Re: [Qemu-devel] [PATCH] kvm-unittest: fix build with gcc 4.3.X and older

[Paolo Bonzini <pbonzini@redhat.com>]

Il 17/10/2013 08:27, Gleb Natapov ha scritto:
> On Wed, Oct 16, 2013 at 10:46:53PM +0300, Michael S. Tsirkin wrote:
>> Old GCC didn't let you reference variable by
>> number if it is listed with a specific register
>> constraint, on the assumption you can just
>> use the register name explicitly.
>>
>> Build fails with errors like this:
>> a.c:6: error: invalid 'asm': invalid operand code 'd'
>>
> Is it worth to support such ancient compiler? Nobody complained till
> now. BTW with your patch I still cannot compile with 4.2:
> 
> x86/s3.c: In function 'main':
> x86/s3.c:145: error: inconsistent operand constraints in an 'asm'
> 
>> To fix, let's just use %eax %al etc.
>>
> Only %d0 does not work and dropping "d" fixes it since compiler can
> figure out correct register from variable size. The patch bellow fixes
> compilation for 4.2.
>  
> diff --git a/lib/x86/pci.c b/lib/x86/pci.c
> index f95cd88..231668a 100644
> --- a/lib/x86/pci.c
> +++ b/lib/x86/pci.c
> @@ -3,13 +3,13 @@
>  
>  static void outl(unsigned short port, unsigned val)
>  {
> -    asm volatile("outl %d0, %w1" : : "a"(val), "Nd"(port));
> +    asm volatile("outl %0, %w1" : : "a"(val), "Nd"(port));
>  }
>  
>  static unsigned inl(unsigned short port)
>  {
>      unsigned data;
> -    asm volatile("inl %w1, %d0" : "=a"(data) : "Nd"(port));
> +    asm volatile("inl %w1, %0" : "=a"(data) : "Nd"(port));
>      return data;
>  }

This is okay.

>  static uint32_t pci_config_read(pcidevaddr_t dev, uint8_t reg)
> diff --git a/x86/s3.c b/x86/s3.c
> index 71d3ff9..d568aa7 100644
> --- a/x86/s3.c
> +++ b/x86/s3.c
> @@ -143,14 +143,14 @@ static inline int rtc_in(u8 reg)
>  {
>      u8 x = reg;
>      asm volatile("outb %b1, $0x70; inb $0x71, %b0"
> -		 : "+a"(x) : "0"(x));
> +		 : "=a"(x) : "0"(x));
>      return x;
>  }

This should be wrong.  GCC should complain that the same operand is used
for both input and output but has an "=" constraint.

>  static inline void rtc_out(u8 reg, u8 val)
>  {
>      asm volatile("outb %b1, $0x70; mov %b2, %b1; outb %b1, $0x71"
> -		 : "+a"(reg) : "0"(reg), "ri"(val));
> +		 : "=a"(reg) : "0"(reg), "ri"(val));
>  }

Same here.

But I'm not sure what is the error message for older GCC for s3.c, as I
wrote in reply to Michael.

>  extern char resume_start, resume_end;
> diff --git a/x86/vmexit.c b/x86/vmexit.c
> index 3b945de..7e9af15 100644
> --- a/x86/vmexit.c
> +++ b/x86/vmexit.c
> @@ -26,7 +26,7 @@ static void outw(unsigned short port, unsigned val)
>  
>  static void outl(unsigned short port, unsigned val)
>  {
> -    asm volatile("outl %d0, %w1" : : "a"(val), "Nd"(port));
> +    asm volatile("outl %0, %w1" : : "a"(val), "Nd"(port));
>  }

Okay.

Paolo

>  static unsigned int inb(unsigned short port)
> --
> 			Gleb.
>