linux-kernel - Re: [PATCH 20/43] x86/entry: Clean up SYSENTER

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Date:   Sat, 25 Nov 2017 09:55:15 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>,
        linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 20/43] x86/entry: Clean up SYSENTER_stack code



> On Nov 25, 2017, at 9:50 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> On Sat, 25 Nov 2017, Borislav Petkov wrote:
>>> -
>>> +    wrmsr(MSR_IA32_SYSENTER_ESP, (unsigned long)(cpu_SYSENTER_stack(cpu) + 1), 0);
>>>    wrmsr(MSR_IA32_SYSENTER_EIP, (unsigned long)entry_SYSENTER_32, 0);
>> 
>> Right, so we have now two TSS thingies, AFAICT:
>> 
>>    tss = &per_cpu(cpu_tss, cpu);
>> 
>> which is cpu_tss and then indirectly, we have also:
>> 
>>    &get_cpu_entry_area((cpu))->tss
>> 
>> And those are two different things in my guest here:
>> 
>> [    0.044002] tss: 0xf5747000
>> [    0.044706] entry area tss: 0xffef1000
>> 
>> What is the logic here? We carry two TSSs per CPU - one which is RO
>> for the entry area and the other is the actual cpu_tss thing? Or am I
>> misreading it?
> 
> entry area tss is a alias mapping of cpu_tss
> 
> +       set_percpu_fixmap_pages(get_cpu_entry_area_index(cpu, tss),
> +                               &per_cpu(cpu_tss, cpu),
> +                               sizeof(struct tss_struct) / PAGE_SIZE,
> +                               PAGE_KERNEL);
> 

Exactly.  And, in the patch I haven't emailed, the alias is RO on x86_64.

Maybe I should rename cpu_tss to cpu_tss_rw in that patch.

> Thanks,
> 
>    tglx