Message-ID: <2d5d540c-6ed5-455f-a895-9a3dd12bae2c@linux.microsoft.com>
Date: Fri, 17 Oct 2025 16:55:24 -0700
From: Mukesh R <mrathor@...ux.microsoft.com>
To: Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com>,
 kys@...rosoft.com, haiyangz@...rosoft.com, wei.liu@...nel.org,
 decui@...rosoft.com
Cc: linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 5/5] Drivers: hv: Add support for movable memory
 regions

On 10/15/25 17:27, Stanislav Kinsburskii wrote:
> Introduce support for movable memory regions in the Hyper-V root partition
> driver, thus improving memory management flexibility and preparing the
> driver for advanced use cases such as dynamic memory remapping.
> 
> Integrate mmu_interval_notifier for movable regions, implement functions to
> handle HMM faults and memory invalidation, and update memory region mapping
> logic to support movable regions.
> 
> While MMU notifiers are commonly used in virtualization drivers, this
> implementation leverages HMM (Heterogeneous Memory Management) for its
> tailored functionality. HMM provides a ready-made framework for mirroring,
> invalidation, and fault handling, avoiding the need to reimplement these
> mechanisms for a single callback. Although MMU notifiers are more generic,
> using HMM reduces boilerplate and ensures maintainability by utilizing a
> mechanism specifically designed for such use cases.
> 
> Signed-off-by: Anirudh Rayabharam <anrayabh@...ux.microsoft.com>
> Signed-off-by: Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com>
> ---
>  drivers/hv/Kconfig          |    1 
>  drivers/hv/mshv_root.h      |    8 +
>  drivers/hv/mshv_root_main.c |  328 ++++++++++++++++++++++++++++++++++++++++++-
>  3 files changed, 327 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/hv/Kconfig b/drivers/hv/Kconfig
> index 0b8c391a0342..5f1637cbb6e3 100644
> --- a/drivers/hv/Kconfig
> +++ b/drivers/hv/Kconfig
> @@ -75,6 +75,7 @@ config MSHV_ROOT
>  	depends on PAGE_SIZE_4KB
>  	select EVENTFD
>  	select VIRT_XFER_TO_GUEST_WORK
> +	select HMM_MIRROR
>  	default n
>  	help
>  	  Select this option to enable support for booting and running as root
> diff --git a/drivers/hv/mshv_root.h b/drivers/hv/mshv_root.h
> index 97e64d5341b6..13367c84497c 100644
> --- a/drivers/hv/mshv_root.h
> +++ b/drivers/hv/mshv_root.h
> @@ -15,6 +15,7 @@
>  #include <linux/hashtable.h>
>  #include <linux/dev_printk.h>
>  #include <linux/build_bug.h>
> +#include <linux/mmu_notifier.h>
>  #include <uapi/linux/mshv.h>
>  
>  /*
> @@ -81,9 +82,14 @@ struct mshv_mem_region {
>  	struct {
>  		u64 large_pages:  1; /* 2MiB */
>  		u64 range_pinned: 1;
> -		u64 reserved:	 62;
> +		u64 is_ram	: 1; /* mem region can be ram or mmio */

In case this gets accepted/merged, this is named 
+               u64 memreg_isram: 1; /* mem region can be ram or mmio */

Keeping the name same will avoid unnecessary diffs when we try to compare
files to see what is missing internally or externally.

