This is a note to let you know that I've just added the patch titled
kselftest/arm64: sve: Do not use non-canonical FFR register value
to the 5.10-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
kselftest-arm64-sve-do-not-use-non-canonical-ffr-reg.patch
and it can be found in the queue-5.10 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 281911083997a077e0b3dbf2906eeef644f02a95
Author: Andre Przywara <andre.przywara@xxxxxxx>
Date: Fri Mar 19 12:01:28 2021 +0000
kselftest/arm64: sve: Do not use non-canonical FFR register value
[ Upstream commit 7011d72588d16a9e5f5d85acbc8b10019809599c ]
The "First Fault Register" (FFR) is an SVE register that mimics a
predicate register, but clears bits when a load or store fails to handle
an element of a vector. The supposed usage scenario is to initialise
this register (using SETFFR), then *read* it later on to learn about
elements that failed to load or store. Explicit writes to this register
using the WRFFR instruction are only supposed to *restore* values
previously read from the register (for context-switching only).
As the manual describes, this register holds only certain values, it:
"... contains a monotonic predicate value, in which starting from bit 0
there are zero or more 1 bits, followed only by 0 bits in any remaining
bit positions."
Any other value is UNPREDICTABLE and is not supposed to be "restored"
into the register.
The SVE test currently tries to write a signature pattern into the
register, which is *not* a canonical FFR value. Apparently the existing
setups treat UNPREDICTABLE as "read-as-written", but a new
implementation actually only stores canonical values. As a consequence,
the sve-test fails immediately when comparing the FFR value:
-----------
# ./sve-test
Vector length: 128 bits
PID: 207
Mismatch: PID=207, iteration=0, reg=48
Expected [cf00]
Got [0f00]
Aborted
-----------
Fix this by only populating the FFR with proper canonical values.
Effectively the requirement described above limits us to 17 unique
values over 16 bits worth of FFR, so we condense our signature down to 4
bits (2 bits from the PID, 2 bits from the generation) and generate the
canonical pattern from it. Any bits describing elements above the
minimum 128 bit are set to 0.
This aligns the FFR usage to the architecture and fixes the test on
microarchitectures implementing FFR in a more restricted way.
Signed-off-by: Andre Przywara <andre.przywara@xxxxxxx>
Reviwed-by: Mark Brown <broonie@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20210319120128.29452-1-andre.przywara@xxxxxxx
Signed-off-by: Will Deacon <will@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/tools/testing/selftests/arm64/fp/sve-test.S b/tools/testing/selftests/arm64/fp/sve-test.S
index f95074c9b48b..07f14e279a90 100644
--- a/tools/testing/selftests/arm64/fp/sve-test.S
+++ b/tools/testing/selftests/arm64/fp/sve-test.S
@@ -284,16 +284,28 @@ endfunction
// Set up test pattern in the FFR
// x0: pid
// x2: generation
+//
+// We need to generate a canonical FFR value, which consists of a number of
+// low "1" bits, followed by a number of zeros. This gives us 17 unique values
+// per 16 bits of FFR, so we create a 4 bit signature out of the PID and
+// generation, and use that as the initial number of ones in the pattern.
+// We fill the upper lanes of FFR with zeros.
// Beware: corrupts P0.
function setup_ffr
mov x4, x30
- bl pattern
+ and w0, w0, #0x3
+ bfi w0, w2, #2, #2
+ mov w1, #1
+ lsl w1, w1, w0
+ sub w1, w1, #1
+
ldr x0, =ffrref
- ldr x1, =scratch
- rdvl x2, #1
- lsr x2, x2, #3
- bl memcpy
+ strh w1, [x0], 2
+ rdvl x1, #1
+ lsr x1, x1, #3
+ sub x1, x1, #2
+ bl memclr
mov x0, #0
ldr x1, =ffrref
