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Re: [Qemu-devel] [PATCH 10/34] linux-user: Support for restarting system


From: Peter Maydell
Subject: Re: [Qemu-devel] [PATCH 10/34] linux-user: Support for restarting system calls for Microblaze targets
Date: Thu, 3 Mar 2016 20:15:13 +0000

Hi Edgar -- I'm just looking back at these signal handling
race condition fix patches, and with this one I have a confusion
about the Microblaze Linux syscall code that I hope you can
clear up for me.

Looking at the kernel entry.S code it looks to me like
the way syscalls work on microblaze is:
 * syscall insn is brki r14
 * the insn itself saves the PC of the brki into r14
 * on entry the kernel advances r14 by 4 to skip the brki
 * then SAVE_REGS saves r14 into the 'PC' slot in the pt_regs
   struct
 * for syscall restart handle_restart() may wind the PC
   value in the pt_regs back by 4
 * in any case, on syscall exit we pull the PC value out of
   pt_regs into r14, and do a return with rtbd r14, 0

I think what this implies is that:
 * r14 is a "used by the kernel, may be corrupted at any
   time, not to be touched by userspace" register
 * on exit from a syscall PC and r14 are always the same
 * this includes do_sigreturn, ie "taking a signal" is one
   of the things that can corrupt r14

Is that right?

(For context, the original patch is this one:
http://patchwork.ozlabs.org/patch/514879/
and I now suspect my review comments at the time to be wrong.)

thanks
-- PMM



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