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[PATCH 20/27] arm64/sve: Add sysctl to set the default vector length for new processes

• From: Dave Martin <Dave dot Martin at arm dot com>
• To: linux-arm-kernel at lists dot infradead dot org
• Cc: Catalin Marinas <catalin dot marinas at arm dot com>,Will Deacon <will dot deacon at arm dot com>,Ard Biesheuvel <ard dot biesheuvel at linaro dot org>,Szabolcs Nagy <szabolcs dot nagy at arm dot com>,Richard Sandiford <richard dot sandiford at arm dot com>,kvmarm at lists dot cs dot columbia dot edu,libc-alpha at sourceware dot org,linux-arch at vger dot kernel dot org
• Date: Wed, 9 Aug 2017 13:05:26 +0100
• Subject: [PATCH 20/27] arm64/sve: Add sysctl to set the default vector length for new processes
• Authentication-results: sourceware.org; auth=none
• References: <1502280338-23002-1-git-send-email-Dave.Martin@arm.com>

Because of the effect of SVE on the size of the signal frame, thedefault vector length used for new processes involves a tradeoffbetween performance of SVE-enabled software on the one hand, andreliability of non-SVE-aware software on the other hand.For this reason, the best choice depends on the repertoire ofuserspace software in use and is thus best left up to distromaintainers, sysadmins and developers.If CONFIG_SYSCTL is enabled, this patch exposes the default vectorlength in /proc/sys/abi/sve_default_vector_length, where bootscripts or the adventurous can poke it.In common with other arm64 ABI sysctls, this control is currentlyglobal: setting it requires CAP_SYS_ADMIN in the root usernamespace, but the value set is effective for subsequent execs inall namespaces.  The control only affects _new_ processes, however:changing it does not affect the vector length of any existingprocess.The intended usage model is that if userspace is known to be fullySVE-tolerant (or a developer is curious to find out) then initscripts can crank this up during startup.Signed-off-by: Dave Martin <Dave.Martin@arm.com>--- arch/arm64/kernel/fpsimd.c | 57 +++++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 56 insertions(+), 1 deletion(-)diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.cindex 4fe1675..d45fcfb 100644--- a/arch/arm64/kernel/fpsimd.c+++ b/arch/arm64/kernel/fpsimd.c@@ -31,6 +31,7 @@ #include <linux/sched/signal.h> #include <linux/signal.h> #include <linux/slab.h>+#include <linux/sysctl.h>  #include <asm/fpsimd.h> #include <asm/cputype.h>@@ -200,6 +201,60 @@ static unsigned int find_supported_vector_length(unsigned int vl) return 16 * bit_to_vq(bit); } +#ifdef CONFIG_SYSCTL++static int sve_proc_do_default_vl(struct ctl_table *table, int write,+  void __user *buffer, size_t *lenp,+  loff_t *ppos)+{+int ret;+int vl = sve_default_vl;+struct ctl_table tmp_table = {+.data = &vl,+.maxlen = sizeof(vl),+};++ret = proc_dointvec(&tmp_table, write, buffer, lenp, ppos);+if (ret || !write)+return ret;++/* Writing -1 has the special meaning "set to max": */+if (vl == -1) {+BUG_ON(find_supported_vector_length(sve_max_vl) != sve_max_vl);+sve_default_vl = sve_max_vl;++return 0;+}++if (!sve_vl_valid(vl))+return -EINVAL;++sve_default_vl = find_supported_vector_length(vl);+return 0;+}++static struct ctl_table sve_default_vl_table[] = {+{+.procname= "sve_default_vector_length",+.mode= 0644,+.proc_handler= sve_proc_do_default_vl,+},+{ }+};++static int __init sve_sysctl_init(void)+{+if (system_supports_sve())+if (!register_sysctl("abi", sve_default_vl_table))+return -EINVAL;++return 0;+}++#else /* ! CONFIG_SYSCTL */+static int __init sve_sysctl_init(void) { return 0; }+#endif /* ! CONFIG_SYSCTL */+ #define ZREG(sve_state, vq, n) ((char *)(sve_state) +\ (SVE_SIG_ZREG_OFFSET(vq, n) - SVE_SIG_REGS_OFFSET)) @@ -1037,6 +1092,6 @@ static int __init fpsimd_init(void) if (!(elf_hwcap & HWCAP_ASIMD)) pr_notice("Advanced SIMD is not implemented\n"); -return 0;+return sve_sysctl_init(); } late_initcall(fpsimd_init);-- 2.1.4

• References:
  • [PATCH 00/27] ARM Scalable Vector Extension (SVE)
    • From: Dave Martin