Re: [PATCH v2 0/2] Add volatile for mNumberToFinish

["Kinney, Michael D" <michael.d.kinney@intel.com>]

Hi Andrew,

Thanks for all the examples!

I have done a more detailed review of the BaseSynchronizationLib.  I do not think there are any missing memory barriers in the implementation.

I have also experimented with adding volatile qualifier to parameters in the Interlocked*() functions, and for the builds I have done so far, that appears to be a compatible change.  I did have to update the internal functions in that lib to also consistently use volatile, but that makes sense based on the ANSI C spec items that Laszlo quoted.

I have also been looking at Jeff's patch to CpuS3.c to add volatile to the mNumberToFinish global variable, and I have a slightly different patch I have evaluated to avoid the type case from a volatile variable to a non-volatile variable in all cases.  It does require that the calls to SwitchStack() and DisablePaging64() treat the parameter as a UINTN address of the semaphore, instead of a volatile pointer to the semaphore.

I will send some patches soon for review.

Mike

From: afish@apple.com [mailto:afish@apple.com]
Sent: Wednesday, November 16, 2016 1:14 PM
To: Laszlo Ersek <lersek@redhat.com>
Cc: Kinney, Michael D <michael.d.kinney@intel.com>; Fan, Jeff <jeff.fan@intel.com>; edk2-devel@ml01.01.org; Paolo Bonzini <pbonzini@redhat.com>; Tian, Feng <feng.tian@intel.com>; Yao, Jiewen <jiewen.yao@intel.com>
Subject: Re: [edk2] [PATCH v2 0/2] Add volatile for mNumberToFinish


On Nov 16, 2016, at 11:21 AM, Laszlo Ersek <lersek@redhat.com<mailto:lersek@redhat.com>> wrote:

On 11/16/16 19:18, Kinney, Michael D wrote:

Laszlo,

Thanks for the details from and ANSI C spec.

For this compiler issue, are there more details on the
assembly code generated by the GCC 5.4 compiler in the
failing mode?

Unfortunately, I have little first-hand experience with this kind of
failure, I've just read different accounts of it. Perhaps Paolo can
comment -- my experience with contacting GCC maintainers directly hasn't
been stellar.


If it reproduces in clang I've had a lot of luck getting help from those folks.

In general the best thing to do if you think you have a compiler bug it to write a small command line program that reproduces the issues. If it is a code generation issue, you don't even need to make the code link or run, you can just compile the code directly to assembler and write up in the bug report why you think the code generation is in error. Basically like the examples I post to this list from time to time. Well unless it is a link time optimization issue...


I also see Liming's observation that the internal
implementation of the SynchronizationLib adds volatile
in some internal worker functions.  Other implementation
artifacts include the use of a read/write memory barrier
to prevent the optimizing compiler from optimizing
across a boundary.  These read/write barriers are used
in the spin lock functions, but not the Interlocked*()
functions.

I want to make sure we have studied the code generation
to see if the issue is related to volatile or a read/write
memory barrier.  It could be adding volatile forced the
compiler to internally add read/write barrier.

In my understanding, MemoryFence()
[MdePkg/Library/BaseLib/X64/GccInline.c] is just a compiler-level
barrier, not a processor level barrier.

 __asm__ __volatile__ ("":::"memory");

Processor level barriers shouldn't be a problem here; as far as I recall
from the SDM from a few days ago, the XCHG instruction "engages the
CPU's locking protocol even without the LOCK prefix", plus I see an
explicit LOCK in InternalSyncDecrement()
[MdePkg/Library/BaseSynchronizationLib/X64/GccInline.c] for example.

I'm pretty sure the compiler opimized away the access because it saw no
intervening access (it doesn't know about the code running on other
CPUs). I think "volatile" and MemoryFence() -- with the latter telling
GCC that all memory should be considered clobbered -- should fix the
issue just the same, in this situation.


There is a good chance it is similar to my example from my previous email when the return value is a constant since the compiler wrote the value and remember the value it wrote. You will need to make the global volatile or place a barrier between the write and the read to tell the compiler what optimizations are legal.

