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authorMark Rutland <mark.rutland@arm.com>2023-10-16 11:24:36 +0100
committerCatalin Marinas <catalin.marinas@arm.com>2023-10-16 14:17:03 +0100
commit34f66c4c4d5518c11bfb7d10defff8f814c9f28a (patch)
treeeb0e07db4a3f26acb9f9b913e78b5a362ef28564 /arch/arm64/kernel/fpsimd.c
parent14567ba42c5747f50584eb463ac8029f2a30e0d1 (diff)
arm64: Use a positive cpucap for FP/SIMD
Currently we have a negative cpucap which describes the *absence* of FP/SIMD rather than *presence* of FP/SIMD. This largely works, but is somewhat awkward relative to other cpucaps that describe the presence of a feature, and it would be nicer to have a cpucap which describes the presence of FP/SIMD: * This will allow the cpucap to be treated as a standard ARM64_CPUCAP_SYSTEM_FEATURE, which can be detected with the standard has_cpuid_feature() function and ARM64_CPUID_FIELDS() description. * This ensures that the cpucap will only transition from not-present to present, reducing the risk of unintentional and/or unsafe usage of FP/SIMD before cpucaps are finalized. * This will allow using arm64_cpu_capabilities::cpu_enable() to enable the use of FP/SIMD later, with FP/SIMD being disabled at boot time otherwise. This will ensure that any unintentional and/or unsafe usage of FP/SIMD prior to this is trapped, and will ensure that FP/SIMD is never unintentionally enabled for userspace in mismatched big.LITTLE systems. This patch replaces the negative ARM64_HAS_NO_FPSIMD cpucap with a positive ARM64_HAS_FPSIMD cpucap, making changes as described above. Note that as FP/SIMD will now be trapped when not supported system-wide, do_fpsimd_acc() must handle these traps in the same way as for SVE and SME. The commentary in fpsimd_restore_current_state() is updated to describe the new scheme. No users of system_supports_fpsimd() need to know that FP/SIMD is available prior to alternatives being patched, so this is updated to use alternative_has_cap_likely() to check for the ARM64_HAS_FPSIMD cpucap, without generating code to test the system_cpucaps bitmap. Signed-off-by: Mark Rutland <mark.rutland@arm.com> Reviewed-by: Mark Brown <broonie@kernel.org> Cc: Suzuki K Poulose <suzuki.poulose@arm.com> Cc: Will Deacon <will@kernel.org> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Diffstat (limited to 'arch/arm64/kernel/fpsimd.c')
-rw-r--r--arch/arm64/kernel/fpsimd.c44
1 files changed, 35 insertions, 9 deletions
diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index 45ea9cabbaa4..d286bcfc3f41 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -1520,8 +1520,17 @@ void do_sme_acc(unsigned long esr, struct pt_regs *regs)
*/
void do_fpsimd_acc(unsigned long esr, struct pt_regs *regs)
{
- /* TODO: implement lazy context saving/restoring */
- WARN_ON(1);
+ /* Even if we chose not to use FPSIMD, the hardware could still trap: */
+ if (!system_supports_fpsimd()) {
+ force_signal_inject(SIGILL, ILL_ILLOPC, regs->pc, 0);
+ return;
+ }
+
+ /*
+ * When FPSIMD is enabled, we should never take a trap unless something
+ * has gone very wrong.
+ */
+ BUG();
}
/*
@@ -1762,13 +1771,23 @@ void fpsimd_bind_state_to_cpu(struct cpu_fp_state *state)
void fpsimd_restore_current_state(void)
{
/*
- * For the tasks that were created before we detected the absence of
- * FP/SIMD, the TIF_FOREIGN_FPSTATE could be set via fpsimd_thread_switch(),
- * e.g, init. This could be then inherited by the children processes.
- * If we later detect that the system doesn't support FP/SIMD,
- * we must clear the flag for all the tasks to indicate that the
- * FPSTATE is clean (as we can't have one) to avoid looping for ever in
- * do_notify_resume().
+ * TIF_FOREIGN_FPSTATE is set on the init task and copied by
+ * arch_dup_task_struct() regardless of whether FP/SIMD is detected.
+ * Thus user threads can have this set even when FP/SIMD hasn't been
+ * detected.
+ *
+ * When FP/SIMD is detected, begin_new_exec() will set
+ * TIF_FOREIGN_FPSTATE via flush_thread() -> fpsimd_flush_thread(),
+ * and fpsimd_thread_switch() will set TIF_FOREIGN_FPSTATE when
+ * switching tasks. We detect FP/SIMD before we exec the first user
+ * process, ensuring this has TIF_FOREIGN_FPSTATE set and
+ * do_notify_resume() will call fpsimd_restore_current_state() to
+ * install the user FP/SIMD context.
+ *
+ * When FP/SIMD is not detected, nothing else will clear or set
+ * TIF_FOREIGN_FPSTATE prior to the first return to userspace, and
+ * we must clear TIF_FOREIGN_FPSTATE to avoid do_notify_resume()
+ * looping forever calling fpsimd_restore_current_state().
*/
if (!system_supports_fpsimd()) {
clear_thread_flag(TIF_FOREIGN_FPSTATE);
@@ -2101,6 +2120,13 @@ static inline void fpsimd_hotplug_init(void)
static inline void fpsimd_hotplug_init(void) { }
#endif
+void cpu_enable_fpsimd(const struct arm64_cpu_capabilities *__always_unused p)
+{
+ unsigned long enable = CPACR_EL1_FPEN_EL1EN | CPACR_EL1_FPEN_EL0EN;
+ write_sysreg(read_sysreg(CPACR_EL1) | enable, CPACR_EL1);
+ isb();
+}
+
/*
* FP/SIMD support code initialisation.
*/