linux-kernel - Re: [PATCH -next for tip:x86/pti] x86/tlb: drop unneeded local vars in enable_l1d_flush_for

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Message-ID: <20200929083709.GC2651@hirez.programming.kicks-ass.net>
Date:   Tue, 29 Sep 2020 10:37:09 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Lukas Bulwahn <lukas.bulwahn@...il.com>
Cc:     Balbir Singh <sblbir@...zon.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        kernel-janitors@...r.kernel.org, linux-safety@...ts.elisa.tech
Subject: Re: [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)