linux-kernel - Re: [PATCH] x86/cpu/vmware: use the full form of inl in VMWARE

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Message-ID: <CAKwvOd=hjxJYhU=C4wqkKKnLwuQjjL=wPAh6uBj-M8r8AtDdFA@mail.gmail.com>
Date:   Mon, 7 Oct 2019 12:57:42 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Thomas Hellstrom <thellstrom@...are.com>, pv-drivers@...are.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Kees Cook <keescook@...omium.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        virtualization@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] x86/cpu/vmware: use the full form of inl in VMWARE_PORT

On Mon, Oct 7, 2019 at 12:21 PM 'Sami Tolvanen' via Clang Built Linux
<clang-built-linux@...glegroups.com> wrote:
>
> LLVM's assembler doesn't accept the short form inl (%%dx) instruction,
> but instead insists on the output register to be explicitly specified:
>
>   <inline asm>:1:7: error: invalid operand for instruction
>           inl (%dx)
>              ^
>   LLVM ERROR: Error parsing inline asm
>
> Use the full form of the instruction to fix the build.
>
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>

Thanks Sami, this looks like it addresses:
Link: https://github.com/ClangBuiltLinux/linux/issues/734
Looks like GAS' testsuite has some cases where the second operand is
indeed implicitly %eax if unspecified. (This should still be fixed in
Clang).
Just to triple check that they're equivalent:
$ cat inl.s
  inl (%dx)
  inl (%dx), %eax
$ as inl.s
$ objdump -d a.out

a.out:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <.text>:
   0: ed                    in     (%dx),%eax
   1: ed                    in     (%dx),%eax

Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

> ---
>  arch/x86/kernel/cpu/vmware.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/cpu/vmware.c b/arch/x86/kernel/cpu/vmware.c
> index 9735139cfdf8..46d732696c1c 100644
> --- a/arch/x86/kernel/cpu/vmware.c
> +++ b/arch/x86/kernel/cpu/vmware.c
> @@ -49,7 +49,7 @@
>  #define VMWARE_CMD_VCPU_RESERVED 31
>
>  #define VMWARE_PORT(cmd, eax, ebx, ecx, edx)                           \
> -       __asm__("inl (%%dx)" :                                          \
> +       __asm__("inl (%%dx), %%eax" :                                   \
>                 "=a"(eax), "=c"(ecx), "=d"(edx), "=b"(ebx) :            \
>                 "a"(VMWARE_HYPERVISOR_MAGIC),                           \
>                 "c"(VMWARE_CMD_##cmd),                                  \

-- 
Thanks,
~Nick Desaulniers