linux-kernel - Re: selftests/x86/fsgsbase

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Message-ID: <0b0a3cea-d951-d3be-16de-a0e9d50cb2ba@zytor.com>
Date:   Mon, 29 Jan 2018 10:30:05 -0800
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Dan Rue <dan.rue@...aro.org>, Shuah Khan <shuah@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Dmitry Safonov <dsafonov@...tuozzo.com>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: selftests/x86/fsgsbase_64 test problem

On 01/29/18 10:26, Andy Lutomirski wrote:
>>>
>>> That will utterly suck on non-UMIP machines that have
>>> hypervisor-provided UMIP emulation.
>>
>> Is that a valid thing to optimize for, especially given that paranoid
>> entries aren't the most common anyway?
> 
> A bunch of people seem to care about NMI performance for perf.
>

That wasn't really the question...

> And the current patch set works without this trick.

But I believe the tricks it uses are fragile.

> FWIW, if we switch all entries to the entry text trampoline, we get direct percpu access for free.

That might be a better option.

	-hpa