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

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Date:   Tue, 14 Mar 2017 12:17:06 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Kyle Huey <me@...ehuey.com>,
        "Robert O'Callahan" <robert@...llahan.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, 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>
Cc:     linux-kernel@...r.kernel.org,
        user-mode-linux-devel@...ts.sourceforge.net,
        user-mode-linux-user@...ts.sourceforge.net,
        linux-fsdevel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH v10 6/7] x86/arch_prctl: Add ARCH_[GET|SET]_CPUID

On 03/14/17 12:01, H. Peter Anvin 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.)
> 

Nevermind, I was accidentally looking at v10 not v15 of this patchset.
My bad.

	-hpa