Thanks,
-Mukesh

>  	} flags;
>  	struct mshv_partition *partition;
> +#if defined(CONFIG_MMU_NOTIFIER)
> +	struct mmu_interval_notifier mni;
> +	struct mutex mutex;	/* protects region pages remapping */
> +#endif
>  	struct page *pages[];
>  };
>  
> diff --git a/drivers/hv/mshv_root_main.c b/drivers/hv/mshv_root_main.c
> index c4f114376435..b2738443ac5d 100644
> --- a/drivers/hv/mshv_root_main.c
> +++ b/drivers/hv/mshv_root_main.c
> @@ -29,6 +29,7 @@
>  #include <linux/crash_dump.h>
>  #include <linux/panic_notifier.h>
>  #include <linux/vmalloc.h>
> +#include <linux/hmm.h>
>  
>  #include "mshv_eventfd.h"
>  #include "mshv.h"
> @@ -36,6 +37,8 @@
>  
>  #define VALUE_PMD_ALIGNED(c)			(!((c) & (PTRS_PER_PMD - 1)))
>  
> +#define MSHV_MAP_FAULT_IN_PAGES			HPAGE_PMD_NR
> +
>  MODULE_AUTHOR("Microsoft");
>  MODULE_LICENSE("GPL");
>  MODULE_DESCRIPTION("Microsoft Hyper-V root partition VMM interface /dev/mshv");
> @@ -76,6 +79,11 @@ static int mshv_vp_mmap(struct file *file, struct vm_area_struct *vma);
>  static vm_fault_t mshv_vp_fault(struct vm_fault *vmf);
>  static int mshv_init_async_handler(struct mshv_partition *partition);
>  static void mshv_async_hvcall_handler(void *data, u64 *status);
> +static struct mshv_mem_region
> +	*mshv_partition_region_by_gfn(struct mshv_partition *pt, u64 gfn);
> +static int mshv_region_remap_pages(struct mshv_mem_region *region,
> +				   u32 map_flags, u64 page_offset,
> +				   u64 page_count);
>  
>  static const union hv_input_vtl input_vtl_zero;
>  static const union hv_input_vtl input_vtl_normal = {
> @@ -602,14 +610,197 @@ static long mshv_run_vp_with_root_scheduler(struct mshv_vp *vp)
>  static_assert(sizeof(struct hv_message) <= MSHV_RUN_VP_BUF_SZ,
>  	      "sizeof(struct hv_message) must not exceed MSHV_RUN_VP_BUF_SZ");
>  
> +#ifdef CONFIG_X86_64
> +
> +#if defined(CONFIG_MMU_NOTIFIER)
> +/**
> + * mshv_region_hmm_fault_and_lock - Handle HMM faults and lock the memory region
> + * @region: Pointer to the memory region structure
> + * @range: Pointer to the HMM range structure
> + *
> + * This function performs the following steps:
> + * 1. Reads the notifier sequence for the HMM range.
> + * 2. Acquires a read lock on the memory map.
> + * 3. Handles HMM faults for the specified range.
> + * 4. Releases the read lock on the memory map.
> + * 5. If successful, locks the memory region mutex.
> + * 6. Verifies if the notifier sequence has changed during the operation.
> + *    If it has, releases the mutex and returns -EBUSY to match with
> + *    hmm_range_fault() return code for repeating.
> + *
> + * Return: 0 on success, a negative error code otherwise.
> + */
> +static int mshv_region_hmm_fault_and_lock(struct mshv_mem_region *region,
> +					  struct hmm_range *range)
> +{
> +	int ret;
> +
> +	range->notifier_seq = mmu_interval_read_begin(range->notifier);
> +	mmap_read_lock(region->mni.mm);
> +	ret = hmm_range_fault(range);
> +	mmap_read_unlock(region->mni.mm);
> +	if (ret)
> +		return ret;
> +
> +	mutex_lock(&region->mutex);
> +
> +	if (mmu_interval_read_retry(range->notifier, range->notifier_seq)) {
> +		mutex_unlock(&region->mutex);
> +		cond_resched();
> +		return -EBUSY;
> +	}
> +
> +	return 0;
> +}
> +
> +/**
> + * mshv_region_range_fault - Handle memory range faults for a given region.
> + * @region: Pointer to the memory region structure.
> + * @page_offset: Offset of the page within the region.
> + * @page_count: Number of pages to handle.
> + *
> + * This function resolves memory faults for a specified range of pages
> + * within a memory region. It uses HMM (Heterogeneous Memory Management)
> + * to fault in the required pages and updates the region's page array.
> + *
> + * Return: 0 on success, negative error code on failure.
