linux-kernel - Re: [PATCH v10 6/7] x86/arch_prctl: Add ARCH_[GET|SET]

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Date:   Tue, 14 Mar 2017 12:08:31 -0700
From:   Kyle Huey <me@...ehuey.com>
To:     "H. Peter Anvin" <hpa@...or.com>
Cc:     "Robert O'Callahan" <robert@...llahan.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Jeff Dike <jdike@...toit.com>,
        Richard Weinberger <richard@....at>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Shuah Khan <shuah@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Borislav Petkov <bp@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Len Brown <len.brown@...el.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Dmitry Safonov <dsafonov@...tuozzo.com>,
        David Matlack <dmatlack@...gle.com>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:USER-MODE LINUX (UML)" 
        <user-mode-linux-devel@...ts.sourceforge.net>,
        "open list:USER-MODE LINUX (UML)" 
        <user-mode-linux-user@...ts.sourceforge.net>,
        "open list:FILESYSTEMS (VFS and infrastructure)" 
        <linux-fsdevel@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>, kvm list <kvm@...r.kernel.org>
Subject: Re: [PATCH v10 6/7] x86/arch_prctl: Add ARCH_[GET|SET]_CPUID

On Tue, Mar 14, 2017 at 12:01 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 11/08/16 10:39, Kyle Huey wrote:
>>       }
>>
>> +     if (test_tsk_thread_flag(prev_p, TIF_NOCPUID) ^
>> +         test_tsk_thread_flag(next_p, TIF_NOCPUID)) {
>> +             set_cpuid_faulting(test_tsk_thread_flag(next_p, TIF_NOCPUID));
>> +     }
>> +
>>       if (test_tsk_thread_flag(prev_p, TIF_NOTSC) ^
>>           test_tsk_thread_flag(next_p, TIF_NOTSC)) {
>>               /* prev and next are different */
>>               if (test_tsk_thread_flag(next_p, TIF_NOTSC))
>>                       hard_disable_TSC();
>>               else
>>                       hard_enable_TSC();
>>       }
>
> I'm unhappy about this part: we already do two XORs on these after bit
> extraction, which is quite inefficient; and at least theoretically we
> could be indirecting though the ->stack pointer for every one if gcc
> can't tell it won't have changed (we really need to get thread_info
> moved into the task_struct allocation and away from the kernel stack,
> especially since on x86 the pointer is the same size as the vestigial
> structure it points to.)
>
> It would be so much saner to do one xor and then go onto a common slow path:
>
>         struct thread_info *prev_ti = task_thread_info(prev_p);
>         struct thread_info *next_ti = task_thread_info(next_p);
>
>         tif_flipped = prev_ti->flags ^ next_ti->flags;
>
>         if (unlikely(tif_flipped &
>                 (_TIF_BLOCKSTEP | _TIF_NOTSC | _TIF_NOCPUID))) {
>                 if (tif_flipped & _TIF_BLOCKSTEP) {
>                         ...
>                 }
>                 if (tif_flipped & _TIF_NOTSC) {
>                         ...
>                 }
>                 if (tif_flipped & _TIF_NOCPUID) {
>                         ...
>                 }
>         }
>
> Then we can also replace test_tsk_thread_flag() with
> test_ti_thread_flag() in other places in this function.

That's largely what we ended up doing.  See
https://lkml.org/lkml/2017/2/14/80 and the latest version of this
patch, https://lkml.org/lkml/2017/3/11/197.

- Kyle