linux-kernel - Re: um: PTRACE_SETREGSET failure with XSTATE on Kabylake CPU

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Message-Id: <D352734E-E991-4338-A5B3-94E840039841@m3y3r.de>
Date:   Tue, 20 Jun 2017 22:13:29 +0200
From:   Thomas Meyer <thomas@...3r.de>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:     Richard Weinberger <richard@....at>, elicooper@....com,
        linux-kernel@...r.kernel.org,
        "open list:USER-MODE LINUX (UML)" 
        <user-mode-linux-devel@...ts.sourceforge.net>,
        linux-x86_64@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: um: PTRACE_SETREGSET failure with XSTATE on Kabylake CPU


> Am 20.06.2017 um 21:53 schrieb Yu-cheng Yu <yu-cheng.yu@...el.com>:
> 
>> On Tue, 2017-06-20 at 20:59 +0200, Richard Weinberger wrote:
>> Yu-cheng,
>> 
>>> Am 20.06.2017 um 20:17 schrieb Richard Weinberger:
>>> Yu-cheng,
>>> 
>>> Am 20.06.2017 um 20:04 schrieb Yu-cheng Yu:
>>>>>> So to summarize:
>>>>>> 
>>>>>> - PTRACE_GETREGSET with NT_X86_XSTATE gets 832 and return 832, with no
>>>>>> error.
>>>>>> 
>>>>>> - PTRACE_SETREGSET get 832 (sizeof struct _xstate) but wants at least
>>>>>> 1088, otherwise it will fail with -EFAULT (why not -EINVAL?)
>>>>>> 
>>>>>> Ideas?
>>>> 
>>>> We considered allowing a partial XSAVE buffer for PTRACE_SETREGSET, but
>>>> it was that the XSAVE instruction requires a full-size buffer led to
>>>> this choice.  Using a smaller buffer for XSAVE causes a fault.
>>> 
>>> So, this code is not supposed to work?
>>> 
>>> iov.iov_base = fp_regs;
>>> iov.iov_len = sizeof(struct _xstate);
>>> ptrace(PTRACE_GETREGSET, pid, NT_X86_XSTATE, &iov);
>>> ptrace(PTRACE_SETREGSET, pid, NT_X86_XSTATE, &iov);
>>> 
>>> This is what UML does and on Thomas's new Laptop PTRACE_SETREGSET is failing.
>> 
>> Hmm, I think we need to do what gdb does, it uses a buffer of size X86_XSTATE_MAX_SIZE.
> 
> Linux kernel determines XSAVE buffer size from CPUID:
> http://elixir.free-electrons.com/linux/latest/source/arch/x86/kernel/fpu/xstate.c#L626

Hi,

Is there a user space API to get this value?

> 
> GDB has a fixed X86_XSTATE_MAX_SIZE of 2688.  That can become an issue.
> 
> Yu-cheng
> 
> 

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