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

[Gleb Natapov]

On Thu, Oct 17, 2013 at 12:55:16PM +0200, Paolo Bonzini wrote:
> 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.
> 
x86/s3.c: In function 'main':
x86/s3.c:145: error: inconsistent operand constraints in an 'asm'

And I am puzzled by this too.

> >  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.
> > 

--
                        Gleb.