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Date:   Thu, 12 Nov 2020 09:40:14 -0500
From:   Joel Fernandes <joel@joelfernandes.org>
To:     Alexander Graf <graf@amazon.com>
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Subject: Re: [RFC 1/2] x86/bugs: Disable coresched on hardware that does not
 need it
Message-ID: <20201112144014.GB1549282@google.com>
References: <20201111211011.1381848-1-joel@joelfernandes.org>
 <20201111211011.1381848-2-joel@joelfernandes.org>
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 <b1a1e07d-0df2-72c2-c3da-78e42fa355e8@amazon.com>
 <CAEXW_YRQ_GDcCxFcLrYjwNTG1nDZwUovczPSyOCvxXHq614DFw@mail.gmail.com>
 <CAEXW_YSC+qh8a4nhh6EC2jCaUZd1S59_enWT_rJSXSx5YHjFhw@mail.gmail.com>
 <76aa80c6-b797-f776-90fc-ef4585c41262@amazon.com>
 <20201112134005.GA1549282@google.com>
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On Thu, Nov 12, 2020 at 08:40:05AM -0500, Joel Fernandes wrote:
> On Wed, Nov 11, 2020 at 11:29:37PM +0100, Alexander Graf wrote:
> > 
> > 
> > On 11.11.20 23:15, Joel Fernandes wrote:
> > > 
> > > On Wed, Nov 11, 2020 at 5:13 PM Joel Fernandes <joel@joelfernandes.org> wrote:
> > > > 
> > > > On Wed, Nov 11, 2020 at 5:00 PM Alexander Graf <graf@amazon.com> wrote:
> > > > > On 11.11.20 22:14, Joel Fernandes wrote:
> > > > > > > Some hardware such as certain AMD variants don't have cross-HT MDS/L1TF
> > > > > > > issues. Detect this and don't enable core scheduling as it can
> > > > > > > needlessly slow the device done.
> > > > > > > 
> > > > > > > diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> > > > > > > index dece79e4d1e9..0e6e61e49b23 100644
> > > > > > > --- a/arch/x86/kernel/cpu/bugs.c
> > > > > > > +++ b/arch/x86/kernel/cpu/bugs.c
> > > > > > > @@ -152,6 +152,14 @@ void __init check_bugs(void)
> > > > > > >    #endif
> > > > > > >    }
> > > > > > > 
> > > > > > > +/*
> > > > > > > + * Do not need core scheduling if CPU does not have MDS/L1TF vulnerability.
> > > > > > > + */
> > > > > > > +int arch_allow_core_sched(void)
> > > > > > > +{
> > > > > > > +       return boot_cpu_has_bug(X86_BUG_MDS) || boot_cpu_has_bug(X86_BUG_L1TF);
> > > > > 
> > > > > Can we make this more generic and user settable, similar to the L1 cache
> > > > > flushing modes in KVM?
> > > > > 
> > > > > I am not 100% convinced that there are no other thread sibling attacks
> > > > > possible without MDS and L1TF. If I'm paranoid, I want to still be able
> > > > > to force enable core scheduling.
> > > > > 
> > > > > In addition, we are also using core scheduling as a poor man's mechanism
> > > > > to give customers consistent performance for virtual machine thread
> > > > > siblings. This is important irrespective of CPU bugs. In such a
> > > > > scenario, I want to force enable core scheduling.
> > > > 
> > > > Ok,  I can make it new kernel command line option with:
> > > > coresched=on
> > > > coresched=secure (only if HW has MDS/L1TF)
> > > > coresched=off
> > > 
> > > Also, I would keep "secure" as the default.  (And probably, we should
> > > modify the informational messages in sysfs to reflect this..)
> > 
> > I agree that "secure" should be the default.
> 
> Ok.

