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Subject: Re: [PATCH 08/10] x86/xsaves: Fix PTRACE frames for XSAVES
To: Andy Lutomirski <luto@amacapital.net>, Yu-cheng Yu <yu-cheng.yu@intel.com>
References: <1456167621-3974-1-git-send-email-yu-cheng.yu@intel.com>
 <CALCETrUSQB2FG4HMS=uKaK2cgMxd_X9pg4W4UjUz=nvF0JNFpA@mail.gmail.com>
Cc: X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@suse.de>,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Fenghua Yu <fenghua.yu@intel.com>
From: Dave Hansen <dave.hansen@linux.intel.com>
Message-ID: <56CB914E.3040406@linux.intel.com>
Date: Mon, 22 Feb 2016 14:53:02 -0800
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On 02/22/2016 02:45 PM, Andy Lutomirski wrote:
>> +/*
>> > + * Convert from kernel XSAVES compacted format to standard format and copy
>> > + * to a ptrace buffer. It supports partial copy but pos always starts from
>> > + * zero. This is called from xstateregs_get() and there we check the cpu
>> > + * has XSAVES.
>> > + */
>> > +int copyout_from_xsaves(unsigned int pos, unsigned int count, void *kbuf,
>> > +                       void __user *ubuf, const struct xregs_state *xsave)
> Now that you've written this code, can it be shared with the signal
> handling code?

It could be.  But the signal handler code has the advantage of already
having the data in the registers since it's running on its *own* FPU
state, so it can just call XSAVE(S) directly.

This ptrace code *could* do a kernel_fpu_begin(), XRSTOR the user buffer
into the registers, XRSTOR the ptracee's system state in to the
registers, then XSAVES the whole thing to the kernel buffer, then
kernel_fpu_end().

Or, we could remove the signal handler's ability to XSAVE directly to
userspace.  But it already *had* that and we know it works.