Re: [PATCH 2/2] mshv: Add kexec blocking support

[Stanislav Kinsburskii]

On Thu, Feb 12, 2026 at 02:11:13PM -0800, Mukesh R wrote:
> On 1/28/26 09:42, Stanislav Kinsburskii wrote:
> > Add kexec notifier to prevent kexec when VMs are active or memory
> > is deposited. The notifier blocks kexec operations if:
> > - Active VMs exist in the partition table
> > - Pages are still deposited to the hypervisor
> > 
> > The kernel cannot access hypervisor deposited pages: any access
> > triggers a GPF. Until the deposited page state can be handed over
> > to the next kernel, kexec must be blocked if there is any shared
> > state between kernel and hypervisor.
> > 
> > For L1 host virtualization, attempt to withdraw all deposited memory before
> > allowing kexec to proceed. If withdrawal fails or pages remain deposited
> > block the kexec operation.
> > 
> > Signed-off-by: Stanislav Kinsburskii <skinsburskii@xxxxxxxxxxxxxxxxxxx>
> > ---
> >   drivers/hv/Makefile            |    1 +
> >   drivers/hv/hv_proc.c           |    4 ++
> >   drivers/hv/mshv_kexec.c        |   66 ++++++++++++++++++++++++++++++++++++++++
> >   drivers/hv/mshv_root.h         |   14 ++++++++
> >   drivers/hv/mshv_root_hv_call.c |    2 +
> >   drivers/hv/mshv_root_main.c    |    7 ++++
> >   6 files changed, 94 insertions(+)
> >   create mode 100644 drivers/hv/mshv_kexec.c
> > 
> > diff --git a/drivers/hv/Makefile b/drivers/hv/Makefile
> > index a49f93c2d245..bb72be5cc525 100644
> > --- a/drivers/hv/Makefile
> > +++ b/drivers/hv/Makefile
> > @@ -15,6 +15,7 @@ hv_vmbus-$(CONFIG_HYPERV_TESTING)	+= hv_debugfs.o
> >   hv_utils-y := hv_util.o hv_kvp.o hv_snapshot.o hv_utils_transport.o
> >   mshv_root-y := mshv_root_main.o mshv_synic.o mshv_eventfd.o mshv_irq.o \
> >   	       mshv_root_hv_call.o mshv_portid_table.o mshv_regions.o
> > +mshv_root-$(CONFIG_KEXEC) += mshv_kexec.o
> >   mshv_vtl-y := mshv_vtl_main.o
> >   # Code that must be built-in
> > diff --git a/drivers/hv/hv_proc.c b/drivers/hv/hv_proc.c
> > index 89870c1b0087..39bbbedb0340 100644
> > --- a/drivers/hv/hv_proc.c
> > +++ b/drivers/hv/hv_proc.c
> > @@ -15,6 +15,8 @@
> >    */
> >   #define HV_DEPOSIT_MAX (HV_HYP_PAGE_SIZE / sizeof(u64) - 1)
> > +atomic_t hv_pages_deposited;
> > +
> >   /* Deposits exact number of pages. Must be called with interrupts enabled.  */
> >   int hv_call_deposit_pages(int node, u64 partition_id, u32 num_pages)
> >   {
> > @@ -93,6 +95,8 @@ int hv_call_deposit_pages(int node, u64 partition_id, u32 num_pages)
> >   		goto err_free_allocations;
> >   	}
> > +	atomic_add(page_count, &hv_pages_deposited);
> > +
> >   	ret = 0;
> >   	goto free_buf;
> > diff --git a/drivers/hv/mshv_kexec.c b/drivers/hv/mshv_kexec.c
> > new file mode 100644
> > index 000000000000..5222b2e4ff97
> > --- /dev/null
> > +++ b/drivers/hv/mshv_kexec.c
> > @@ -0,0 +1,66 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +/*
> > + * Copyright (c) 2026, Microsoft Corporation.
> > + *
> > + * Live update orchestration management for mshv_root module.
> > + *
> > + * Author: Stanislav Kinsburskii <skinsburskii@xxxxxxxxxxxxxxxxxxx>
> > + */
> > +
> > +#include <linux/kexec.h>
> > +#include <linux/notifier.h>
> > +#include <asm/mshyperv.h>
> > +#include "mshv_root.h"
> > +
> > +static BLOCKING_NOTIFIER_HEAD(overlay_notify_chain);
> > +
> > +static int mshv_block_kexec_notify(struct notifier_block *nb,
> > +				   unsigned long action, void *arg)
> > +{
> > +	if (!hash_empty(mshv_root.pt_htable)) {
> > +		pr_warn("mshv: Cannot perform kexec while VMs are active\n");
> > +		return -EBUSY;
> > +	}
> > +
> > +	if (hv_l1vh_partition()) {
> > +		int err;
> > +
> > +		/* Attempt to withdraw all the deposited pages */
> > +		err = hv_call_withdraw_memory(U64_MAX, NUMA_NO_NODE,
> > +					      hv_current_partition_id);
> > +		if (err) {
> > +			pr_err("mshv: Failed to withdraw memory from L1 virtualization: %d\n",
> > +			       err);
> > +			return err;
> > +		}
> > +	}
> > +
> > +	if (atomic_read(&hv_pages_deposited)) {
> > +		pr_warn("mshv: Cannot perform kexec while pages are deposited\n");
> > +		return -EBUSY;
> > +	}
> > +	return 0;
> > +}
> > +
> 
> What guarantees another deposit won't happen after this. Are all cpus
> "locked" in kexec path and not doing anything at this point?
> 

Yeah, this should be guarded.

