[PATCH] mm/Kconfig: additional help text for HMM_MIRROR option

[john.hubbard@gmail.com]

From: John Hubbard <jhubbard@nvidia.com>

The HMM_MIRROR option in Kconfig is a little underdocumented and
mysterious, and leaves people wondering whether to enable it.

Add text explaining just a little bit more about HMM, and also
mention which hardware would benefit from having HMM_MIRROR
enabled.

Suggested-by: Pavel Machek <pavel@ucw.cz>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---

Hi Pavel and all, does this help? I've tried to capture the key missing bits
of documentation, but still keep it small, for Kconfig.

thanks,
John Hubbard
NVIDIA

 mm/Kconfig | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/mm/Kconfig b/mm/Kconfig
index 56cec636a1fc..2fcb92e7f696 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -681,11 +681,18 @@ config HMM_MIRROR
 	depends on MMU && 64BIT
 	select MMU_NOTIFIER
 	help
-	  Select HMM_MIRROR 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.
+	  This is Heterogeneous Memory Management (HMM) process address space
+	  mirroring.
+
+	  HMM_MIRROR provides a way to mirror ranges of the CPU page tables
+	  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.
+
+	  Select HMM_MIRROR if you have hardware that meets the above
+	  description. An early, partial list of such hardware is:
+	  an NVIDIA GPU >= Pascal, Mellanox IB >= mlx5, or an AMD GPU.
 
 config DEVICE_PRIVATE
 	bool "Unaddressable device memory (GPU memory, ...)"
-- 
2.22.0