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SubjectRe: [PATCH -next for tip:x86/pti] x86/tlb: drop unneeded local vars in enable_l1d_flush_for_task()
On Tue, Sep 29, 2020 at 09:12:11AM +0200, Peter Zijlstra wrote:
> On Mon, Sep 28, 2020 at 02:44:57PM +0200, Lukas Bulwahn wrote:
> > diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> > index 6b0f4c88b07c..90515c04d90a 100644
> > --- a/arch/x86/mm/tlb.c
> > +++ b/arch/x86/mm/tlb.c
> > @@ -316,7 +316,7 @@ EXPORT_SYMBOL_GPL(leave_mm);
> >
> > int enable_l1d_flush_for_task(struct task_struct *tsk)
> > {
> > - int cpu, ret = 0, i;
> > + int i;
> >
> > /*
> > * Do not enable L1D_FLUSH_OUT if
> > @@ -329,7 +329,7 @@ int enable_l1d_flush_for_task(struct task_struct *tsk)
> > !static_cpu_has(X86_FEATURE_FLUSH_L1D))
> > return -EINVAL;
> >
> > - cpu = get_cpu();
> > + get_cpu();
> >
> > for_each_cpu(i, &tsk->cpus_mask) {
> > if (cpu_data(i).smt_active == true) {
> > @@ -340,7 +340,7 @@ int enable_l1d_flush_for_task(struct task_struct *tsk)
> >
> > set_ti_thread_flag(&tsk->thread_info, TIF_SPEC_L1D_FLUSH);
> > put_cpu();
> > - return ret;
> > + return 0;
> > }
>
> If you don't use the return value of get_cpu(), then change it over to
> preempt_{dis,en}able(), but this got me looking at the function, wtf is
> that garbage supposed to do in the first place
>
> What do we need to disable preemption for?
>
> Please explain the desired semantics against sched_setaffinity().

Here, I fixed it..

---
arch/x86/mm/tlb.c | 24 +++---------------------
1 file changed, 3 insertions(+), 21 deletions(-)

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 6b0f4c88b07c..f02a2f1909da 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -316,31 +316,13 @@ EXPORT_SYMBOL_GPL(leave_mm);

int enable_l1d_flush_for_task(struct task_struct *tsk)
{
- int cpu, ret = 0, i;
-
- /*
- * 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
- * c. The task's affinity is on cores with SMT on.
- */
-
if (!boot_cpu_has_bug(X86_BUG_L1TF) ||
- !static_cpu_has(X86_FEATURE_FLUSH_L1D))
+ !boot_cpu_has(X86_FEATURE_FLUSH_L1D) ||
+ sched_smt_active());
return -EINVAL;

- cpu = get_cpu();
-
- for_each_cpu(i, &tsk->cpus_mask) {
- if (cpu_data(i).smt_active == true) {
- put_cpu();
- return -EINVAL;
- }
- }
-
set_ti_thread_flag(&tsk->thread_info, TIF_SPEC_L1D_FLUSH);
- put_cpu();
- return ret;
+ return 0;
}

int disable_l1d_flush_for_task(struct task_struct *tsk)
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