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/*
* Copyright 2013 Red Hat Inc.
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* Authors: Jérôme Glisse <jglisse@redhat.com>
*/
/*
* Heterogeneous Memory Management (HMM)
*
* See Documentation/vm/hmm.txt for reasons and overview of what HMM is and it
* is for. Here we focus on the HMM API description, with some explanation of
* the underlying implementation.
*
* Short description: HMM provides a set of helpers to share a virtual address
* space between CPU and a device, so that the device can access any valid
* address of the process (while still obeying memory protection). HMM also
* provides helpers to migrate process memory to device memory, and back. Each
* set of functionality (address space mirroring, and migration to and from
* device memory) can be used independently of the other.
*
*
* HMM address space mirroring API:
*
* Use HMM address space mirroring if you want to mirror range of the CPU page
* table of a process into a device page table. Here, "mirror" means "keep
* synchronized". Prerequisites: the device must provide the ability to write-
* protect its page tables (at PAGE_SIZE granularity), and must be able to
* recover from the resulting potential page faults.
*
* HMM guarantees that at any point in time, a given virtual address points to
* either the same memory in both CPU and device page tables (that is: CPU and
* device page tables each point to the same pages), or that one page table (CPU
* or device) points to no entry, while the other still points to the old page
* for the address. The latter case happens when the CPU page table update
* happens first, and then the update is mirrored over to the device page table.
* This does not cause any issue, because the CPU page table cannot start
* pointing to a new page until the device page table is invalidated.
*
* HMM uses mmu_notifiers to monitor the CPU page tables, and forwards any
* updates to each device driver that has registered a mirror. It also provides
* some API calls to help with taking a snapshot of the CPU page table, and to
* synchronize with any updates that might happen concurrently.
*
*
* HMM migration to and from device memory:
*
* HMM provides a set of helpers to hotplug device memory as ZONE_DEVICE, with
* a new MEMORY_DEVICE_PRIVATE type. This provides a struct page for each page
* of the device memory, and allows the device driver to manage its memory
* using those struct pages. Having struct pages for device memory makes
* migration easier. Because that memory is not addressable by the CPU it must
* never be pinned to the device; in other words, any CPU page fault can always
* cause the device memory to be migrated (copied/moved) back to regular memory.
*
* A new migrate helper (migrate_vma()) has been added (see mm/migrate.c) that
* allows use of a device DMA engine to perform the copy operation between
* regular system memory and device memory.
*/
#ifndef LINUX_HMM_H
#define LINUX_HMM_H
#include <linux/kconfig.h>
#if IS_ENABLED(CONFIG_HMM)
/*
* hmm_pfn_t - HMM uses its own pfn type to keep several flags per page
*
* Flags:
* HMM_PFN_VALID: pfn is valid
* HMM_PFN_WRITE: CPU page table has write permission set
*/
typedef unsigned long hmm_pfn_t;
#define HMM_PFN_VALID (1 << 0)
#define HMM_PFN_WRITE (1 << 1)
#define HMM_PFN_SHIFT 2
/*
* hmm_pfn_t_to_page() - return struct page pointed to by a valid hmm_pfn_t
* @pfn: hmm_pfn_t to convert to struct page
* Returns: struct page pointer if pfn is a valid hmm_pfn_t, NULL otherwise
*
* If the hmm_pfn_t is valid (ie valid flag set) then return the struct page
* matching the pfn value stored in the hmm_pfn_t. Otherwise return NULL.
*/
static inline struct page *hmm_pfn_t_to_page(hmm_pfn_t pfn)
{
if (!(pfn & HMM_PFN_VALID))
return NULL;
return pfn_to_page(pfn >> HMM_PFN_SHIFT);
}
/*
* hmm_pfn_t_to_pfn() - return pfn value store in a hmm_pfn_t
* @pfn: hmm_pfn_t to extract pfn from
* Returns: pfn value if hmm_pfn_t is valid, -1UL otherwise
*/
static inline unsigned long hmm_pfn_t_to_pfn(hmm_pfn_t pfn)
{
if (!(pfn & HMM_PFN_VALID))
return -1UL;
return (pfn >> HMM_PFN_SHIFT);
}
/*
* hmm_pfn_t_from_page() - create a valid hmm_pfn_t value from struct page
* @page: struct page pointer for which to create the hmm_pfn_t
* Returns: valid hmm_pfn_t for the page
*/
static inline hmm_pfn_t hmm_pfn_t_from_page(struct page *page)
{
return (page_to_pfn(page) << HMM_PFN_SHIFT) | HMM_PFN_VALID;
}
/*
* hmm_pfn_t_from_pfn() - create a valid hmm_pfn_t value from pfn
* @pfn: pfn value for which to create the hmm_pfn_t
* Returns: valid hmm_pfn_t for the pfn
*/
static inline hmm_pfn_t hmm_pfn_t_from_pfn(unsigned long pfn)
{
return (pfn << HMM_PFN_SHIFT) | HMM_PFN_VALID;
}
/* Below are for HMM internal use only! Not to be used by device driver! */
void hmm_mm_destroy(struct mm_struct *mm);
static inline void hmm_mm_init(struct mm_struct *mm)
{
mm->hmm = NULL;
}
#else /* IS_ENABLED(CONFIG_HMM) */
/* Below are for HMM internal use only! Not to be used by device driver! */
static inline void hmm_mm_destroy(struct mm_struct *mm) {}
static inline void hmm_mm_init(struct mm_struct *mm) {}
#endif /* IS_ENABLED(CONFIG_HMM) */
#endif /* LINUX_HMM_H */
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