Re: [RFC 1/2] x86/bugs: Disable coresched on hardware that does not need it

[Alexander Graf <graf@amazon.com>]

On 12.11.20 15:40, Joel Fernandes wrote:
> 
> 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;


Instead of calling the matching function over and over again, can we 
just configure a static branch (see below) based on the command line 
setting here? Or do we not know about the bugs yet?

> +}
> +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;

I would expect core scheduling to be basically an option that you set 
once and never flip. This sounds like a prefect use case for a static 
branch to me?


Alex

>          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; }
> 



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