linux-kernel - Re: [PATCH v3 1/3] x86/idle: Disable IBRS when cpu is offline

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Date:   Wed, 21 Jun 2023 22:13:04 -0400
From:   Waiman Long <longman@...hat.com>
To:     Randy Dunlap <rdunlap@...radead.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Len Brown <lenb@...nel.org>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-pm@...r.kernel.org, Robin Jarry <rjarry@...hat.com>,
        Joe Mario <jmario@...hat.com>
Subject: Re: [PATCH v3 1/3] x86/idle: Disable IBRS when cpu is offline


On 6/21/23 22:05, Randy Dunlap wrote:
>
> On 6/21/23 17:36, Waiman Long wrote:
>> Commit bf5835bcdb96 ("intel_idle: Disable IBRS during long idle")
>> disables IBRS when the CPU enters long idle. However, when a CPU
>> becomes offline, the IBRS bit is still set when X86_FEATURE_KERNEL_IBRS
>> is enabled. That will impact the performance of a sibling CPU. Mitigate
>> this performance impact by clearing all the mitigation bits in SPEC_CTRL
>> MSR when offline. When the CPU is online again, it will be re-initialized
>> and so restoring the SPEC_CTRL value isn't needed.
>>
>> Add a comment to say that native_play_dead() is a __noreturn function,
>> but it can't be marked as such to avoid confusion about the missing
>> MSR restoration code.
>>
>> Signed-off-by: Waiman Long <longman@...hat.com>
>> ---
>>   arch/x86/kernel/smpboot.c | 10 ++++++++++
>>   1 file changed, 10 insertions(+)
>>
>> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
>> index 352f0ce1ece4..7bc33885518c 100644
>> --- a/arch/x86/kernel/smpboot.c
>> +++ b/arch/x86/kernel/smpboot.c
>> @@ -84,6 +84,7 @@
>>   #include <asm/hw_irq.h>
>>   #include <asm/stackprotector.h>
>>   #include <asm/sev.h>
>> +#include <asm/nospec-branch.h>
>>   
>>   /* representing HT siblings of each logical CPU */
>>   DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_sibling_map);
>> @@ -1836,8 +1837,17 @@ void __noreturn hlt_play_dead(void)
>>   	}
>>   }
>>   
>> +/*
>> + * naitve_play_dead() is essentially a __noreturn function, but it can't
> typo: native_play_dead()

Thanks for spotting the typo. Will fix it in the next version.

Cheers,
Longman

>
>> + * be marked as such as the compiler may complain about it.
>> + */
>>   void native_play_dead(void)
>>   {
>> +	if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS)) {
>> +		this_cpu_write(x86_spec_ctrl_current, 0);
>> +		native_wrmsrl(MSR_IA32_SPEC_CTRL, 0);
>> +	}
>> +
>>   	play_dead_common();
>>   	tboot_shutdown(TB_SHUTDOWN_WFS);
>>