[PATCH 4.17 060/220] cxl: Disable prefault_mode in Radix mode

From: Greg Kroah-Hartman
Date: Sun Jul 01 2018 - 13:17:50 EST

4.17-stable review patch. If anyone has any objections, please let me know.


From: Vaibhav Jain <vaibhav@xxxxxxxxxxxxx>

commit b6c84ba22ff3a198eb8d5552cf9b8fda1d792e54 upstream.

Currently we see a kernel-oops reported on Power-9 while attaching a
context to an AFU, with radix-mode and sysfs attr 'prefault_mode' set
to anything other than 'none'. The backtrace of the oops is of this

Unable to handle kernel paging request for data at address 0x00000080
Faulting instruction address: 0xc00800000bcf3b20
cpu 0x1: Vector: 300 (Data Access) at [c00000037f003800]
pc: c00800000bcf3b20: cxl_load_segment+0x178/0x290 [cxl]
lr: c00800000bcf39f0: cxl_load_segment+0x48/0x290 [cxl]
sp: c00000037f003a80
msr: 9000000000009033
dar: 80
dsisr: 40000000
current = 0xc00000037f280000
paca = 0xc0000003ffffe600 softe: 3 irq_happened: 0x01
pid = 3529, comm = afp_no_int
cxl_prefault+0xfc/0x248 [cxl]
process_element_entry_psl9+0xd8/0x1a0 [cxl]
cxl_attach_dedicated_process_psl9+0x44/0x130 [cxl]
native_attach_process+0xc0/0x130 [cxl]
afu_ioctl+0x3f4/0x5e0 [cxl]

The issue is caused as on Power-8 the AFU attr 'prefault_mode' was
used to improve initial storage fault performance by prefaulting
process segments. However on Power-9 with radix mode we don't have
Storage-Segments that we can prefault. Also prefaulting process Pages
will be too costly and fine-grained.

Hence, since the prefaulting mechanism doesn't makes sense of
radix-mode, this patch updates prefault_mode_store() to not allow any
other value apart from CXL_PREFAULT_NONE when radix mode is enabled.

Fixes: f24be42aab37 ("cxl: Add psl9 specific code")
Cc: stable@xxxxxxxxxxxxxxx # v4.12+
Signed-off-by: Vaibhav Jain <vaibhav@xxxxxxxxxxxxx>
Acked-by: Frederic Barrat <fbarrat@xxxxxxxxxxxxxxxxxx>
Acked-by: Andrew Donnellan <andrew.donnellan@xxxxxxxxxxx>
Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

Documentation/ABI/testing/sysfs-class-cxl | 4 +++-
drivers/misc/cxl/sysfs.c | 16 ++++++++++++----
2 files changed, 15 insertions(+), 5 deletions(-)

--- a/Documentation/ABI/testing/sysfs-class-cxl
+++ b/Documentation/ABI/testing/sysfs-class-cxl
@@ -69,7 +69,9 @@ Date: September 2014
Contact: linuxppc-dev@xxxxxxxxxxxxxxxx
Description: read/write
Set the mode for prefaulting in segments into the segment table
- when performing the START_WORK ioctl. Possible values:
+ when performing the START_WORK ioctl. Only applicable when
+ running under hashed page table mmu.
+ Possible values:
none: No prefaulting (default)
work_element_descriptor: Treat the work element
descriptor as an effective address and
--- a/drivers/misc/cxl/sysfs.c
+++ b/drivers/misc/cxl/sysfs.c
@@ -353,12 +353,20 @@ static ssize_t prefault_mode_store(struc
struct cxl_afu *afu = to_cxl_afu(device);
enum prefault_modes mode = -1;

- if (!strncmp(buf, "work_element_descriptor", 23))
- if (!strncmp(buf, "all", 3))
if (!strncmp(buf, "none", 4))
+ else {
+ if (!radix_enabled()) {
+ /* only allowed when not in radix mode */
+ if (!strncmp(buf, "work_element_descriptor", 23))
+ if (!strncmp(buf, "all", 3))
+ } else {
+ dev_err(device, "Cannot prefault with radix enabled\n");
+ }
+ }

if (mode == -1)
return -EINVAL;