Messages in this thread | | | From | Fenghua Yu <> | Subject | [PATCH v3 00/10] x86/split_lock: Enable #AC exception for split locked accesses | Date | Fri, 1 Feb 2019 21:14:41 -0800 |
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==Introduction==
A split lock is any atomic operation whose operand crosses two cache lines. Since the operand spans two cache lines and the operation must be atomic, the system locks the bus while the CPU accesses the two cache lines.
During bus locking, request from other CPUs or bus agents for control of the bus are blocked. Blocking bus access from other CPUs plus overhead of configuring bus locking protocol degrade not only the performance of one CPU but overall system performance.
If operand is cacheable and completely contained in one cache line, atomic operation is optimized by less expensive cache locking on Intel P6 and recent processors. If split lock is detected and the two cache lines in the operand can be merged into one cache line, cache locking instead of more expensive bus locking will be used for atomic operation. Removing split lock can improve overall performance.
Instructions that may cause split lock issue include lock add, lock btc, xchg, lsl, far call, ltr, etc.
More information about split lock, bus locking, and cache locking can be found in the latest Intel 64 and IA-32 Architecture Software Developer's Manual.
==#AC for split lock==
Currently we can trace split lock event counter sq_misc.split_lock for debug purpose. But for system deployed in the field, this event counter after the fact is insufficient. We need a mechanism that detects split lock before it happens to ensure that bus lock is never incurred due to split lock.
Intel introduces a mechanism to detect split lock via Alignment Check (#AC) exception before badly aligned atomic instructions might impact whole system performance in Tremont and other future processors.
This capability is critical for real time system designers who build consolidated real time systems. These systems run hard real time code on some cores and run "untrusted" user processes on some other cores. The hard real time cannot afford to have any bus lock from the untrusted processes to hurt real time performance. To date the designers have been unable to deploy these solutions as they have no way to prevent the "untrusted" user code from generating split lock and bus lock to block the hard real time code to access memory during bus locking.
This capability may also find usage in cloud. A user process with split lock running in one guest can block other cores from accessing shared memory during its split locked memory access. That may cause overall system performance degradation.
Split lock may open a security hole where malicious user code may slow down overall system by executing instructions with split lock.
==Enable #AC for Split Lock==
A control bit (bit 29) in TEST_CTL MSR 0x33 will be introduced in future x86 processors. When the bit 29 is set, the processor causes #AC exception for split locked accesses at all CPL.
User uses "setcpuid=ac_split_lock" or "setcpuid=255" to enable the feature.
Please note: The feature could be enumerated through MSR IA32_CORE_CAPABILITY (0xCF). But the enumeration is not completely published yet. So this patch set doesn't include the method.
The bit 29 specification in MSR TEST_CTL is published in the latest Intel Architecture Instruction Set Extensions and Future Features Programming Reference.
==Handle Split Lock===
There may be different considerations to handle split lock, e.g. how to handle split lock issue in firmware after kernel enables #AC for split lock.
But we use a simple way to handle split lock which is suggested by Thomas Gleixner and Dave Hansen:
- If split lock happens in kernel, a warning is issued and #AC for split lock is disabled on the local CPU. The split lock issue should be fixed in kernel.
- If split lock happens in user process, the process is killed by SIGBUS. Unless the issue is fixed, the process cannot run in the system.
- If split lock happens in firmware, system may hang in firmware. The issue should be fixed in firmware.
- User can force to enable the feature if he knows the feature is available.
==Patches== Patch 1-4: Fix a few split lock issues. Patch 5-7: Extend kernel option "clearcpuid=" to support multiple options and CPU cap flags. Patch 8: Add a new kernel option "setcpuid=" to force enable features. Patch 9-10: Handle split lock
==Changelog== v3: - Handle split lock as suggested by Thomas Gleixner. - Fix a few potential spit lock issues suggested by Thomas Gleixner. - Support kernel option "setcpuid=" suggested by Dave Hanson and Thomas Gleixner. - Support flag string in "clearcpuid=" suggested by Dave Hanson and Thomas Gleixner.
v2: - Remove code that handles split lock issue in firmware and fix x86_capability issue mainly based on comments from Thomas Gleixner and Peter Zijlstra.
In previous version: Comments from Dave Hansen: - Enumerate feature in X86_FEATURE_SPLIT_LOCK_AC - Separate #AC handler from do_error_trap - Use CONFIG to configure inherit BIOS setting, enable, or disable split lock. Remove kernel parameter "split_lock_ac=" - Change config interface to debugfs from sysfs - Fix a few bisectable issues - Other changes.
Comment from Tony Luck and Dave Hansen: - Dump right information in #AC handler
Comment from Alan Cox and Dave Hansen: - Description of split lock in patch 0
Others: - Remove tracing because we can trace split lock in existing sq_misc.split_lock. - Add CONFIG to configure either panic or re-execute faulting instruction for split lock in kernel. - other minor changes.
Fenghua Yu (10): x86/common: Align cpu_caps_cleared and cpu_caps_set to unsigned long drivers/net/b44: Align pwol_mask to unsigned long for better performance wlcore: Align reg_ch_conf_pending and tmp_ch_bitmap to unsigned long for better performance x86/split_lock: Align x86_capability to unsigned long to avoid split locked access x86/clearcpuid: Support multiple clearcpuid options x86/clearcpuid: Support feature flag string in kernel option clearcpuid Change document for kernel option clearcpuid x86/setcpuid: Add kernel option setcpuid x86/split_lock: Define #AC for split lock feature x86/split_lock: Handle #AC exception for split lock
Documentation/admin-guide/kernel-parameters.txt | 21 +++++++- arch/x86/include/asm/cmdline.h | 3 ++ arch/x86/include/asm/cpu.h | 2 + arch/x86/include/asm/cpufeature.h | 2 + arch/x86/include/asm/cpufeatures.h | 1 + arch/x86/include/asm/msr-index.h | 4 ++ arch/x86/include/asm/processor.h | 4 +- arch/x86/kernel/cpu/common.c | 5 +- arch/x86/kernel/cpu/cpuid-deps.c | 68 +++++++++++++++++++++---- arch/x86/kernel/cpu/intel.c | 25 +++++++++ arch/x86/kernel/fpu/init.c | 40 +++++++++++---- arch/x86/kernel/setup.c | 3 ++ arch/x86/kernel/smpboot.c | 3 ++ arch/x86/kernel/traps.c | 30 ++++++++++- arch/x86/lib/cmdline.c | 17 ++++++- drivers/net/ethernet/broadcom/b44.c | 3 +- drivers/net/wireless/ti/wlcore/cmd.c | 3 +- drivers/net/wireless/ti/wlcore/wlcore.h | 6 ++- 18 files changed, 208 insertions(+), 32 deletions(-)
-- 2.7.4
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