Messages in this thread | | | From | Andy Lutomirski <> | Date | Mon, 22 Feb 2016 14:55:50 -0800 | Subject | Re: [PATCH 08/10] x86/xsaves: Fix PTRACE frames for XSAVES |
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On Mon, Feb 22, 2016 at 2:53 PM, Dave Hansen <dave.hansen@linux.intel.com> wrote: > 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.
Some day I kind of want to delete all this xsave/xrstor directly on user buffers code. I've never been thrilled with the concept, and it has messy (although AFAICT not presently buggy [1]) interactions with context switches, and it can't run with preemption disabled because it can take page faults.
In the mean time, it's fine as far as I know, but maybe it would be cleaner if it used the software copy code. Or maybe we can change it later if a good reason shows up.
[1] actually there's a minor bug in the 32-bit compat code that Borislav has a patch for.
-- Andy Lutomirski AMA Capital Management, LLC
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