> + */
> +static int mshv_region_range_fault(struct mshv_mem_region *region,
> +				   u64 page_offset, u64 page_count)
> +{
> +	struct hmm_range range = {
> +		.notifier = &region->mni,
> +		.default_flags = HMM_PFN_REQ_FAULT | HMM_PFN_REQ_WRITE,
> +	};
> +	unsigned long *pfns;
> +	int ret;
> +	u64 i;
> +
> +	pfns = kmalloc_array(page_count, sizeof(unsigned long), GFP_KERNEL);
> +	if (!pfns)
> +		return -ENOMEM;
> +
> +	range.hmm_pfns = pfns;
> +	range.start = region->start_uaddr + page_offset * HV_HYP_PAGE_SIZE;
> +	range.end = range.start + page_count * HV_HYP_PAGE_SIZE;
> +
> +	do {
> +		ret = mshv_region_hmm_fault_and_lock(region, &range);
> +	} while (ret == -EBUSY);
> +
> +	if (ret)
> +		goto out;
> +
> +	for (i = 0; i < page_count; i++)
> +		region->pages[page_offset + i] = hmm_pfn_to_page(pfns[i]);
> +
> +	if (PageHuge(region->pages[page_offset]))
> +		region->flags.large_pages = true;
> +
> +	ret = mshv_region_remap_pages(region, region->hv_map_flags,
> +				      page_offset, page_count);
> +
> +	mutex_unlock(&region->mutex);
> +out:
> +	kfree(pfns);
> +	return ret;
> +}
> +#else /* CONFIG_MMU_NOTIFIER */
> +static int mshv_region_range_fault(struct mshv_mem_region *region,
> +				   u64 page_offset, u64 page_count)
> +{
> +	return -ENODEV;
> +}
> +#endif /* CONFIG_MMU_NOTIFIER */
> +
> +static bool mshv_region_handle_gfn_fault(struct mshv_mem_region *region, u64 gfn)
> +{
> +	u64 page_offset, page_count;
> +	int ret;
> +
> +	if (WARN_ON_ONCE(region->flags.range_pinned))
> +		return false;
> +
> +	/* Align the page offset to the nearest MSHV_MAP_FAULT_IN_PAGES. */
> +	page_offset = ALIGN_DOWN(gfn - region->start_gfn,
> +				 MSHV_MAP_FAULT_IN_PAGES);
> +
> +	/* Map more pages than requested to reduce the number of faults. */
> +	page_count = min(region->nr_pages - page_offset,
> +			 MSHV_MAP_FAULT_IN_PAGES);
> +
> +	ret = mshv_region_range_fault(region, page_offset, page_count);
> +
> +	WARN_ONCE(ret,
> +		  "p%llu: GPA intercept failed: region %#llx-%#llx, gfn %#llx, page_offset %llu, page_count %llu\n",
> +		  region->partition->pt_id, region->start_uaddr,
> +		  region->start_uaddr + (region->nr_pages << HV_HYP_PAGE_SHIFT),
> +		  gfn, page_offset, page_count);
> +
> +	return !ret;
> +}
> +
> +/**
> + * mshv_handle_gpa_intercept - Handle GPA (Guest Physical Address) intercepts.
> + * @vp: Pointer to the virtual processor structure.
> + *
> + * This function processes GPA intercepts by identifying the memory region
> + * corresponding to the intercepted GPA, aligning the page offset, and
> + * mapping the required pages. It ensures that the region is valid and
> + * handles faults efficiently by mapping multiple pages at once.
> + *
> + * Return: true if the intercept was handled successfully, false otherwise.