There can also be other insidious things in place that hide bugs. For example in this code:

  mNumberToFinish = mAcpiCpuData.NumberOfCpus - 1;
  mExchangeInfo->ApFunction  = (VOID *) (UINTN) EarlyMPRendezvousProcedure;

  //
  // Send INIT IPI - SIPI to all APs
  //
  SendInitSipiSipiAllExcludingSelf ((UINT32)mAcpiCpuData.StartupVector);

  while (mNumberToFinish > 0) {
    CpuPause ();
  }

What if some versions of SendInitSipiSipiAllExcludingSelf() have side effects that make it hard to optimize and some do not. You could get a different result...

Anyway I think this optimization is more subtle than my previous examples. If you just compile this code to assembler it looks correct (you see a loop). But if you do link time optimization the compiler will make more assumptions about how mNumberToFinish is used globally and be more aggressive about optimizing it  out. You can get the same behavior from the assembler generation if you make mNumberToFinish a static local.

~/work/Compiler>cat barriers.c

#define _ReadWriteBarrier() do { __asm__ __volatile__ ("": : : "memory"); } while(0)

unsigned int mNumberToFinish = 0;

void SendInit2()
{
   *(int *)0x12345678 = 0;
}

int main()
{
  mNumberToFinish = 3;

  SendInit2();
  while (mNumberToFinish > 0) {
    ;
  }

  return 0;
}


~/work/Compiler>clang -Os -flto  -g barriers.c
~/work/Compiler>lldb a.out
(lldb) target create "a.out"
Current executable set to 'a.out' (x86_64).
(lldb) dis -n main -b
a.out`main:
a.out[0x100000fa7] <+0>:  55                                pushq  %rbp
a.out[0x100000fa8] <+1>:  48 89 e5                          movq   %rsp, %rbp
a.out[0x100000fab] <+4>:  c7 04 25 78 56 34 12 00 00 00 00  movl   $0x0, 0x12345678
a.out[0x100000fb6] <+15>: eb fe                             jmp    0x100000fb6               ; <+15> at barriers.c:16


~/work/Compiler>clang -Os -S  barriers.c
~/work/Compiler>cat barriers.s

             .globl       _SendInit2
_SendInit2:                             ## @SendInit2
             pushq       %rbp
             movq       %rsp, %rbp
             movl        $0, 305419896
             popq        %rbp
             retq


             .globl       _main
_main:                                  ## @main
             pushq       %rbp
             movq       %rsp, %rbp
             movl        $3, _mNumberToFinish(%rip)
             xorl         %eax, %eax
             movl        %eax, 305419896
             cmpl        %eax, _mNumberToFinish(%rip)
             je             LBB1_2
LBB1_1:                                 ## =>This Inner Loop Header: Depth=1
             jmp          LBB1_1
LBB1_2:
             xorl         %eax, %eax
             popq        %rbp
             retq

             .globl       _mNumberToFinish        ## @mNumberToFinish
.zerofill __DATA,__common,_mNumberToFinish,4,2


Live in fear of the optimizer as it is getting smarter every day. Sorry if I picked the wrong bit of code as the example.

Thanks,

Andrew Fish

PS This is a good example of how barriers don't do what you may think. Notice how the barrier only adds a single read and then a return or an infinite loop. It makes sense if you think about it.... After the barrier the compiler is clueless as to what is going on so it reads the global. Given the global can't change it decides to return or fall into the infinite loop. This is really good example of the difference between a barrier and a volatile variable. This really highlights that barrier is a one shot vs. the volatile is a property for the variable.

~/work/Compiler>cat barriers.c

#define _ReadWriteBarrier() do { __asm__ __volatile__ ("": : : "memory"); } while(0)

unsigned int mNumberToFinish = 0;

void SendInit2()
{
   *(int *)0x12345678 = 0;
}

int main()
{
  mNumberToFinish = 3;

