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Date:   Sun, 10 May 2020 17:48:42 -0700
From:   Andi Kleen <ak@linux.intel.com>
To:     Vegard Nossum <vegard.nossum@oracle.com>
Cc:     Sasha Levin <sashal@kernel.org>, linux-kernel@vger.kernel.org,
        tglx@linutronix.de, luto@kernel.org, tony.luck@intel.com,
        chang.seok.bae@intel.com
Subject: Re: [PATCH v10 00/18] Enable FSGSBASE instructions
Message-ID: <20200511004842.GJ3538@tassilo.jf.intel.com>
References: <20200423232207.5797-1-sashal@kernel.org>
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> [ 6402.786418] ------------[ cut here ]------------
> [ 6402.787769] WARNING: CPU: 0 PID: 13802 at arch/x86/kernel/traps.c:811
> do_debug+0x16c/0x210
...

> [ 6402.848299]  ? trace_hardirqs_off_thunk+0x1a/0x33
> [ 6402.849593]  trace_hardirqs_off_caller+0xa6/0xd0
> [ 6402.850862]  ? debug+0x4e/0x70
> [ 6402.851727]  trace_hardirqs_off_thunk+0x1a/0x33
> [ 6402.852983]  debug+0x53/0x70
> [ 6402.853785] RIP: 0033:0x400060dd

So this is a check that checks if you're running in user mode if
you have a debug trap with single step, but somehow it triggered
for a user segment.

Probably the regs got corrupted. 

Sasha, I suspect you're missing a mov %rsp,%rdi somewhere in the
debug entry path that sets up the regs argument for the C code.

-Andi