[PATCH v3 3/6] x86/hyperv: Skip LP/VP creation on kexec

Jork Loeser
2026-04-08T01:37:06Z

After a kexec the logical processors and virtual processors already
exist in the hypervisor because they were created by the previous
kernel. Attempting to add them again causes either a BUG_ON or
corrupted VP state leading to MCEs in the new kernel.

Add hv_lp_exists() to probe whether an LP is already present by
calling HVCALL_GET_LOGICAL_PROCESSOR_RUN_TIME. When it succeeds the
LP exists and we skip the add-LP and create-VP loops entirely.

Also add hv_call_notify_all_processors_started() which informs the
hypervisor that all processors are online. This is required after
adding LPs (fresh boot) and is a no-op on kexec since we skip that
path.

Co-developed-by: Anirudh Rayabharam <anrayabh@linux.microsoft.com>
Signed-off-by: Anirudh Rayabharam <anrayabh@linux.microsoft.com>
Co-developed-by: Stanislav Kinsburskii <stanislav.kinsburskii@gmail.com>
Signed-off-by: Stanislav Kinsburskii <stanislav.kinsburskii@gmail.com>
Co-developed-by: Mukesh Rathor <mrathor@linux.microsoft.com>
Signed-off-by: Mukesh Rathor <mrathor@linux.microsoft.com>
Signed-off-by: Jork Loeser <jloeser@linux.microsoft.com>
---
arch/x86/kernel/cpu/mshyperv.c | 7 +++++
drivers/hv/hv_proc.c | 47 ++++++++++++++++++++++++++++++++++
include/asm-generic/mshyperv.h | 10 ++++++++
include/hyperv/hvgdk_mini.h | 1 +
include/hyperv/hvhdk_mini.h | 12 +++++++++
5 files changed, 77 insertions(+)

diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index e498b6b2ef19..b5b6a58b67b0 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -431,6 +431,10 @@ static void __init hv_smp_prepare_cpus(unsigned int max_cpus)
}

#ifdef CONFIG_X86_64
+ /* If AP LPs exist, we are in a kexec'd kernel and VPs already exist */
+ if (num_present_cpus() == 1 || hv_lp_exists(1))
+ return;
+
for_each_present_cpu(i) {
if (i == 0)
continue;
@@ -438,6 +442,9 @@ static void __init hv_smp_prepare_cpus(unsigned int max_cpus)
BUG_ON(ret);
}

+ ret = hv_call_notify_all_processors_started();
+ WARN_ON(ret);
+
for_each_present_cpu(i) {
if (i == 0)
continue;
diff --git a/drivers/hv/hv_proc.c b/drivers/hv/hv_proc.c
index 3cb4b2a3035c..57b2c64197cb 100644
--- a/drivers/hv/hv_proc.c
+++ b/drivers/hv/hv_proc.c
@@ -239,3 +239,50 @@ int hv_call_create_vp(int node, u64 partition_id, u32 vp_index, u32 flags)
return ret;
}
EXPORT_SYMBOL_GPL(hv_call_create_vp);
+
+int hv_call_notify_all_processors_started(void)
+{
+ struct hv_input_notify_partition_event *input;
+ u64 status;
+ unsigned long irq_flags;
+ int ret = 0;
+
+ local_irq_save(irq_flags);
+ input = *this_cpu_ptr(hyperv_pcpu_input_arg);
+ memset(input, 0, sizeof(*input));
+ input->event = HV_PARTITION_ALL_LOGICAL_PROCESSORS_STARTED;
+ status = hv_do_hypercall(HVCALL_NOTIFY_PARTITION_EVENT,
+ input, NULL);
+ local_irq_restore(irq_flags);
+
+ if (!hv_result_success(status)) {
+ hv_status_err(status, "\n");
+ ret = hv_result_to_errno(status);
+ }
+ return ret;
+}
+
+bool hv_lp_exists(u32 lp_index)
+{
+ struct hv_input_get_logical_processor_run_time *input;
+ struct hv_output_get_logical_processor_run_time *output;
+ unsigned long flags;
+ u64 status;
+
+ local_irq_save(flags);
+ input = *this_cpu_ptr(hyperv_pcpu_input_arg);
+ output = *this_cpu_ptr(hyperv_pcpu_output_arg);
+
+ input->lp_index = lp_index;
+ status = hv_do_hypercall(HVCALL_GET_LOGICAL_PROCESSOR_RUN_TIME,
+ input, output);
+ local_irq_restore(flags);
+
+ if (!hv_result_success(status) &&
+ hv_result(status) != HV_STATUS_INVALID_LP_INDEX) {
+ hv_status_err(status, "\n");
+ BUG();
+ }
+
+ return hv_result_success(status);
+}
diff --git a/include/asm-generic/mshyperv.h b/include/asm-generic/mshyperv.h
index d37b68238c97..bf601d67cecb 100644
--- a/include/asm-generic/mshyperv.h
+++ b/include/asm-generic/mshyperv.h
@@ -347,6 +347,8 @@ bool hv_result_needs_memory(u64 status);
int hv_deposit_memory_node(int node, u64 partition_id, u64 status);
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_notify_all_processors_started(void);
+bool hv_lp_exists(u32 lp_index);
int hv_call_create_vp(int node, u64 partition_id, u32 vp_index, u32 flags);

