else
 			ret = hv_call_deposit_pages(NUMA_NO_NODE,
-						    partition->pt_id, 1);
+						    pt_id, 1);
 	} while (!ret);
 
 	args.status = hv_result(status);
@@ -2050,6 +2052,9 @@ static long mshv_dev_ioctl(struct file *filp, unsigned int ioctl,
 	case MSHV_CREATE_PARTITION:
 		return mshv_ioctl_create_partition((void __user *)arg,
 						misc->this_device);
+	case MSHV_ROOT_HVCALL:
+		return mshv_ioctl_passthru_hvcall(NULL, false,
+					(void __user *)arg);
 	}
 
 	return -ENOTTY;

base-commit: db7df69995ffbe806d60ad46d5fb9d959da9e549
-- 
2.34.1