Re: [PATCH v2 2/5] x86/idle: Disable IBRS when cpu is offline

[Waiman Long <longman@redhat.com>]

On 6/21/23 10:36, Peter Zijlstra wrote:
> On Wed, Jun 21, 2023 at 09:59:52AM -0400, Waiman Long wrote:
>> On 6/21/23 03:23, Peter Zijlstra wrote:
>>> On Tue, Jun 20, 2023 at 10:06:22AM -0400, 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 and restoring the value of the MSR when it becomes
>>>> online again.
>>>>
>>>> Signed-off-by: Waiman Long <longman@redhat.com>
>>>> ---
>>>>    arch/x86/kernel/smpboot.c | 13 +++++++++++++
>>>>    1 file changed, 13 insertions(+)
>>>>
>>>> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
>>>> index 352f0ce1ece4..5ff82fef413c 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);
>>>> @@ -1838,12 +1839,24 @@ void __noreturn hlt_play_dead(void)
>>>>    void native_play_dead(void)
>>>>    {
>>>> +	u64 spec_ctrl = spec_ctrl_current();
>>>> +
>>>> +	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);
>>>>    	mwait_play_dead();
>>>>    	if (cpuidle_play_dead())
>>>>    		hlt_play_dead();
>>>> +
>>>> +	if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS)) {
>>>> +		native_wrmsrl(MSR_IA32_SPEC_CTRL, spec_ctrl);
>>>> +		this_cpu_write(x86_spec_ctrl_current, spec_ctrl);
>>>> +	}
>>>>    }
>>> play_dead() is marked __noreturn
>> There are different versions of play_dead() in the kernel. Some of them are
>> indeed marked __noreturn like the non-SMP one in arch/x86/kernel/process.c.
>> The native_play_dead() that I am patching isn't one of those.
> mostly by accident I think, hlt_play_dead() is, so I'm thinking
> everybody (all compiler and objtool) managed to figure out
> native_play_dead() is __noreturn too.

Well, hlt_play_dead() is only called if cpuidle_play_dead() returns an 
error which is not the typical case. My testing does confirm that this 
patch is able to keep the IBRS bit off when a CPU is offline via its 
online sysfs file.

Cheers,
Longman