Andrew:
I agree this is a generic feature. Your change is OK. If we add it, I think
we should apply them for all similar functions, and apply them in MSFT and GCC
assembly code both.
So far, I find the following APIs have the similar usage.
SwitchStack()
AsmEnablePaging32()
AsmEnablePaging64()
AsmDisablePaging32()
AsmDisablePaging64()
Thanks
Liming
From: Andrew Fish [mailto:[email protected]]
Sent: Friday, June 20, 2014 11:57 PM
To: [email protected]
Subject: Re: [edk2] MdePkg maintainer: We need InternalX86EnablePaging64() to
work like SwitchStack CpuExceptionHandlerLib
On Jun 20, 2014, at 3:15 AM, Gao, Liming
<[email protected]<mailto:[email protected]>> wrote:
Andrew:
Seemly, this is a debug feature. Which debugger supports it? Or, this is a
generic function (stop walking the frame) in debugger?
Xcode clang/lldb use this. This is what the C runtime does on OS X.
The compiler maintains a frame pointer so you can unwind the stack without
symbols.
pushq %rbp
movq %rsp, %rbp
...
popq %rbp
ret
If you know the PC and frame pointer (RBP) you can walk the stack, as the stack
has the PC and FP of the caller. Hitting a return address of zero is used by
the algorithm to terminate the stack walk. If you don't hit the zero the stack
walk sees the return address from the call and keeps going. In any frame
pointer scheme you need a way to stop, and this is usually hitting a PC of
zero. So I think this is a generic feature.
This means that a stack walk is possible without the debug information. For
example it is possible for a CpuExceptionHandlerLib to print the complete stack
frame on an exception, and lookup the module name, and offset in the module of
the PC for the entire frame. This means you get a full stack trace printed out
for every ASSERT()! We have an lldb script that walks the frame and loads
symbols for only the modules in the frame. It is also a useful feature for
profiling, and leak detection.
You can only walk the stack in Visual Studio if you have symbols. I'm guessing
that the stack unwind just stops when you hit an address that does not have
symbols.
I assume there is a flag in GCC to emit a frame pointer for X64, so this would
be useful in that case too.
Thanks,
Andrew Fish
~/work/Compiler>cat a.c
int
main ()
{
return 0;
}
~/work/Compiler>clang -S -Os a.c
~/work/Compiler>cat a.S
.section __TEXT,__text,regular,pure_instructions
.globl _main
_main: ## @main
.cfi_startproc
## BB#0:
pushq %rbp
Ltmp2:
.cfi_def_cfa_offset 16
Ltmp3:
.cfi_offset %rbp, -16
movq %rsp, %rbp
Ltmp4:
.cfi_def_cfa_register %rbp
xorl %eax, %eax
popq %rbp
ret
Thanks
Liming
From: Andrew Fish [mailto:[email protected]]
Sent: Wednesday, June 18, 2014 10:55 PM
To: [email protected]<mailto:[email protected]>
Subject: [edk2] MdePkg maintainer: We need InternalX86EnablePaging64() to work
like SwitchStack
InternalX86EnablePaging64() does a call to the new 64-bit entry point. This
call breaks our debugger stack walking code. We need it to be a push $0/jmp. In
place of a call. The push $0/jmp makes the return address from the call be 0,
and this tells the debugger to stop walking the frame. In clang and some
flavors of GCC all C functions spill enough state to the stack so that you can
walk the stack via an algorithm (on VC++ for X64 you need symbols).
If this change is OK with folks I can submit a patch.
https://svn.code.sf.net/p/edk2/code/trunk/edk2/MdePkg/Library/BaseLib/X64/SwitchStack.S
#
# Reserve space for register parameters (rcx, rdx, r8 & r9) on the stack,
# in case the callee wishes to spill them.
#
lea -0x20(%r9), %rsp
pushq $0 // stop gdb stack unwind
jmp *%rax // call EntryPoint ()
https://svn.code.sf.net/p/edk2/code/trunk/edk2/MdePkg/Library/BaseLib/Ia32/EnablePaging64.S
.byte 0x48
addl $0x-0x20, %esp # add rsp, -20h
call *%ebx # call rbx
jmp . # no one should get here
So what we would like to see is:
.byte 0x48
addl $0x-0x20, %esp # add rsp, -20h
push $0 // stop gdb stack unwind
jmp *%ebx // call EntryPoint ()
Thanks,
Andrew Fish
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