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Subject: Re: selftests/x86/fsgsbase_64 test problem
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Dan Rue <dan.rue@linaro.org>, Shuah Khan <shuah@kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Dmitry Safonov <dsafonov@virtuozzo.com>,
        Borislav Petkov <bp@alien8.de>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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From:   "H. Peter Anvin" <hpa@zytor.com>
Message-ID: <9630de94-83ba-85ad-c5f3-a515f7aa7243@zytor.com>
Date:   Fri, 26 Jan 2018 14:51:19 -0800
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On 01/26/18 11:46, Andy Lutomirski wrote:
> 
> Under QEMU TCG, gs stays equal to 1.  On native or KVM, on Skylake, it
> changes to 0.
> 
> On KVM or native, I do not observe do_debug getting called with %gs ==
> 1.  On TCG, I do.  I don't think that's precisely the problem that's
> causing the test to fail, since the test doesn't use TF or ptrace, but
> I wouldn't be shocked if it's related.
> 
> hpa, any insight?
> 
> (NB: if you want to play with this as I've described it, you may need
> to make invalid_selector() in ptrace.c always return false.  The
> current implementation is too strict and causes problems.)
> 

Looking at it.  I want to grok this in the general context of fsgsbase
as well, of course.

	-hpa