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Date:   Mon, 13 Feb 2017 14:18:36 -0800
To:     Andy Lutomirski <>,
        Dmitry Safonov <>
CC:     "" <>,
        Dmitry Safonov <>,
        Shuah Khan <>,
        Thomas Gleixner <>,
        Ingo Molnar <>,
        Andy Lutomirski <>,
        Borislav Petkov <>, X86 ML <>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
Subject: Re: [PATCHv3] x86/selftests: add clobbers for int80 on x86_64

On February 13, 2017 11:06:04 AM PST, Andy Lutomirski <> wrote:
>On Mon, Feb 13, 2017 at 2:13 AM, Dmitry Safonov
><> wrote:
>> Kernel erases R8..R11 registers prior returning to userspace
>> from int80:
>> GCC can reuse this registers and doesn't expect them to change
>> during syscall invocation. I met this kind of bug in CRIU once
>> gcc 6.1 and clang stored local variables in those registers
>> and the kernel zerofied them during syscall:
>> By that reason I suggest to add those registers to clobbers
>> in selftests.  Also, as notted by Andy - removed unneeded clobber
>> for flags in int 0x80 inline asm.
>Acked-by: Andy Lutomirski <>
>Although these days we could preserve these regs in the kernel at the
>cost of (I think) 4 bytes of text and zero runtime overhead.

If we can do so that cheaply I think we should.
Sent from my Android device with K-9 Mail. Please excuse my brevity.

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