Re: [PATCH v5] x86: use builtins to read eflags

[H. Peter Anvin]

On March 15, 2022 12:19:40 AM PDT, Bill Wendling <morbo@xxxxxxxxxx> wrote:
>On Mon, Mar 14, 2022 at 6:08 PM H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>>
>> My only concern is this: how does this affect the placement of these sequences in relation to the things they need to protect?
>>
>With clang (and I assume it's similar with gcc), the
>__builtin_ia32_readeflags_u{64|32} builtins specify that the EFLAGS
>register is used (and the stack pointer is modified). Instructions
>that may change EFLAGS are marked as defining EFLAGS, and clang and
>gcc are careful not to move instructions that access EFLAGS past them.
>
>One change you may see due to this patch is the compiler moving the
>"pop %..." instruction away from the "pushf" instruction. This could
>happen if the compiler determines that it could produce better code by
>doing so---e.g. to reduce register pressure. The "gcc -O2" example
>below shows this code movement.
>
>-bw
>
>> On March 14, 2022 4:07:23 PM PDT, Bill Wendling <morbo@xxxxxxxxxx> wrote:
>>>
>>> On Tue, Mar 1, 2022 at 12:19 PM Bill Wendling <morbo@xxxxxxxxxx> wrote:
>>>>
>>>>
>>> Bump for review.
>>>
>>> -bw
>>>
>>>>  This issue arose due to Clang's issue with the "=rm" constraint. Clang
>>>>  chooses to be conservative in these situations and always uses memory
>>>>  instead of registers, resulting in sub-optimal code. (This is a known
>>>>  issue, which is currently being addressed.)
>>>>
>>>>  This function has gone through numerous changes over the years:
>>>>
>>>>    - The original version of this function used the "=g" constraint,
>>>>      which has the following description:
>>>>
>>>>        Any register, memory or immediate integer operand is allowed,
>>>>        except for registers that are not general registers.
>>>>
>>>>    - This was changed to "=r" in commit f1f029c7bfbf ("x86: fix assembly
>>>>      constraints in native_save_fl()"), because someone noticed that:
>>>>
>>>>        the offset of the flags variable from the stack pointer will
>>>>        change when the pushf is performed. gcc doesn't attempt to
>>>>        understand that fact, and address used for pop will still be the
>>>>        same. It will write to somewhere near flags on the stack but not
>>>>        actually into it and overwrite some other value.
>>>>
>>>>    - However, commit f1f029c7bfbf ("x86: fix assembly constraints in
>>>>      native_save_fl()") was partially reverted in commit ab94fcf528d1
>>>>      ("x86: allow "=rm" in native_save_fl()"), because the original
>>>>      reporter of the issue was using a broken x86 simulator. The
>>>>      justification for this change was:
>>>>
>>>>        "=rm" is allowed in this context, because "pop" is explicitly
>>>>        defined to adjust the stack pointer *before* it evaluates its
>>>>        effective address, if it has one.  Thus, we do end up writing to
>>>>        the correct address even if we use an on-stack memory argument.
>>>>
>>>>  Clang generates good code when the builtins are used. On one benchmark,
>>>>  a hotspot in kmem_cache_free went from using 5.18% of cycles popping to
>>>>  a memory address to 0.13% popping to a register. This benefit is
>>>>  magnified given that this code is inlined in numerous places in the
>>>>  kernel.
>>>>
>>>>  The builtins also help GCC. It allows GCC (and Clang) to reduce register
>>>>  pressure and, consequently, register spills by rescheduling
>>>>  instructions. It can't happen with instructions in inline assembly,
>>>>  because compilers view inline assembly blocks as "black boxes," whose
>>>>  instructions can't be rescheduled.
>>>>
>>>>  Another benefit of builtins over asm blocks is that compilers are able
>>>>  to make more precise inlining decisions, since they no longer need to
>>>>  rely on imprecise measures based on newline counts.
>>>>
>>>>  A trivial example demonstrates this code motion.
>>>>
