linux-kernel - Re: [PATCH 10/18] x86/asm/32: Pull MSR_IA32_SYSENTER_CS update code out of native_load

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Date:   Fri, 27 Oct 2017 08:50:04 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        Borislav Petkov <bpetkov@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 10/18] x86/asm/32: Pull MSR_IA32_SYSENTER_CS update code
 out of native_load_sp0()

On Fri, Oct 27, 2017 at 6:51 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Thu, 26 Oct 2017, Andy Lutomirski wrote:
>> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
>> index b390ff76e58f..0167e3e35a57 100644
>> --- a/arch/x86/include/asm/processor.h
>> +++ b/arch/x86/include/asm/processor.h
>> @@ -520,13 +520,6 @@ static inline void
>>  native_load_sp0(struct tss_struct *tss, struct thread_struct *thread)
>>  {
>>       tss->x86_tss.sp0 = thread->sp0;
>> -#ifdef CONFIG_X86_32
>> -     /* Only happens when SEP is enabled, no need to test "SEP"arately: */
>> -     if (unlikely(tss->x86_tss.ss1 != thread->sysenter_cs)) {
>> -             tss->x86_tss.ss1 = thread->sysenter_cs;
>> -             wrmsr(MSR_IA32_SYSENTER_CS, thread->sysenter_cs, 0);
>> -     }
>> -#endif
>>  }
>>
>>  static inline void native_swapgs(void)
>> diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
>> index fcc5cd387fd1..f3fa19925ae1 100644
>> --- a/arch/x86/include/asm/switch_to.h
>> +++ b/arch/x86/include/asm/switch_to.h
>> @@ -72,4 +72,15 @@ do {                                                                       \
>>       ((last) = __switch_to_asm((prev), (next)));                     \
>>  } while (0)
>>
>> +#ifdef CONFIG_X86_32
>> +static inline void refresh_sysenter_cs(struct thread_struct *thread)
>> +{
>> +     /* Only happens when SEP is enabled, no need to test "SEP"arately: */
>> +     if (unlikely(this_cpu_read(cpu_tss.x86_tss.ss1) == thread->sysenter_cs))
>> +             return;
>> +
>> +     this_cpu_write(cpu_tss.x86_tss.ss1, thread->sysenter_cs);
>
> You lost the wrmsr() on the way... Ideally you move the code unmodified
> first and then do the this_cpu_ change on top.

I don't think that would work.  The old location of that code had a
local variable "tss", and the new location doesn't.  I could make the
nonsensical one-line change as a preparatory patch, but I don't think
that really makes what's changing any clearer.


Anyway, I fixed it.

>
> Thanks,
>
>         tglx