Currently our SVE syscall ABI documentation does not reflect the actual implemented ABI, it says that register state not shared with FPSIMD becomes undefined on syscall when in reality we always clear it. Since changing this would cause a change in the observed kernel behaviour there is a substantial desire to avoid taking advantage of the documented ABI so instead let's document what we actually do so it's clear that it is in reality an ABI.
There has been some pushback on tightening the documentation in the past but it is hard to see who that helps, it makes the implementation decisions less clear and makes it harder for people to discover and make use of the actual ABI. The main practical concern is that qemu's user mode does not currently flush the registers.
Mark Brown (3): kselftest/arm64: Correct buffer allocation for SVE Z registers arm64/sve: Document our actual ABI for clearing registers on syscall kselftest/arm64: Enforce actual ABI for SVE syscalls
Documentation/arm64/sve.rst | 2 +- .../testing/selftests/arm64/abi/syscall-abi.c | 61 ++++++++++++------- 2 files changed, 41 insertions(+), 22 deletions(-)
base-commit: a111daf0c53ae91e71fd2bfe7497862d14132e3e
The buffer used for verifying SVE Z registers allocated enough space for 16 maximally sized registers rather than 32 due to using the macro for the number of P registers. In practice this didn't matter since for historical reasons the maximum VQ defined in the ABI is greater the architectural maximum so we will always allocate more space than is needed even with emulated platforms implementing the architectural maximum. Still, we should use the right define.
Signed-off-by: Mark Brown broonie@kernel.org --- tools/testing/selftests/arm64/abi/syscall-abi.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/tools/testing/selftests/arm64/abi/syscall-abi.c b/tools/testing/selftests/arm64/abi/syscall-abi.c index b632bfe9e022..95229fa73232 100644 --- a/tools/testing/selftests/arm64/abi/syscall-abi.c +++ b/tools/testing/selftests/arm64/abi/syscall-abi.c @@ -113,8 +113,8 @@ static int check_fpr(struct syscall_cfg *cfg, int sve_vl, int sme_vl, }
static uint8_t z_zero[__SVE_ZREG_SIZE(SVE_VQ_MAX)]; -uint8_t z_in[SVE_NUM_PREGS * __SVE_ZREG_SIZE(SVE_VQ_MAX)]; -uint8_t z_out[SVE_NUM_PREGS * __SVE_ZREG_SIZE(SVE_VQ_MAX)]; +uint8_t z_in[SVE_NUM_ZREGS * __SVE_ZREG_SIZE(SVE_VQ_MAX)]; +uint8_t z_out[SVE_NUM_ZREGS * __SVE_ZREG_SIZE(SVE_VQ_MAX)];
static void setup_z(struct syscall_cfg *cfg, int sve_vl, int sme_vl, uint64_t svcr)
Currently our ABI documentation says that the state of the bits in the Z registers not shared with the V registers becomes undefined on syscall but our actual implementation unconditionally clears these bits. Taking advantage of the flexibility of our documented ABI would be a change in the observable ABI so there is concern around doing so, instead document the actual behaviour so that it is more discoverable for userspace programmers who might be able to take advantage of it and to record our decision about not changing the kernel ABI.
This makes qemu's user mode implementation buggy since it does not clear these bits.
Signed-off-by: Mark Brown broonie@kernel.org --- Documentation/arm64/sve.rst | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/Documentation/arm64/sve.rst b/Documentation/arm64/sve.rst index 93c2c2990584..e39acf95d157 100644 --- a/Documentation/arm64/sve.rst +++ b/Documentation/arm64/sve.rst @@ -111,7 +111,7 @@ the SVE instruction set architecture.
* On syscall, V0..V31 are preserved (as without SVE). Thus, bits [127:0] of Z0..Z31 are preserved. All other bits of Z0..Z31, and all of P0..P15 and FFR - become unspecified on return from a syscall. + become zero on return from a syscall.
* The SVE registers are not used to pass arguments to or receive results from any syscall.
Currently syscall-abi permits the bits in Z registers not shared with the V registers as well as all of the predicate registers to be preserved on syscall but the actual implementation has always cleared them and our documentation has now been updated to make that the documented ABI so update the syscall-abi test to match.
