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@arm.com --- tools/testing/selftests/arm64/fp/sve-test.S | 22 ++++++++++++++++----- 1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/tools/testing/selftests/arm64/fp/sve-test.S b/tools/testing/selftests/arm64/fp/sve-test.S index 9210691aa998..e3e08d9c7020 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