linux-kernel - Re: selftests/x86/fsgsbase

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Message-ID: <20180227225955.s2vs4g7oc7tdi72i@xps>
Date:   Tue, 27 Feb 2018 16:59:55 -0600
From:   Dan Rue <dan.rue@...aro.org>
To:     "H. Peter Anvin" <hpa@...or.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>, 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 Mon, Jan 29, 2018 at 10:30:05AM -0800, H. Peter Anvin wrote:
> 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.

Has there been any conclusion to this thread? I can still reproduce the
issue on mainline and next.

Thanks,
Dan