Signed-off-by: Mark Brown broonie@kernel.org --- .../testing/selftests/arm64/abi/syscall-abi.c | 57 ++++++++++++------- 1 file changed, 38 insertions(+), 19 deletions(-)
diff --git a/tools/testing/selftests/arm64/abi/syscall-abi.c b/tools/testing/selftests/arm64/abi/syscall-abi.c index 95229fa73232..dd7ebe536d05 100644 --- a/tools/testing/selftests/arm64/abi/syscall-abi.c +++ b/tools/testing/selftests/arm64/abi/syscall-abi.c @@ -112,6 +112,8 @@ static int check_fpr(struct syscall_cfg *cfg, int sve_vl, int sme_vl, return errors; }
+#define SVE_Z_SHARED_BYTES (128 / 8) + static uint8_t z_zero[__SVE_ZREG_SIZE(SVE_VQ_MAX)]; uint8_t z_in[SVE_NUM_ZREGS * __SVE_ZREG_SIZE(SVE_VQ_MAX)]; uint8_t z_out[SVE_NUM_ZREGS * __SVE_ZREG_SIZE(SVE_VQ_MAX)]; @@ -133,22 +135,39 @@ static int check_z(struct syscall_cfg *cfg, int sve_vl, int sme_vl, if (!sve_vl) return 0;
- /* - * After a syscall the low 128 bits of the Z registers should - * be preserved and the rest be zeroed or preserved, except if - * we were in streaming mode in which case the low 128 bits may - * also be cleared by the transition out of streaming mode. - */ for (i = 0; i < SVE_NUM_ZREGS; i++) { - void *in = &z_in[reg_size * i]; - void *out = &z_out[reg_size * i]; - - if ((memcmp(in, out, SVE_VQ_BYTES) != 0) && - !((svcr & SVCR_SM_MASK) && - memcmp(z_zero, out, SVE_VQ_BYTES) == 0)) { - ksft_print_msg("%s SVE VL %d Z%d low 128 bits changed\n", - cfg->name, sve_vl, i); - errors++; + uint8_t *in = &z_in[reg_size * i]; + uint8_t *out = &z_out[reg_size * i]; + + if (svcr & SVCR_SM_MASK) { + /* + * In streaming mode the whole register should + * be cleared by the transition out of + * streaming mode. + */ + if (memcmp(z_zero, out, reg_size) != 0) { + ksft_print_msg("%s SVE VL %d Z%d non-zero\n", + cfg->name, sve_vl, i); + errors++; + } + } else { + /* + * For standard SVE the low 128 bits should be + * preserved and any additional bits cleared. + */ + if (memcmp(in, out, SVE_Z_SHARED_BYTES) != 0) { + ksft_print_msg("%s SVE VL %d Z%d low 128 bits changed\n", + cfg->name, sve_vl, i); + errors++; + } + + if (reg_size > SVE_Z_SHARED_BYTES && + (memcmp(z_zero, out + SVE_Z_SHARED_BYTES, + reg_size - SVE_Z_SHARED_BYTES) != 0)) { + ksft_print_msg("%s SVE VL %d Z%d high bits non-zero\n", + cfg->name, sve_vl, i); + errors++; + } } }
@@ -176,9 +195,9 @@ static int check_p(struct syscall_cfg *cfg, int sve_vl, int sme_vl, if (!sve_vl) return 0;
- /* After a syscall the P registers should be preserved or zeroed */ + /* After a syscall the P registers should be zeroed */ for (i = 0; i < SVE_NUM_PREGS * reg_size; i++) - if (p_out[i] && (p_in[i] != p_out[i])) + if (p_out[i]) errors++; if (errors) ksft_print_msg("%s SVE VL %d predicate registers non-zero\n", @@ -226,9 +245,9 @@ static int check_ffr(struct syscall_cfg *cfg, int sve_vl, int sme_vl, !(getauxval(AT_HWCAP2) & HWCAP2_SME_FA64)) return 0;
- /* After a syscall the P registers should be preserved or zeroed */ + /* After a syscall FFR should be zeroed */ for (i = 0; i < reg_size; i++) - if (ffr_out[i] && (ffr_in[i] != ffr_out[i])) + if (ffr_out[i]) errors++; if (errors) ksft_print_msg("%s SVE VL %d FFR non-zero\n",
On Wed, Jul 20, 2022 at 09:50:10PM +0100, Mark Brown wrote:
Currently our SVE syscall ABI documentation does not reflect the actual implemented ABI, it says that register state not shared with FPSIMD becomes undefined on syscall when in reality we always clear it. Since changing this would cause a change in the observed kernel behaviour there is a substantial desire to avoid taking advantage of the documented ABI so instead let's document what we actually do so it's clear that it is in reality an ABI.
There has been some pushback on tightening the documentation in the past but it is hard to see who that helps, it makes the implementation decisions less clear and makes it harder for people to discover and make use of the actual ABI. The main practical concern is that qemu's user mode does not currently flush the registers.
Mark Brown (3): kselftest/arm64: Correct buffer allocation for SVE Z registers arm64/sve: Document our actual ABI for clearing registers on syscall kselftest/arm64: Enforce actual ABI for SVE syscalls
For the series:
Acked-by: Catalin Marinas catalin.marinas@arm.com
linux-kselftest-mirror@lists.linaro.org
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Catalin Marinas -
Mark Brown