  SendInit2();
  _ReadWriteBarrier();
  while (mNumberToFinish > 0) {
    ;
  }

  return 0;
}


~/work/Compiler>clang -Os -flto  -g barriers.c
~/work/Compiler>lldb a.out
(lldb) target create "a.out"
Current executable set to 'a.out' (x86_64).
(lldb) dis -n main -b
a.out`main:
a.out[0x100000f8d] <+0>:  55                                pushq  %rbp
a.out[0x100000f8e] <+1>:  48 89 e5                          movq   %rsp, %rbp
a.out[0x100000f91] <+4>:  c6 05 68 00 00 00 01              movb   $0x1, 0x68(%rip)
a.out[0x100000f98] <+11>: c7 04 25 78 56 34 12 00 00 00 00  movl   $0x0, 0x12345678
a.out[0x100000fa3] <+22>: 0f b6 05 56 00 00 00              movzbl 0x56(%rip), %eax          ; mNumberToFinish
a.out[0x100000faa] <+29>: 83 e0 01                          andl   $0x1, %eax
a.out[0x100000fad] <+32>: 83 f8 01                          cmpl   $0x1, %eax
a.out[0x100000fb0] <+35>: 75 02                             jne    0x100000fb4               ; <+39> at barriers.c:21
a.out[0x100000fb2] <+37>: eb fe                             jmp    0x100000fb2               ; <+37> at barriers.c:17
a.out[0x100000fb4] <+39>: 31 c0                             xorl   %eax, %eax
a.out[0x100000fb6] <+41>: 5d                                popq   %rbp
a.out[0x100000fb7] <+42>: c3                                retq





Also, given the implementation I see in the SynchronizationLib
I am not sure the cast to (UINT32 *) is required in the
proposed patch.  Do we get compiler warnings/errors if those
casts are not included?

Yes.

Passing an argument to a function whose prototype is visible invokes
type conversion just like in assignment:

 6.5.2.2 Function calls

 7 If the expression that denotes the called function has a type that
   does include a prototype, the arguments are implicitly converted,
   as if by assignment, to the types of the corresponding parameters,
   taking the type of each parameter to be the unqualified version
   of its declared type. [...]

(IMPORTANT: the de-qualification mentioned above refers to the pointer
object itself, not to the pointed-to object! It just means if you have a
function parameter "const long x", and pass the argument 5 (which has
type int), then it is converted to "long", and not "const long".)

So, passing a (volatile UINT32 *) argument to a (UINT32 *) parameter is
the same as the similar assignment

 volatile UINT32 *x;
 UINT32 *y;
 y = x;

Then, regarding assignment, we have

 6.5.16.1 Simple assignment

 Constraints

 1 One of the following shall hold:

   [...]
   - both operands are pointers to qualified or unqualified versions
     of compatible types, and the type pointed to by the left has all
     the qualifiers of the type pointed to by the right;
   [...]

Finally we have diagnostics,

 5.1.1.3 Diagnostics

 1 A conforming implementation shall produce at least one diagnostic
   message (identified in an implementation-defined manner) if a
   preprocessing translation unit or translation unit contains a
   violation of any syntax rule or constraint, even if the behavior is
   also explicitly specified as undefined or implementation-defined.
   [...]

In summary, without adding the explicit cast, we break the constraint on
simple assignment, which requires the compiler to emit a diagnostic message.

(In turn if we add the explicit cast, we break other requirements, as
discussed in my other email -- and here we should investigate whether we
care about breaking that exact rule.)


The second topic is what the SynchronizationLib API interfaces
should have been from the beginning.  In retrospect, I think
they should have been defined with volatile pointers.

I agree!


The spin
lock APIs do use volatile pointers, but that is embedded in the
typedef for SPIN_LOCK, so it is not as obvious.

Thanks
Laszlo


-----Original Message-----
From: Laszlo Ersek [mailto:lersek@redhat.com]
Sent: Tuesday, November 15, 2016 8:10 AM
To: Fan, Jeff <jeff.fan@intel.com<mailto:jeff.fan@intel.com>>; edk2-devel@ml01.01.org<mailto:edk2-devel@ml01.01.org>
Cc: Paolo Bonzini <pbonzini@redhat.com<mailto:pbonzini@redhat.com>>; Yao, Jiewen <jiewen.yao@intel.com<mailto:jiewen.yao@intel.com>>; Tian,
Feng <feng.tian@intel.com<mailto:feng.tian@intel.com>>; Kinney, Michael D <michael.d.kinney@intel.com<mailto:michael.d.kinney@intel.com>>
Subject: Re: [PATCH v2 0/2] Add volatile for mNumberToFinish

Jeff,

On 11/15/16 15:08, Jeff Fan wrote:

v2:
 Add patch #1 per Laszlo's comments
 at https://lists.01.org/pipermail/edk2-devel/2016-November/004697.html

About the comments updated SynchronizationLib to add volatile for
input parameter, I will send in another serial patches.

