On Tue, Jan 15, 2013 at 4:34 PM, David Daney <ddaney.cavm@xxxxxxxxx> wrote: > On 01/15/2013 01:07 PM, Steven Rostedt wrote: >> >> On Tue, 2013-01-15 at 09:55 -0800, David Daney wrote: >> >>>> There's nothing that states what the ftrace caller must be. We can have >>>> it do a proper stack update. That is, only at boot up do we need to >>>> handle the defined mcount. After that, those instructions are just place >>>> holders for our own algorithms. If the addiu was needed for the defined >>>> mcount, there's no reason to keep it for our own ftrace_caller. >>>> >>>> Would that work? >>> >>> >>> ... either do as you suggest and dynamically change the ABI of the >>> target function. >> >> >> We already change the ABI. We have it call ftrace_caller instead of >> mcount. >> >> BTW, I've just compiled with gcc 4.6.3 against mips, and I don't see the >> issue. I have: >> >> 0000000000000000 <account_kernel_stack>: >> 0: 03e0082d move at,ra >> 4: 0c000000 jal 0 <account_kernel_stack> >> 4: R_MIPS_26 _mcount >> 4: R_MIPS_NONE *ABS* >> 4: R_MIPS_NONE *ABS* >> 8: 0000602d move t0,zero >> c: 2402000d li v0,13 >> 10: 3c030000 lui v1,0x0 >> 10: R_MIPS_HI16 mem_section >> 10: R_MIPS_NONE *ABS* >> 10: R_MIPS_NONE *ABS* >> 14: 000216fc dsll32 v0,v0,0x1b >> 18: 64630000 daddiu v1,v1,0 >> >> Is it dependent on the config? > > > Yes. > > You need to select a 32-bit kernel (which in turn may require selecting a > board type that also supports it). > > The ABI is different for 32-bit and 64-bit _mcount. > > David Daney > Building for MIPS malta will show the problem. > > >> >>> >>> Or add support to GCC for a better tracing ABI (as I already said we did >>> for mips64). >> >> >> I wouldn't waste time changing gcc for this. If you're going to change >> gcc than please implement the -mfentry option. Look at x86_64 to >> understand this more. > > > A good point. But I don't really plan on doing any work related to 32-bit > mips things at this point, so any such change would have to be done by > someone else. > > David Daney > I love the idea of removing the useless stack adjust stuff at run time! The issue still remains for the initial writing of the 2 nops. It looks like the initial call to write the nops is done from ftrace_init which is called before SMP is up, so if I write the 2 nops via a single call to a function with interrupts disabled it should be safe. I also need to do this for modules at insmod time. This has been GREAT feedback! Thanks