#else /* CONFIG_MSHV_ROOT */
@@ -366,6 +368,14 @@ static inline int hv_call_add_logical_proc(int node, u32 lp_index, u32 acpi_id)
{
return -EOPNOTSUPP;
}
+static inline int hv_call_notify_all_processors_started(void)
+{
+ return -EOPNOTSUPP;
+}
+static inline bool hv_lp_exists(u32 lp_index)
+{
+ return false;
+}
static inline int hv_call_create_vp(int node, u64 partition_id, u32 vp_index, u32 flags)
{
return -EOPNOTSUPP;
diff --git a/include/hyperv/hvgdk_mini.h b/include/hyperv/hvgdk_mini.h
index f9600f87186a..6a4e8b9d570f 100644
--- a/include/hyperv/hvgdk_mini.h
+++ b/include/hyperv/hvgdk_mini.h
@@ -435,6 +435,7 @@ union hv_vp_assist_msr_contents { /* HV_REGISTER_VP_ASSIST_PAGE */
/* HV_CALL_CODE */
#define HVCALL_FLUSH_VIRTUAL_ADDRESS_SPACE 0x0002
#define HVCALL_FLUSH_VIRTUAL_ADDRESS_LIST 0x0003
+#define HVCALL_GET_LOGICAL_PROCESSOR_RUN_TIME 0x0004
#define HVCALL_NOTIFY_LONG_SPIN_WAIT 0x0008
#define HVCALL_SEND_IPI 0x000b
#define HVCALL_ENABLE_VP_VTL 0x000f
diff --git a/include/hyperv/hvhdk_mini.h b/include/hyperv/hvhdk_mini.h
index 091c03e26046..b4cb2fa26e9b 100644
--- a/include/hyperv/hvhdk_mini.h
+++ b/include/hyperv/hvhdk_mini.h
@@ -362,6 +362,7 @@ union hv_partition_event_input {

enum hv_partition_event {
HV_PARTITION_EVENT_ROOT_CRASHDUMP = 2,
+ HV_PARTITION_ALL_LOGICAL_PROCESSORS_STARTED = 4,
};

struct hv_input_notify_partition_event {
@@ -369,6 +370,17 @@ struct hv_input_notify_partition_event {
union hv_partition_event_input input;
} __packed;

+struct hv_input_get_logical_processor_run_time {
+ u32 lp_index;
+} __packed;
+
+struct hv_output_get_logical_processor_run_time {
+ u64 global_time;
+ u64 local_run_time;
+ u64 rsvdz0;
+ u64 hypervisor_time;
+} __packed;
+
struct hv_lp_startup_status {
u64 hv_status;
u64 substatus1;
--
2.43.0