> + */
> +static bool mshv_handle_gpa_intercept(struct mshv_vp *vp)
> +{
> +	struct mshv_partition *p = vp->vp_partition;
> +	struct mshv_mem_region *region;
> +	struct hv_x64_memory_intercept_message *msg;
> +	u64 gfn;
> +
> +	msg = (struct hv_x64_memory_intercept_message *)
> +		vp->vp_intercept_msg_page->u.payload;
> +
> +	gfn = HVPFN_DOWN(msg->guest_physical_address);
> +
> +	region = mshv_partition_region_by_gfn(p, gfn);
> +	if (!region)
> +		return false;
> +
> +	if (WARN_ON_ONCE(!region->flags.is_ram))
> +		return false;
> +
> +	if (WARN_ON_ONCE(region->flags.range_pinned))
> +		return false;
> +
> +	return mshv_region_handle_gfn_fault(region, gfn);
> +}
> +
> +#else	/* CONFIG_X86_64 */
> +
> +static bool mshv_handle_gpa_intercept(struct mshv_vp *vp) { return false; }
> +
> +#endif	/* CONFIG_X86_64 */
> +
> +static bool mshv_vp_handle_intercept(struct mshv_vp *vp)
> +{
> +	switch (vp->vp_intercept_msg_page->header.message_type) {
> +	case HVMSG_GPA_INTERCEPT:
> +		return mshv_handle_gpa_intercept(vp);
> +	}
> +	return false;
> +}
> +
>  static long mshv_vp_ioctl_run_vp(struct mshv_vp *vp, void __user *ret_msg)
>  {
>  	long rc;
>  
> -	if (hv_scheduler_type == HV_SCHEDULER_TYPE_ROOT)
> -		rc = mshv_run_vp_with_root_scheduler(vp);
> -	else
> -		rc = mshv_run_vp_with_hyp_scheduler(vp);
> +	do {
> +		if (hv_scheduler_type == HV_SCHEDULER_TYPE_ROOT)
> +			rc = mshv_run_vp_with_root_scheduler(vp);
> +		else
> +			rc = mshv_run_vp_with_hyp_scheduler(vp);
> +	} while (rc == 0 && mshv_vp_handle_intercept(vp));
>  
>  	if (rc)
>  		return rc;
> @@ -1209,6 +1400,110 @@ mshv_partition_region_by_uaddr(struct mshv_partition *partition, u64 uaddr)
>  	return NULL;
>  }
>  
> +#if defined(CONFIG_MMU_NOTIFIER)
> +static void mshv_region_movable_fini(struct mshv_mem_region *region)
> +{
> +	if (region->flags.range_pinned)
> +		return;
> +
> +	mmu_interval_notifier_remove(&region->mni);
> +}
> +
> +/**
> + * mshv_region_interval_invalidate - Invalidate a range of memory region
> + * @mni: Pointer to the mmu_interval_notifier structure
> + * @range: Pointer to the mmu_notifier_range structure
> + * @cur_seq: Current sequence number for the interval notifier
> + *
> + * This function invalidates a memory region by remapping its pages with
> + * no access permissions. It locks the region's mutex to ensure thread safety
> + * and updates the sequence number for the interval notifier. If the range
> + * is blockable, it uses a blocking lock; otherwise, it attempts a non-blocking
> + * lock and returns false if unsuccessful.
> + *
> + * NOTE: Failure to invalidate a region is a serious error, as the pages will
> + * be considered freed while they are still mapped by the hypervisor.
> + * Any attempt to access such pages will likely crash the system.
> + *
> + * Return: true if the region was successfully invalidated, false otherwise.