Something like so then:

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index dece79e4d1e9..3c2457d47f54 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -152,6 +152,21 @@ void __init check_bugs(void)
 #endif
 }
 
+/*
+ * When coresched=secure, do not need coresched if CPU does not have MDS/L1TF bugs.
+ */
+int arch_allow_core_sched(void)
+{
+	/*
+	 * x86: Disallow coresched if it is in secure mode and the CPU does not
+	 * have vulnerabilities.
+	 */
+	if (coresched_cmd_secure())
+		return boot_cpu_has_bug(X86_BUG_MDS) || boot_cpu_has_bug(X86_BUG_L1TF);
+	else
+		return true;
+}
+
 void
 x86_virt_spec_ctrl(u64 guest_spec_ctrl, u64 guest_virt_spec_ctrl, bool setguest)
 {
diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index d6428aaf67e7..1be5cf85a4a6 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -228,4 +228,7 @@ static inline int cpuhp_smt_disable(enum cpuhp_smt_control ctrlval) { return 0;
 extern bool cpu_mitigations_off(void);
 extern bool cpu_mitigations_auto_nosmt(void);
 
+extern bool coresched_cmd_off(void);
+extern bool coresched_cmd_secure(void);
+
 #endif /* _LINUX_CPU_H_ */
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 6ff2578ecf17..674edf534cc5 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -2552,3 +2552,46 @@ bool cpu_mitigations_auto_nosmt(void)
 	return cpu_mitigations == CPU_MITIGATIONS_AUTO_NOSMT;
 }
 EXPORT_SYMBOL_GPL(cpu_mitigations_auto_nosmt);
+
+/*
+ * These are used for a global "coresched=" cmdline option for controlling
+ * core scheduling. Note that core sched may be needed for usecases other
+ * than security as well.
+ */
+enum coresched_cmds {
+	CORE_SCHED_OFF,
+	CORE_SCHED_SECURE,
+	CORE_SCHED_ON,
+};
+
+static enum coresched_cmds coresched_cmd __ro_after_init = CORE_SCHED_SECURE;
+
+static int __init coresched_parse_cmdline(char *arg)
+{
+	if (!strcmp(arg, "off"))
+		coresched_cmd = CORE_SCHED_OFF;
+	else if (!strcmp(arg, "on"))
+		coresched_cmd = CORE_SCHED_ON;
+	else if (!strcmp(arg, "secure"))
+		coresched_cmd = CORE_SCHED_SECURE;
+	else
+		pr_crit("Unsupported coresched=%s, defaulting to secure.\n",
+			arg);
+
+	return 0;
+}
+early_param("coresched", coresched_parse_cmdline);
+
+/* coresched=off */
+bool coresched_cmd_off(void)
+{
+	return coresched_cmd == CORE_SCHED_OFF;
+}
+EXPORT_SYMBOL_GPL(coresched_cmd_off);
+
+/* coresched=secure */
+bool coresched_cmd_secure(void)
+{
+	return coresched_cmd == CORE_SCHED_SECURE;
+}
+EXPORT_SYMBOL_GPL(coresched_cmd_secure);
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 5ed26b469ed6..6f586d221ddb 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -333,8 +333,23 @@ static void __sched_core_disable(void)
 	printk("core sched disabled\n");
 }
 
+static bool __coresched_supported(void)
+{
+	/* coreched=off command line option. */
+	if (coresched_cmd_off())
+		return false;
+
+	/*
+	 * Some arch may not need coresched, example some x86 may not need
+	 * coresched if coresched=secure option is passed (=secure is default).
+	 */
+	return arch_allow_core_sched();
+}
+
 void sched_core_get(void)
 {
+	if (!__coresched_supported())
+		return;
 	mutex_lock(&sched_core_mutex);
 	if (!sched_core_count++)
 		__sched_core_enable();
@@ -343,6 +358,8 @@ void sched_core_get(void)
 
 void sched_core_put(void)
 {
+	if (!__coresched_supported())
+		return;
 	mutex_lock(&sched_core_mutex);
 	if (!--sched_core_count)
 		__sched_core_disable();
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index ada56d8ce56f..20d2aa53336e 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1206,6 +1206,11 @@ int cpu_core_tag_color_write_u64(struct cgroup_subsys_state *css,
 
 bool cfs_prio_less(struct task_struct *a, struct task_struct *b, bool fi);
 
+int __weak arch_allow_core_sched(void)
+{
+	return true;
+}
+
 #else /* !CONFIG_SCHED_CORE */
 
 static inline bool sched_core_enqueued(struct task_struct *task) { return false; }