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Date: Thu, 17 Oct 2013 14:31:56 +0300
From: "Michael S. Tsirkin" <mst@redhat.com>
Message-ID: <20131017113156.GC18678@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] kvm-unittest: fix build with gcc 4.3.X and
	older
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: kvm@vger.kernel.org, Gleb Natapov <gleb@redhat.com>, qemu-devel@nongnu.org

On Thu, Oct 17, 2013 at 01:02:17PM +0200, Paolo Bonzini wrote:
> Il 17/10/2013 12:58, Gleb Natapov ha scritto:
> >>> > > @@ -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.
> 
> Hmm, looks like my version is the incorrect one and, if you use "+a" you
> need not use the matching input constraint.  So it's either your
> version, or one that removes the "0" altogether.
> 
> Thus,
> 
> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

Yea, you can add Reviewed-by: Michael S. Tsirkin <mst@redhat.com> too.

But maybe we can add %k constraints like Paolo said?
Being extra careful ...


> Paolo