Messages in this thread Patch in this message |  | | Date | Sat, 28 Mar 2020 09:32:01 -0700 | From | Sean Christopherson <> | Subject | Re: [PATCH v7 1/2] x86/split_lock: Rework the initialization flow of split lock detection |
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On Wed, Mar 25, 2020 at 11:09:23AM +0800, Xiaoyao Li wrote: > static void __init split_lock_setup(void) > { > + enum split_lock_detect_state state = sld_warn; > char arg[20]; > int i, ret; > > - setup_force_cpu_cap(X86_FEATURE_SPLIT_LOCK_DETECT); > - sld_state = sld_warn; > + if (!split_lock_verify_msr(false)) { > + pr_info("MSR access failed: Disabled\n");
A few nits on the error handling.
The error message for this is a bit wonky, lots of colons and it's not super clear what "Disabled" refers to.
[ 0.000000] x86/split lock detection: MSR access failed: Disabled
Maybe this, so that it reads "split lock detection disabled because the MSR access failed".
pr_info("Disabled, MSR access failed\n");
And rather than duplicate the error message, maybe use a goto, e.g.
if (!split_lock_verify_msr(false)) goto msr_failed;
...
if (!split_lock_verify_msr(true)) goto msr_failed;
> + return; > + } > > ret = cmdline_find_option(boot_command_line, "split_lock_detect", > arg, sizeof(arg)); > if (ret >= 0) { > for (i = 0; i < ARRAY_SIZE(sld_options); i++) { > if (match_option(arg, ret, sld_options[i].option)) { > - sld_state = sld_options[i].state; > + state = sld_options[i].state; > break; > } > } > } > > - switch (sld_state) { > + switch (state) { > case sld_off: > pr_info("disabled\n"); > - break; > - > + return; > case sld_warn: > pr_info("warning about user-space split_locks\n"); > break; > - > case sld_fatal: > pr_info("sending SIGBUS on user-space split_locks\n"); > break; > } > + > + if (!split_lock_verify_msr(true)) { > + pr_info("MSR access failed: Disabled\n"); > + return; > + } > + > + sld_state = state; > + setup_force_cpu_cap(X86_FEATURE_SPLIT_LOCK_DETECT); > } > > /* > - * Locking is not required at the moment because only bit 29 of this > - * MSR is implemented and locking would not prevent that the operation > - * of one thread is immediately undone by the sibling thread. > - * Use the "safe" versions of rdmsr/wrmsr here because although code > - * checks CPUID and MSR bits to make sure the TEST_CTRL MSR should > - * exist, there may be glitches in virtualization that leave a guest > - * with an incorrect view of real h/w capabilities. > + * MSR_TEST_CTRL is per core, but we treat it like a per CPU MSR. Locking > + * is not implemented as one thread could undo the setting of the other > + * thread immediately after dropping the lock anyway. > */ > -static bool __sld_msr_set(bool on) > +static void sld_update_msr(bool on) > { > u64 test_ctrl_val; > > - if (rdmsrl_safe(MSR_TEST_CTRL, &test_ctrl_val)) > - return false; > + rdmsrl(MSR_TEST_CTRL, test_ctrl_val); > > if (on) > test_ctrl_val |= MSR_TEST_CTRL_SPLIT_LOCK_DETECT; > else > test_ctrl_val &= ~MSR_TEST_CTRL_SPLIT_LOCK_DETECT; > > - return !wrmsrl_safe(MSR_TEST_CTRL, test_ctrl_val); > + wrmsrl(MSR_TEST_CTRL, test_ctrl_val); > } > > static void split_lock_init(void) > { > - if (sld_state == sld_off) > - return; > - > - if (__sld_msr_set(true)) > - return; > - > - /* > - * If this is anything other than the boot-cpu, you've done > - * funny things and you get to keep whatever pieces. > - */ > - pr_warn("MSR fail -- disabled\n"); > - sld_state = sld_off; > + split_lock_verify_msr(sld_state != sld_off);
I think it'd be worth a WARN_ON() if this fails with sld_state != off. If the WRMSR fails, then presumably SLD is off when it's expected to be on. The implied WARN on the unsafe WRMSR in sld_update_msr() won't fire unless a task generates an #AC on a non-buggy core and then gets migrated to the buggy core. Even if the WARNs are redundant, if something is wrong it'd be a lot easier for a user to triage/debug if there is a WARN in boot as opposed to a runtime WARN that requires a misbehaving application and scheduler behavior.
> } > > bool handle_user_split_lock(struct pt_regs *regs, long error_code) > @@ -1071,7 +1083,7 @@ bool handle_user_split_lock(struct pt_regs *regs, long error_code) > * progress and set TIF_SLD so the detection is re-enabled via > * switch_to_sld() when the task is scheduled out. > */ > - __sld_msr_set(false); > + sld_update_msr(false); > set_tsk_thread_flag(current, TIF_SLD); > return true; > } > @@ -1085,7 +1097,7 @@ bool handle_user_split_lock(struct pt_regs *regs, long error_code) > */ > void switch_to_sld(unsigned long tifn) > { > - __sld_msr_set(!(tifn & _TIF_SLD)); > + sld_update_msr(!(tifn & _TIF_SLD)); > } > > #define SPLIT_LOCK_CPU(model) {X86_VENDOR_INTEL, 6, model, X86_FEATURE_ANY} > -- > 2.20.1 >
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