Messages in this thread |  | | Date | Sat, 5 Dec 2020 13:56:23 +1100 | From | Balbir Singh <> | Subject | Re: [PATCH v3 4/5] prctl: Hook L1D flushing in via prctl |
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On Fri, Dec 04, 2020 at 11:19:17PM +0100, Thomas Gleixner wrote: > > Balbir, > > On Fri, Nov 27 2020 at 17:59, Balbir Singh wrote: > > +enum l1d_flush_out_mitigations { > > + L1D_FLUSH_OUT_OFF, > > + L1D_FLUSH_OUT_ON, > > +}; > > + > > +static enum l1d_flush_out_mitigations l1d_flush_out_mitigation __ro_after_init = L1D_FLUSH_OUT_ON; > > Why default on and why stays it on when the CPU is not affected by L1TF ... >
Because we don't set the PRCTL is the processor is not affected by the bug
> > /* Default mitigation for TAA-affected CPUs */ > > static enum taa_mitigations taa_mitigation __ro_after_init = TAA_MITIGATION_VERW; > > static bool taa_nosmt __ro_after_init; > > @@ -379,6 +386,18 @@ static void __init taa_select_mitigation(void) > > pr_info("%s\n", taa_strings[taa_mitigation]); > > } > > > > +static int __init l1d_flush_out_parse_cmdline(char *str) > > +{ > > + if (!boot_cpu_has_bug(X86_BUG_L1TF)) > > + return 0; > > ... while here you check for L1TF. > > Also shouldn't it be default off and enabled via command line? >
I chose the other way because the prctl is an opt-in as well
> > +static int l1d_flush_out_prctl_get(struct task_struct *task) > > +{ > > + int ret; > > + > > + if (l1d_flush_out_mitigation == L1D_FLUSH_OUT_OFF) > > + return PR_SPEC_FORCE_DISABLE; > > + > > + ret = test_ti_thread_flag(&task->thread_info, TIF_SPEC_L1D_FLUSH); > > That ret indirection is pointless. Just make it if (test_....)
Sure, will do
> > > +static int l1d_flush_out_prctl_set(struct task_struct *task, unsigned long ctrl) > > +{ > > + > > + if (l1d_flush_out_mitigation == L1D_FLUSH_OUT_OFF) > > + return -EPERM; > > So here you check for off and then... >
Yes
> > int enable_l1d_flush_for_task(struct task_struct *tsk) > > { > > + /* > > + * Do not enable L1D_FLUSH_OUT if > > + * b. The CPU is not affected by the L1TF bug > > + * c. The CPU does not have L1D FLUSH feature support > > + */ > > + > > + if (!boot_cpu_has_bug(X86_BUG_L1TF) || > > + !boot_cpu_has(X86_FEATURE_FLUSH_L1D)) > > + return -EINVAL; > > ... you check for the feature bits with a malformatted condition at some > other place. It's not supported when these conditions are not there. So > why having this check here? > > > + > > set_ti_thread_flag(&tsk->thread_info, TIF_SPEC_L1D_FLUSH); > > return 0; > > } > > Aside of that, why is this in tlb.c and not in bugs.c? There is nothing > tlb specific in these enable/disable functions. They just fiddle with > the TIF bit. >
I can move them over.
> > +/* > > + * Sent to a task that opts into L1D flushing via the prctl interface > > + * but ends up running on an SMT enabled core. > > + */ > > +static void l1d_flush_kill(struct callback_head *ch) > > +{ > > + force_sig(SIGBUS); > > +} > > + > > static inline unsigned long mm_mangle_tif_spec_bits(struct task_struct *next) > > { > > unsigned long next_tif = task_thread_info(next)->flags; > > unsigned long spec_bits = (next_tif >> TIF_SPEC_IB) & LAST_USER_MM_SPEC_MASK; > > + unsigned long next_mm; > > > > BUILD_BUG_ON(TIF_SPEC_L1D_FLUSH != TIF_SPEC_IB + 1); > > - return (unsigned long)next->mm | spec_bits; > > + next_mm = (unsigned long)next->mm | spec_bits; > > + > > + if ((next_mm & LAST_USER_MM_L1D_FLUSH) && this_cpu_read(cpu_info.smt_active)) { > > Wheeee. Yet more unconditional checks on every context switch.
A task can only get here if it is affected by the bug (processor has L1TF and L1D_FLUSH support) and the task opted in, I think what your suggesting is that we avoid the check for all tasks (the signgle next_mm & LAST_USER_MM_L1D_FLUSH) check as well?
> > > + clear_ti_thread_flag(&next->thread_info, TIF_SPEC_L1D_FLUSH); > > + next->l1d_flush_kill.func = l1d_flush_kill; > > + task_work_add(next, &next->l1d_flush_kill, true); > > int task_work_add(struct task_struct *task, struct callback_head *twork, > enum task_work_notify_mode mode); > > true is truly not a valid enum constant ....
:) I might really have added it when we were transitioning from true to TWA_RESUME, I am surprised the compiler did not catch it, I'll move it over.
> > > + } > > So you really want to have: > > DEFINE_STATIC_KEY_FALSE(l1dflush_enabled); > static bool l1dflush_mitigation __init_data; > > and then with the command line option you set l1dflush_mitigation and in > check_bugs() you invoke l1dflush_select_mitigation() which does: > > if (!l1dflush_mitigation || !boot_cpu_has_bug(X86_BUG_L1TF) || > !boot_cpu_has(X86_FEATURE_FLUSH_L1D)) > return; > > static_branch_enable(&l1dflush_enabled); > > and then in l1d_flush_out_prctl_set() > > if (!static_branch_unlikely(&l1dflush_enabled)) > return -ENOTSUPP; > > Then make the whole switch machinery do: > > if (static_branch_unlikely(&l1dflush_enabled)) { > if (unlikely((next_mm | prev_mm) & LAST_USER_MM_L1D_FLUSH)) > l1dflush_evaluate(next_mm, prev_mm); > } > > and l1dflush_evaluate() > > if (prev_mm & LAST_USER_MM_L1D_FLUSH) > l1d_flush(); > > if ((next_mm & LAST_USER_MM_L1D_FLUSH) && > this_cpu_read(cpu_info.smt_active)) { > > clear_ti_thread_flag(&next->thread_info, TIF_SPEC_L1D_FLUSH); > next->l1d_flush_kill.func = l1d_flush_kill; > task_work_add(next, &next->l1d_flush_kill, TWA_RESUME); > } > > That way the overhead is on systems where the admin decides to enable it > and if enabled the evaluation of prev_mm and next_mm is pushed out of > line. >
OK, I'll rewrite it and see how it looks
Thanks for the review, Balbir Singh
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