Thanks,
Stanislav

> Thanks,
> -Mukesh
> 
> 
> 
> > +static struct notifier_block mshv_kexec_notifier = {
> > +	.notifier_call = mshv_block_kexec_notify,
> > +};
> > +
> > +int __init mshv_kexec_init(void)
> > +{
> > +	int err;
> > +
> > +	err = kexec_block_notifier_register(&mshv_kexec_notifier);
> > +	if (err) {
> > +		pr_err("mshv: Could not register kexec notifier: %pe\n",
> > +		       ERR_PTR(err));
> > +		return err;
> > +	}
> > +
> > +	return 0;
> > +}
> > +
> > +void __exit mshv_kexec_exit(void)
> > +{
> > +	(void)kexec_block_notifier_unregister(&mshv_kexec_notifier);
> > +}
> > diff --git a/drivers/hv/mshv_root.h b/drivers/hv/mshv_root.h
> > index 3c1d88b36741..311f76262d10 100644
> > --- a/drivers/hv/mshv_root.h
> > +++ b/drivers/hv/mshv_root.h
> > @@ -17,6 +17,7 @@
> >   #include <linux/build_bug.h>
> >   #include <linux/mmu_notifier.h>
> >   #include <uapi/linux/mshv.h>
> > +#include <hyperv/hvhdk.h>
> >   /*
> >    * Hypervisor must be between these version numbers (inclusive)
> > @@ -319,6 +320,7 @@ int hv_call_get_partition_property_ex(u64 partition_id, u64 property_code, u64 a
> >   extern struct mshv_root mshv_root;
> >   extern enum hv_scheduler_type hv_scheduler_type;
> >   extern u8 * __percpu *hv_synic_eventring_tail;
> > +extern atomic_t hv_pages_deposited;
> >   struct mshv_mem_region *mshv_region_create(u64 guest_pfn, u64 nr_pages,
> >   					   u64 uaddr, u32 flags);
> > @@ -333,4 +335,16 @@ bool mshv_region_handle_gfn_fault(struct mshv_mem_region *region, u64 gfn);
> >   void mshv_region_movable_fini(struct mshv_mem_region *region);
> >   bool mshv_region_movable_init(struct mshv_mem_region *region);
> > +#if IS_ENABLED(CONFIG_KEXEC)
> > +int mshv_kexec_init(void);
> > +void mshv_kexec_exit(void);
> > +#else
> > +static inline int mshv_kexec_init(void)
> > +{
> > +	return 0;
> > +}
> > +
> > +static inline void mshv_kexec_exit(void) { }
> > +#endif
> > +
> >   #endif /* _MSHV_ROOT_H_ */
> > diff --git a/drivers/hv/mshv_root_hv_call.c b/drivers/hv/mshv_root_hv_call.c
> > index 06f2bac8039d..4203af5190ee 100644
> > --- a/drivers/hv/mshv_root_hv_call.c
> > +++ b/drivers/hv/mshv_root_hv_call.c
> > @@ -73,6 +73,8 @@ int hv_call_withdraw_memory(u64 count, int node, u64 partition_id)
> >   		for (i = 0; i < completed; i++)
> >   			__free_page(pfn_to_page(output_page->gpa_page_list[i]));
> > +		atomic_sub(completed, &hv_pages_deposited);
> > +
> >   		if (!hv_result_success(status)) {
> >   			if (hv_result(status) == HV_STATUS_NO_RESOURCES)
> >   				status = HV_STATUS_SUCCESS;
> > diff --git a/drivers/hv/mshv_root_main.c b/drivers/hv/mshv_root_main.c
> > index 5fc572e31cd7..d55aa69d130c 100644
> > --- a/drivers/hv/mshv_root_main.c
> > +++ b/drivers/hv/mshv_root_main.c
> > @@ -2330,6 +2330,10 @@ static int __init mshv_parent_partition_init(void)
> >   	if (ret)
> >   		goto deinit_root_scheduler;
> > +	ret = mshv_kexec_init();
> > +	if (ret)
> > +		goto deinit_irqfd_wq;
> > +
> >   	spin_lock_init(&mshv_root.pt_ht_lock);
> >   	hash_init(mshv_root.pt_htable);
> > @@ -2337,6 +2341,8 @@ static int __init mshv_parent_partition_init(void)
> >   	return 0;
> > +deinit_irqfd_wq:
> > +	mshv_irqfd_wq_cleanup();
> >   deinit_root_scheduler:
> >   	root_scheduler_deinit();
> >   exit_partition:
> > @@ -2356,6 +2362,7 @@ static void __exit mshv_parent_partition_exit(void)
> >   	hv_setup_mshv_handler(NULL);
> >   	mshv_port_table_fini();
> >   	misc_deregister(&mshv_dev);
> > +	mshv_kexec_exit();
> >   	mshv_irqfd_wq_cleanup();
> >   	root_scheduler_deinit();
> >   	if (hv_root_partition())
> > 
> > 
>