Cc: Paolo Bonzini <pbonzini@redhat.com<mailto:pbonzini@redhat.com>>
Cc: Laszlo Ersek <lersek@redhat.com<mailto:lersek@redhat.com>>
Cc: Jiewen Yao <jiewen.yao@intel.com<mailto:jiewen.yao@intel.com>>
Cc: Feng Tian <feng.tian@intel.com<mailto:feng.tian@intel.com>>
Cc: Michael D Kinney <michael.d.kinney@intel.com<mailto:michael.d.kinney@intel.com>>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jeff Fan <jeff.fan@intel.com<mailto:jeff.fan@intel.com>>

Jeff Fan (2):
 UefiCpuPkg/PiSmmCpuDxeSmm: Add volatile for parameter NumberToFinish
 UefiCpuPkg/PiSmmCpuDxeSmm: Add volatile for mNumberToFinish

UefiCpuPkg/PiSmmCpuDxeSmm/CpuS3.c             | 4 ++--
UefiCpuPkg/PiSmmCpuDxeSmm/Ia32/SmmFuncsArch.c | 4 ++--
UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.h    | 2 +-
UefiCpuPkg/PiSmmCpuDxeSmm/X64/SmmFuncsArch.c  | 2 +-
4 files changed, 6 insertions(+), 6 deletions(-)

if you want to keep GCC5.4 from optimizing away the access, the
synchronization object itself, and all pointers  to it must remain
volatile. Wherever you cast away the volatile qualifier, for example in
a function call, GCC can break code on the next level, even if you don't
actually access the object through that pointer (i.e., if you cast the
pointer back to volatile just in time for the access).

So, the safe way to go about this is to change function prototypes from
callee towards callers -- first change the callee (because both volatile
and non-volatile can be accepted as volatile), then change the caller
(make sure what you pass in is volatile, and propagate it outwards).

It is also okay to convert the original volatile pointer to UINTN, and
to pass it to assembly code like that, or to convert it back to a
volatile pointer from UINTN before use.

>From the C99 standard:

6.3 Conversions
6.3.2.3 Pointers

 2 For any qualifier q, a pointer to a non-q-qualified type may be
   converted to a pointer to the q-qualified version of the type; the
   values stored in the original and converted pointers shall compare
   equal.

 5 An integer may be converted to any pointer type. Except as
   previously specified, the result is implementation-defined, might
   not be correctly aligned, might not point to an entity of the
   referenced type, and might be a trap representation.

 6 Any pointer type may be converted to an integer type. Except as
   previously specified, the result is implementation-defined. If the
   result cannot be represented in the integer type, the behavior is
   undefined. The result need not be in the range of values of any
   integer type.

6.7.3 Type qualifiers, paragraph 5:

   If an attempt is made to modify an object defined with a
   const-qualified type through use of an lvalue with
   non-const-qualified type, the behavior is undefined. If an attempt
   is made to refer to an object defined with a volatile-qualified
   type through use of an lvalue with non-volatile-qualified type, the
   behavior is undefined.

In summary:

- casting away "volatile" even just temporarily (without actual
 accesses) may give gcc license to break the code (6.3.2.3 p2)

- accessing without volatile is known to break the code (6.7.3 p5)

- you can always cast from non-volatile to volatile (6.3.2.3 p2),
 but not the other way around!

- you can cast from (volatile VOID *) to UINTN and back as much as
 you want (6.3.2.3 p5 p6), because our execution environment makes
 that safe ("implementation-defined")

We might want to play loose with 6.3.2.3 p2 -- that is, cast away
volatile from the pointer only temporarily, and cast it back just before
accessing the object through the pointer --, but that could be unsafe in
the long term. The *really* safe method is to cast it to UINTN, and then
back the same way.

Yes, this would affect functions like SwitchStack() too -- the Context1
and Context2 parameters would have to change their types to UINTN.

I think what we should discuss at this point is whether we'd like to
care about 6.3.2.3 p2; that is, whether we consider casting away
volatile temporarily.

The direction I've been experiencing with GCC is that its optimization
methods are becoming more aggressive. For some optimizations, there are
flags that disable them; I'm not sure they provide a specific flag for
preventing GCC from exploiting violations of 6.3.2.3 p2.

Thanks
Laszlo

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