Re: [PATCH v3 4/5] prctl: Hook L1D flushing in via prctl

Balbir Singh
2020-12-05T02:57:38Z

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