arm64/sve: Add sysctl to set the default vector length for new processes (4ffa09a9) · Commits · 戴 / test

Because of the effect of SVE on the size of the signal frame, the default vector length used for new processes involves a tradeoff between performance of SVE-enabled software on the one hand, and reliability of non-SVE-aware software on the other hand. For this reason, the best choice depends on the repertoire of userspace software in use and is thus best left up to distro maintainers, sysadmins and developers. If CONFIG_SYSCTL and CONFIG_PROC_SYSCTL are enabled, this patch exposes the default vector length in /proc/sys/abi/sve_default_vector_length, where boot scripts or the adventurous can poke it. In common with other arm64 ABI sysctls, this control is currently global: setting it requires CAP_SYS_ADMIN in the root user namespace, but the value set is effective for subsequent execs in all namespaces. The control only affects _new_ processes, however: changing it does not affect the vector length of any existing process. The intended usage model is that if userspace is known to be fully SVE-tolerant (or a developer is curious to find out) then this parameter can be cranked up during system startup. Signed-off-by:

Dave Martin <Dave.Martin@arm.com> Reviewed-by:

Alex Bennée <alex.bennee@linaro.org> Reviewed-by:

Catalin Marinas <catalin.marinas@arm.com> Signed-off-by:

Will Deacon <will.deacon@arm.com>

arch/arm64/kernel/fpsimd.c

+61 −1

Original line number	Diff line number	Diff line
		@@ -37,6 +37,7 @@
		#include <linux/sched/task_stack.h>
		#include <linux/signal.h>
		#include <linux/slab.h>
		#include <linux/sysctl.h>

		#include <asm/fpsimd.h>
		#include <asm/cputype.h>
		@@ -335,6 +336,65 @@ static unsigned int find_supported_vector_length(unsigned int vl)
		return sve_vl_from_vq(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) {
		/* Fail safe if sve_max_vl wasn't initialised */
		if (WARN_ON(!sve_vl_valid(sve_max_vl)))
		vl = SVE_VL_MIN;
		else
		vl = sve_max_vl;

		goto chosen;
		}

		if (!sve_vl_valid(vl))
		return -EINVAL;

		vl = find_supported_vector_length(vl);
		chosen:
		sve_default_vl = 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))

		@@ -1209,6 +1269,6 @@ 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);

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