>>>>          void y(void);
>>>>          unsigned long x(void) {
>>>>                  unsigned long v = __builtin_ia32_readeflags_u64();
>>>>                  y();
>>>>                  return v;
>>>>          }
>>>>
>>>>  GCC at -O1:
>>>>          pushq   %rbx
>>>>          pushfq
>>>>          popq    %rbx
>>>>          movl    $0, %eax
>>>>          call    y
>>>>          movq    %rbx, %rax
>>>>          popq    %rbx
>>>>          ret
>>>>
>>>>  GCC at -O2:
>>>>          pushq   %r12
>>>>          pushfq
>>>>          xorl    %eax, %eax
>>>>          popq    %r12
>>>>          call    y
>>>>          movq    %r12, %rax
>>>>          popq    %r12
>>>>          ret
>>>>
>>>>  Link: https://gist.github.com/nickdesaulniers/b4d0f6e26f8cbef0ae4c5352cfeaca67
>>>>  Link: https://github.com/llvm/llvm-project/issues/20571
>>>>  Link: https://gcc.gnu.org/onlinedocs/gcc/Simple-Constraints.html#Simple-Constraints
>>>>  Link: https://godbolt.org/z/5n3Eov1xT
>>>>  Signed-off-by: Bill Wendling <morbo@xxxxxxxxxx>
>>>>  Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
>>>> ________________________________
>>>>  v5: - Incorporate Nick's suggestion to limit the change to Clang >= 14.0 and
>>>>        GCC.
>>>>  v4: - Clang now no longer generates stack frames when using these builtins.
>>>>      - Corrected misspellings.
>>>>  v3: - Add blurb indicating that GCC's output hasn't changed.
>>>>  v2: - Kept the original function to retain the out-of-line symbol.
>>>>      - Improved the commit message.
>>>>      - Note that I couldn't use Nick's suggestion of
>>>>
>>>>          return IS_ENABLED(CONFIG_X86_64) ? ...
>>>>
>>>>        because Clang complains about using __builtin_ia32_readeflags_u32 in
>>>>        64-bit mode.
>>>> ________________________________
>>>>   arch/x86/include/asm/irqflags.h | 10 ++++++++++
>>>>   1 file changed, 10 insertions(+)
>>>>
>>>>  diff --git a/arch/x86/include/asm/irqflags.h b/arch/x86/include/asm/irqflags.h
>>>>  index 87761396e8cc..2eded855f6ab 100644
>>>>  --- a/arch/x86/include/asm/irqflags.h
>>>>  +++ b/arch/x86/include/asm/irqflags.h
>>>>  @@ -19,6 +19,11 @@
>>>>   extern inline unsigned long native_save_fl(void);
>>>>   extern __always_inline unsigned long native_save_fl(void)
>>>>   {
>>>>  +#if defined(CC_IS_CLANG) && defined(UNWINDER_ORC) && CLANG_VERSION < 140000
>>>>  +       /*
>>>>  +        * Clang forced frame pointers via the builtins until Clang-14. Use
>>>>  +        * this as a fall-back until the minimum Clang version is >= 14.0.
>>>>  +        */
>>>>          unsigned long flags;
>>>>
>>>>          /*
>>>>  @@ -33,6 +38,11 @@ extern __always_inline unsigned long native_save_fl(void)
>>>>                       : "memory");
>>>>
>>>>          return flags;
>>>>  +#elif defined(CONFIG_X86_64)
>>>>  +       return __builtin_ia32_readeflags_u64();
>>>>  +#else
>>>>  +       return __builtin_ia32_readeflags_u32();
>>>>  +#endif
>>>>   }
>>>>
>>>>   static __always_inline void native_irq_disable(void)
>>>>  --
>>>>  2.35.1.574.g5d30c73bfb-goog
>>>>

EFLAGS is a mishmash of different things, only some of which are visible to the compiler, and the semantics of which are totally different.

Changing IF, changing DF, changing AC, and changing the arithmetic flags have *enormously* different properties. The compiler can't know how the semantics of a particular instance is, at least without being explicitly told (giving it a some kind of mask of bits that could change.) The memory barrier is needed for IF changes, for example.

This feels like "let's fix LLVM by hacking the kernel in dangerous ways" once again! We rely on "memory" as compiler barriers *all over the place*. This is especially so since this appears to be a suboptimal code generation issue and not a correctness issue; your change is likely to promote the former (underoptimizing) to the latter (overoptimizing.)

To whatever extent my vote matters these days:

Nacked-by: H. Peter Anvin (Intel) <hpa@xxxxxxxxx>