From: Mark Brown <broonie@kernel.org> To: Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org> Cc: Marc Zyngier <maz@kernel.org>, Zhang Lei <zhang.lei@jp.fujitsu.com>, Mark Brown <broonie@kernel.org>, Andre Przywara <andre.przywara@arm.com>, kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org Subject: [PATCH v2 7/7] arm64/sve: Don't zero non-FPSIMD register state on syscall by default Date: Mon, 20 Jun 2022 13:41:58 +0100 [thread overview] Message-ID: <20220620124158.482039-8-broonie@kernel.org> (raw) In-Reply-To: <20220620124158.482039-1-broonie@kernel.org> The documented syscall ABI specifies that the SVE state not shared with FPSIMD is undefined after a syscall. Currently we implement this by always flushing this register state to zero, ensuring consistent behaviour but introducing some overhead in the case where we can return directly to userspace without otherwise needing to update the register state. Take advantage of the flexibility offered by the documented ABI and instead leave the SVE registers untouched in the case where can return directly to userspace. Since this is a user visible change a new sysctl abi.sve_syscall_clear_regs is provided which will restore the current behaviour of flushing the unshared register state unconditionally when enabled. This can be enabled for testing or to work around problems with applications that have been relying on the current flushing behaviour. The sysctl is disabled by default since it is anticipated that the risk of disruption to userspace is low. As well as being within the documented ABI this new behaviour mirrors the standard function call ABI for SVE in the AAPCS which should mean that compiler generated code is unlikely to rely on the current behaviour, the main risk is from hand coded assembly which directly invokes syscalls. The new behaviour is also what is currently implemented by qemu user mode emulation. Signed-off-by: Mark Brown <broonie@kernel.org> --- arch/arm64/kernel/syscall.c | 36 +++++++++++++++++++++++++++++++++++- 1 file changed, 35 insertions(+), 1 deletion(-) diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c index 69b4c06f2e39..29ef3d65cf12 100644 --- a/arch/arm64/kernel/syscall.c +++ b/arch/arm64/kernel/syscall.c @@ -158,6 +158,40 @@ static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr, syscall_trace_exit(regs); } + +static unsigned int sve_syscall_regs_clear; + +#ifdef CONFIG_ARM64_SVE +/* + * Global sysctl to control if we force the SVE register state not + * shared with FPSIMD to be cleared on every syscall. If this is not + * enabled then we will leave the state unchanged unless we need to + * reload from memory (eg, after a context switch). + */ + +static struct ctl_table sve_syscall_sysctl_table[] = { + { + .procname = "sve_syscall_clear_regs", + .mode = 0644, + .data = &sve_syscall_regs_clear, + .maxlen = sizeof(int), + .proc_handler = proc_dointvec_minmax, + .extra1 = SYSCTL_ZERO, + .extra2 = SYSCTL_ONE, + }, + { } +}; + +static int __init sve_syscall_sysctl_init(void) +{ + if (!register_sysctl("abi", sve_syscall_sysctl_table)) + return -EINVAL; + return 0; +} + +core_initcall(sve_syscall_sysctl_init); +#endif /* CONFIG_ARM64_SVE */ + /* * As per the ABI exit SME streaming mode and clear the SVE state not * shared with FPSIMD on syscall entry. @@ -183,7 +217,7 @@ static inline void fp_user_discard(void) if (!system_supports_sve()) return; - if (test_thread_flag(TIF_SVE)) { + if (sve_syscall_regs_clear && test_thread_flag(TIF_SVE)) { unsigned int sve_vq_minus_one; sve_vq_minus_one = sve_vq_from_vl(task_get_sve_vl(current)) - 1; -- 2.30.2 _______________________________________________ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
WARNING: multiple messages have this Message-ID (diff)
From: Mark Brown <broonie@kernel.org> To: Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org> Cc: Marc Zyngier <maz@kernel.org>, Zhang Lei <zhang.lei@jp.fujitsu.com>, James Morse <james.morse@arm.com>, Alexandru Elisei <alexandru.elisei@arm.com>, Andre Przywara <andre.przywara@arm.com>, kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org, Mark Brown <broonie@kernel.org> Subject: [PATCH v2 7/7] arm64/sve: Don't zero non-FPSIMD register state on syscall by default Date: Mon, 20 Jun 2022 13:41:58 +0100 [thread overview] Message-ID: <20220620124158.482039-8-broonie@kernel.org> (raw) In-Reply-To: <20220620124158.482039-1-broonie@kernel.org> The documented syscall ABI specifies that the SVE state not shared with FPSIMD is undefined after a syscall. Currently we implement this by always flushing this register state to zero, ensuring consistent behaviour but introducing some overhead in the case where we can return directly to userspace without otherwise needing to update the register state. Take