> + */
> +static bool
> +mshv_region_interval_invalidate(struct mmu_interval_notifier *mni,
> +				const struct mmu_notifier_range *range,
> +				unsigned long cur_seq)
> +{
> +	struct mshv_mem_region *region = container_of(mni,
> +						struct mshv_mem_region,
> +						mni);
> +	u64 page_offset, page_count;
> +	unsigned long mstart, mend;
> +	int ret = -EPERM;
> +
> +	if (mmu_notifier_range_blockable(range))
> +		mutex_lock(&region->mutex);
> +	else if (!mutex_trylock(&region->mutex))
> +		goto out_fail;
> +
> +	mmu_interval_set_seq(mni, cur_seq);
> +
> +	mstart = max(range->start, region->start_uaddr);
> +	mend = min(range->end, region->start_uaddr +
> +		   (region->nr_pages << HV_HYP_PAGE_SHIFT));
> +
> +	page_offset = HVPFN_DOWN(mstart - region->start_uaddr);
> +	page_count = HVPFN_DOWN(mend - mstart);
> +
> +	ret = mshv_region_remap_pages(region, HV_MAP_GPA_NO_ACCESS,
> +				      page_offset, page_count);
> +	if (ret)
> +		goto out_fail;
> +
> +	mshv_region_invalidate_pages(region, page_offset, page_count);
> +
> +	mutex_unlock(&region->mutex);
> +
> +	return true;
> +
> +out_fail:
> +	WARN_ONCE(ret,
> +		  "Failed to invalidate region %#llx-%#llx (range %#lx-%#lx, event: %u, pages %#llx-%#llx, mm: %#llx): %d\n",
> +		  region->start_uaddr,
> +		  region->start_uaddr + (region->nr_pages << HV_HYP_PAGE_SHIFT),
> +		  range->start, range->end, range->event,
> +		  page_offset, page_offset + page_count - 1, (u64)range->mm, ret);
> +	return false;
> +}
> +
> +static const struct mmu_interval_notifier_ops mshv_region_mni_ops = {
> +	.invalidate = mshv_region_interval_invalidate,
> +};
> +
> +static bool mshv_region_movable_init(struct mshv_mem_region *region)
> +{
> +	int ret;
> +
> +	ret = mmu_interval_notifier_insert(&region->mni, current->mm,
> +					   region->start_uaddr,
> +					   region->nr_pages << HV_HYP_PAGE_SHIFT,
> +					   &mshv_region_mni_ops);
> +	if (ret)
> +		return false;
> +
> +	mutex_init(&region->mutex);
> +
> +	return true;
> +}
> +#else
> +static inline void mshv_region_movable_fini(struct mshv_mem_region *region)
> +{
> +}
> +
> +static inline bool mshv_region_movable_init(struct mshv_mem_region *region)
> +{
> +	return false;
> +}
> +#endif
> +
>  /*
>   * NB: caller checks and makes sure mem->size is page aligned
>   * Returns: 0 with regionpp updated on success, or -errno
> @@ -1241,9 +1536,14 @@ static int mshv_partition_create_region(struct mshv_partition *partition,
>  	if (mem->flags & BIT(MSHV_SET_MEM_BIT_EXECUTABLE))
>  		region->hv_map_flags |= HV_MAP_GPA_EXECUTABLE;
>  
> -	/* Note: large_pages flag populated when we pin the pages */
> -	if (!is_mmio)
> -		region->flags.range_pinned = true;
> +	/* Note: large_pages flag populated when pages are allocated. */
> +	if (!is_mmio) {
> +		region->flags.is_ram = true;
> +
> +		if (mshv_partition_encrypted(partition) ||
> +		    !mshv_region_movable_init(region))
> +			region->flags.range_pinned = true;
> +	}
>  
>  	region->partition = partition;
>  
> @@ -1363,9 +1663,16 @@ mshv_map_user_memory(struct mshv_partition *partition,
>  	if (is_mmio)
>  		ret = hv_call_map_mmio_pages(partition->pt_id, mem.guest_pfn,
>  					     mmio_pfn, HVPFN_DOWN(mem.size));
> -	else
> +	else if (region->flags.range_pinned)
>  		ret = mshv_prepare_pinned_region(region);
> -
> +	else
> +		/*
> +		 * For non-pinned regions, remap with no access to let the
> +		 * hypervisor track dirty pages, enabling pre-copy live
> +		 * migration.
> +		 */
> +		ret = mshv_region_remap_pages(region, HV_MAP_GPA_NO_ACCESS,
> +					      0, region->nr_pages);
>  	if (ret)
>  		goto errout;
>  
> @@ -1388,6 +1695,9 @@ static void mshv_partition_destroy_region(struct mshv_mem_region *region)
>  
>  	hlist_del(&region->hnode);
>  
> +	if (region->flags.is_ram)
> +		mshv_region_movable_fini(region);
> +
>  	if (mshv_partition_encrypted(partition)) {
>  		ret = mshv_partition_region_share(region);
>  		if (ret) {
> 
> 

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