advantage of the flexibility offered by the documented ABI and instead leave the SVE registers untouched in the case where can return directly to userspace. Since this is a user visible change a new sysctl abi.sve_syscall_clear_regs is provided which will restore the current behaviour of flushing the unshared register state unconditionally when enabled. This can be enabled for testing or to work around problems with applications that have been relying on the current flushing behaviour. The sysctl is disabled by default since it is anticipated that the risk of disruption to userspace is low. As well as being within the documented ABI this new behaviour mirrors the standard function call ABI for SVE in the AAPCS which should mean that compiler generated code is unlikely to rely on the current behaviour, the main risk is from hand coded assembly which directly invokes syscalls. The new behaviour is also what is currently implemented by qemu user mode emulation. Signed-off-by: Mark Brown <broonie@kernel.org> --- arch/arm64/kernel/syscall.c | 36 +++++++++++++++++++++++++++++++++++- 1 file changed, 35 insertions(+), 1 deletion(-) diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c index 69b4c06f2e39..29ef3d65cf12 100644 --- a/arch/arm64/kernel/syscall.c +++ b/arch/arm64/kernel/syscall.c @@ -158,6 +158,40 @@ static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr, syscall_trace_exit(regs); } + +static unsigned int sve_syscall_regs_clear; + +#ifdef CONFIG_ARM64_SVE +/* + * Global sysctl to control if we force the SVE register state not + * shared with FPSIMD to be cleared on every syscall. If this is not + * enabled then we will leave the state unchanged unless we need to + * reload from memory (eg, after a context switch). + */ + +static struct ctl_table sve_syscall_sysctl_table[] = { + { + .procname = "sve_syscall_clear_regs", + .mode = 0644, + .data = &sve_syscall_regs_clear, + .maxlen = sizeof(int), + .proc_handler = proc_dointvec_minmax, + .extra1 = SYSCTL_ZERO, + .extra2 = SYSCTL_ONE, + }, + { } +}; + +static int __init sve_syscall_sysctl_init(void) +{ + if (!register_sysctl("abi", sve_syscall_sysctl_table)) + return -EINVAL; + return 0; +} + +core_initcall(sve_syscall_sysctl_init); +#endif /* CONFIG_ARM64_SVE */ + /* * As per the ABI exit SME streaming mode and clear the SVE state not * shared with FPSIMD on syscall entry. @@ -183,7 +217,7 @@ static inline void fp_user_discard(void) if (!system_supports_sve()) return; - if (test_thread_flag(TIF_SVE)) { + if (sve_syscall_regs_clear && test_thread_flag(TIF_SVE)) { unsigned int sve_vq_minus_one; sve_vq_minus_one = sve_vq_from_vl(task_get_sve_vl(current)) - 1; -- 2.30.2 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
next prev parent reply other threads:[~2022-06-20 12:42 UTC|newest] Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top 2022-06-20 12:41 [PATCH v2 0/7] arm64/sve: Clean up KVM integration and optimise syscalls Mark Brown 2022-06-20 12:41 ` Mark Brown 2022-06-20 12:41 ` [PATCH v2 1/7] KVM: arm64: Discard any SVE state when entering KVM guests Mark Brown 2022-06-20 12:41 ` Mark Brown 2022-06-20 12:41 ` [PATCH v2 2/7] arm64/fpsimd: Track the saved FPSIMD state type separately to TIF_SVE Mark Brown 2022-06-20 12:41 ` Mark Brown 2022-07-11 9:40 ` Marc Zyngier 2022-07-11 9:40 ` Marc Zyngier 2022-07-11 11:39 ` Mark Brown 2022-07-11 11:39 ` Mark Brown 2022-07-11 14:33 ` Marc Zyngier 2022-07-11 14:33 ` Marc Zyngier 2022-07-11 15:53 ` Mark Brown 2022-07-11 15:53 ` Mark Brown 2022-07-20 9:40 ` Marc Zyngier 2022-07-20 9:40 ` Marc Zyngier 2022-07-20 13:51 ` Mark Brown 2022-07-20 13:51 ` Mark Brown 2022-06-20 12:41 ` [PATCH v2 3/7] arm64/fpsimd: Have KVM explicitly say which FP registers to save Mark Brown 2022-06-20 12:41 ` Mark Brown 2022-06-20 12:41 ` [PATCH v2 4/7] arm64/fpsimd: Stop using TIF_SVE to manage register saving in KVM Mark Brown 2022-06-20 12:41 ` Mark Brown 2022-06-20 12:41 ` [PATCH v2 5/7] arm64/fpsimd: Load FP state based on recorded data type Mark Brown 2022-06-20 12:41 ` Mark Brown 2022-06-20 12:41 ` [PATCH v2 6/7] arm64/sve: Leave SVE enabled on syscall if we don't context switch Mark Brown 2022-06-20 12:41 ` Mark Brown 2022-06-20 12:41 ` Mark Brown [this message] 2022-06-20 12:41 ` [PATCH v2 7/7] arm64/sve: Don't zero non-FPSIMD register state on syscall by default Mark Brown 2022-07-19 17:35 ` Catalin Marinas 2022-07-19 17:35 ` Catalin Marinas 2022-07-19 19:35 ` Mark Brown 2022-07-19 19:35 ` Mark Brown 2022-07-20 9:20 ` Will Deacon 2022-07-20 9:20 ` Will Deacon 2022-07-20 12:32 ` Mark Brown 2022-07-20 12:32 ` Mark Brown 2022-07-20 9:29 ` Marc Zyngier 2022-07-20 9:29 ` Marc Zyngier 2022-07-20 14:31 ` Mark Brown 2022-07-20 14:31 ` Mark Brown
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